After reading this chapter, the student will be able to:
Human movement is controlled and monitored by the nervous system. The nature of this control is such that many muscles may have to be activated to perform a vigorous movement such as sprinting, or only a few muscles may have to be activated to push a doorbell or make a phone call. The nervous system is responsible for identifying the muscles that will be activated for a particular movement and then generating the stimulus to develop the level of force that will be required from that muscle.
Many human movements require stabilization of adjacent segments while a fine motor skill is performed. This requires a great deal of coordination on the part of the nervous system to stabilize such segments as the arm and forearm while very small, coordinated movements are created with the fingers, as in the act of writing.
Accuracy of movement is another task with which the nervous system is faced. The nervous system coordinates the muscles to throw a baseball with just the right amount of muscular force so that the throw is successful. Recognizing the difficulty of being accurate with a physical movement contributes to an appreciation of the complexity of neural control.
The neural network is extensive because each muscle fiber is individually innervated by a branch of the nervous system. Information exits the muscle and provides input to the nervous system, and information enters the muscle to initiate a muscle activity of a specific nature and magnitude. Through this loop system, which is interconnected with many other loops from other muscles and with central nervous control, the nervous system is able to coordinate the activity of many muscles at once. Specific levels of force may be generated in several muscles simultaneously so that a skill such as kicking may be performed accurately and forcefully. Knowledge of the nervous system is helpful in improving muscular output, refining a skill or task, rehabilitating an injury, and stretching a muscle group.
FIGURE 4-1 The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of all of the nerves that lie outside the spinal cord. The 31 pairs of spinal nerves exit and enter the spinal cord at the various vertebral levels servicing specific regions of the body. Motor information leaves the spinal cord through the ventral root (anterior), and sensory information enters the spinal cord through the dorsal root (posterior).
General Organization of the Nervous System
The nervous system consists of two parts, the central nervous system and the peripheral nervous system, both illustrated in Figure 4-1. The central nervous system consists of the brain and the spinal cord and should be viewed as the means by which human movement is initiated, controlled, and monitored.
The peripheral nervous system consists of all of the branches of nerves that lie outside the spinal cord. The peripheral nerves primarily responsible for muscular action are the spinal nerves, which enter on the posterior, or dorsal, side of the vertebral column and exit on the anterior, or ventral, side at each vertebral level of the spinal cord. Eight pairs of nerves enter and exit the cervical region, 12 pairs at the thoracic region, five
at the lumbar region, five in the sacral region, and one in the coccygeal region. The pathways of the nerves are presented for the upper and lower extremities in Figures 4-2 and 4-3, respectively.
FIGURE 4-2 The upper extremity nerves. Nine nerves innervate the muscles of the upper extremity.
The nerves entering the spinal cord on the dorsal, or back, side of the cord are called sensory neurons because they transmit information into the system from the muscle. This pathway is termed the afferent pathway and carries all incoming information. The nerves exiting on the ventral, or front, side of the body are called motoneurons because they carry impulses away from the system to the muscle. This pathway is termed the efferent pathway and carries all outgoing information. Nerves from the dorsal and ventral roots join together as they exit so that sensory and motor neurons are mixed together to form a spinal nerve that can carry information in and out of the spinal cord.
Areas of the body supplied by the spinal nerves
FIGURE 4-3 The lower extremity nerves. Twelve nerves innervate the muscles of the lower extremity.
Structure of the Motoneuron
The neuron is the functional unit of the nervous system that carries information to and from the nervous system. The structure of a neuron, specifically the motoneuron, warrants examination to clarify the process of muscular contraction. Figure 4-4 shows a close up view of the neuron and the neuromuscular junction.
FIGURE 4-4 The cell body, or soma (A), of the neuron is in or just outside the spinal cord. Traveling from the soma is the axon (B), which is myelinated by Schwann cells (C), separated by gaps, the nodes of Ranvier (D). On the ends of each axon, the branches become unmyelinated to form the motor endplates (E) that terminate at the neuromuscular junction (F) on the muscle. Neurons receive information from other neurons through collateral branches (G).
The motoneuron consists of a cell body containing the nucleus of the nerve cell. The cell body, or soma, of a motoneuron is usually contained within the gray matter of the spinal cord or in bundles of cell bodies just outside the cord, referred to as ganglia. The cell bodies are arranged in pools spanning one to three levels of the spinal cord and innervate portions of a single muscle or selected synergists.
Projections on the cell body, called dendrites, serve as receivers and bring information into the neuron from other neurons. The dendrites are bunched to form small bundles. A bundle contains dendrites from other neurons and may consist of dendrites from different spinal cord levels or different neuron pools. The composition of the bundle changes as dendrites are added and subtracted. This arrangement facilitates cross-talk between neurons.
A large nerve fiber, the axon, branches out from the cell body and exits the spinal cord via the ventral root, where it is bundled together with other peripheral nerves.
The axon of the motoneuron is fairly large, making it capable of transmitting nerve impulses at high velocities, up to 100 m/sec. This large and rapidly transmitting motoneuron is also called an alpha motoneuron. The axon of the motoneuron is myelinated, or covered with an insulated shell. The myelination is sectioned, with Schwann cells insulating and enveloping a specific length along the axon, followed by a gap, termed the node of Ranvier, and then a repeat of the insulated Schwann cell covering.
When the myelinated motoneuron approaches a muscle fiber, it breaks off into unmyelinated terminals, or branches, called motor endplates, which embed into fissures, or clefts, near the center of the muscle fiber. This site is called the neuromuscular junction. The neuron does not make contact with the actual muscle fiber; instead, a small gap, termed the synaptic gap or synapse, exists between the terminal branch of the neuron and the muscle. This is the reason muscular contraction involves a chemical transmission–the only way for a nerve impulse to reach the actual muscle fiber is some type of chemical transmission across the gap.
The nerve impulse travels down the axon in the form of an action potential (Fig. 4-5). As reviewed in the Chapter 3, each action potential generates a twitch response in the muscle. If action potentials are in close enough sequence, the tensions generated by one muscle twitch are summed with other twitches to form a tetanus, or constant tension in the muscle fiber (see Fig. 3-9). This level of tension declines as the motor unit becomes incapable of regenerating the individual twitch responses fast enough.
The action potential is a propagated impulse, meaning that the amplitude of the impulse remains the same as it travels down the axon to the motor endplate. At the motor endplate, the action potential traveling down through the nerve becomes a muscle action potential traveling through the muscle. Externally, these two action potentials are indistinguishable. Eventually, the muscle action potential initiates the development of the cross-bridging and shortening within the muscle sarcomere. The total process is referred to as excitation-contraction coupling (see Chapter 3).
The Motor Unit
The structure of the motor unit was introduced in Chapter 3, in which we concentrated on the action of the muscles in the motor unit. In this section, we concentrate on the nervous system portion of the motor unit. The neuron, cell body, dendrites, axon, branches, and muscle fibers constitute the motor unit (Fig. 4-6). A neuron may terminate on as many as 2000 fibers in muscles, as in the gluteus maximus, or as few as five or six fibers, as in the orbicularis oculi of the eye. The typical ratio of neurons to muscle fibers is 1:10 for the eye muscles, 1:1600 for the gastrocnemius, 1:500 for the tibialis anterior, 1:1000 for the biceps brachii, 1:300 for the dorsal interossei in the hand, and 1:96 for the lumbricales in the hand (4). The average number of fibers per neuron is between 100 and 200 (4,53). The number of fibers controlled by one neuron is termed the innervation ratio. Whereas fibers with a small innervation ratio are capable of exerting fine motor control, those with a large innervation ratio are only capable of gross motor control. The fibers innervated by each motor unit are not bunched together and are not all in the same fascicle; rather, they are spread throughout the muscle.
FIGURE 4-5 The action potential travels down the nerve as the permeability of the nerve membrane changes, allowing an exchange of sodium (Na+) and potassium (K+) ions across the membrane. This creates a voltage differential that is negative on the outside of the membrane. This negative voltage, or action potential, travels down the nerve until it reaches the muscle and stimulates a muscle action potential that can be recorded.
When a motor unit is activated sufficiently, all of the muscle fibers belonging to it contract within a few milliseconds. This is referred to as the all-or-none principle. A muscle that has motor units with very low ratios of nerve to fiber, such as is seen in eye and hand movements,
allows finer control of the movement characteristics. Many lower extremity muscles have large neuron-to-fiber ratios suitable to functions in which large amounts of muscular output are required, such as in weight bearing and walking.
Events in the action potential
Muscle fibers of different motor units are intermingled so that the force applied to the tendon remains constant even with different muscle fibers are contracting or relaxing. Muscle tone is maintained in the resting muscle as random motor units contract.
The activity in the motor unit is determined from all of the inputs it receives. These include motor commands causing excitation via the alpha motorneuron as well as excitatory and inhibitory inputs the motor unit receives from other neurons. This is discussed later in the section on receptors.
Motor Unit Types
Three different types of motor units exist, corresponding to the three fiber types discussed in the previous chapter: slow-twitch oxidative (type I or S), fast-twitch oxidative (type IIa or FR), and fast-twitch glycolytic (type IIb or FF). Performance and size differences are illustrated in Figure 4-7. All three types of muscle fibers are found in all muscles, but the proportion of fiber types within a muscle varies. Whereas certain muscles, such as the soleus, consist primarily of type I muscle fibers and motor units muscles such as the vastus lateralis are approximately 50% type I and the remainder type II.
All of the muscle fibers in a motor unit are of the same type. The fast-twitch glycolytic motor units (type IIb) are innervated by very large alpha motoneurons that conduct the impulses at very fast velocities (100 m/sec), creating
rapid contraction times in the muscle (approximately 30 to 40 ms) (13). As a result, these large motor units generate muscular activity that contracts fast, develops high tensions, and fatigues quickly. These motor units usually have large neuron-to-fiber ratios and are found in some of the largest muscles in the body, such as the quadriceps femoris group. These motor units are useful in activities such as sprinting, jumping, and weight lifting.
FIGURE 4-6 A. The motor unit consists of a neuron and all of the fibers innervated by that neuron. The motoneurons exit the anterior side of the spinal cord and branch out, terminating on a muscle fiber. B. Fine motor movements can occur when the motor unit services only a small number of muscle fibers, such as in the eye. C. When the motor unit terminates on large numbers of muscle fibers, such as in the gastrocnemius, finer movement capabilities are lost at the gain of more overall muscle activity.
The fast-twitch oxidative motor units (type IIa) also have fast contraction times (approximately 30 to 50 ms), but they
have the advantage over fast-twitch glycolytic motor units because they are more fatigue resistant (13). These moderately sized motor units are capable of generating moderate tensions over longer periods. The activity from these motor units is useful in activities such as swimming and bicycling and in job tasks in factories and among longshoremen.
FIGURE 4-7 A. The type I slow-twitch (S) motor unit is smaller and is capable of generating sustained contractions and lower levels of force. B. The type IIa fast-twitch oxidative (FR) motor unit can also generate a sustained contractions at higher force levels than the type Ia. C. The type IIb fast-twitch glycolytic (FF) cannot sustain a contraction for any length of time but is capable of generating the highest force levels.
Motor unit properties
The slow-twitch oxidative motor units (type I) transmit the impulses slowly (approximately 80 m/sec), generating slow contraction times in the muscle (70/ms) (13). These motor units are capable of generating very little tension but can sustain this tension over a long time. Type I fibers are more efficient than the other two fiber types. Consequently, the slow-twitch motor units, the smallest of the three types, are useful in maintaining postures, stabilizing joints, and doing repetitive activities such as typing and gross muscular activities such as jogging.
Neural Control of Force Output
Chapter 3 explored a number of factors such as muscle cross-section that determines maximal force produced by a muscle. We also stated that the force exerted by a motor unit is determined by the number of fibers innervated by the motor unit and the rate at which the motor unit discharges the impulse or action potential (19). When a muscle is producing its maximal force, all motor units are activated and all muscle fibers are active.
Groups of neurons in the spinal cord that innervate a single muscle are termed a motor pool. Pool sizes range from a few hundred to a thousand depending on the size of the muscle. Motor neurons in the pool vary in electrical properties, amplitude of the input they receive, and in contractile properties (e.g., speed, force generation, fatigue resistance) (19).
The tension or force generated by a muscle is determined by the number of motor units actively stimulated at the same time and by the frequency at which the motor units are firing. Recruitment, the term used to describe the order of activation of the motor units, is the prime mechanism for force production in the muscle. Force produced by a muscle can be increased by increasing the number of active motor units to increase the active cross-sectional area of the muscle. Recruitment usually follows an orderly pattern in which pools of motor units are sequentially recruited (14). There is a functional pool of motor units for each task, whereby separate recruitment sequences can be initiated to stimulate the three different types of motor units (types I, IIa, IIb) for the performance of different actions within the same muscle.
The sequence of motor unit recruitment usually follows the size principle, whereby the small, slow-twitch motoneurons are recruited first, followed by recruitment of the fast-twitch oxidative and finally the large, fast-twitch glycolytic motor units (14). This is because the small motoneurons have lower thresholds than the large ones. Thus, the small motoneurons are used over a broad tension range before the moderate or large fibers are recruited.
In walking, for example, the low-threshold motor units are used for most of the gait cycle, except for some brief recruitment of the intermediate motor units during peak activation times. The high threshold, fast-twitch motor units are not usually recruited unless a rapid change of direction or a stumble takes place.
In running, more motor units are recruited, with some high-threshold units recruited for the peak output times in the cycle. Furthermore, the low-threshold units are recruited for activities such as walking and jogging, and the fast-twitch fibers are recruited in activities such as
weight lifting (14,25). Recruitment sequences for walking and for different exercise intensities are presented in Figure 4-8.
FIGURE 4-8 The order of activation of the motor units, termed recruitment, usually follows the size principle: The small slow-twitch fibers are recruited first, followed by the fast-twitch oxidative and last by the fast-twitch glycolytic fibers. A. The muscle activity for the three muscle types for three support phases in walking. Slow-twitch fibers are used for most of the gait cycle, with some recruitment of fast-twitch fibers at peak activation times. (Reprinted with permission from Grimby, L. . Single motor unit discharge during voluntary contraction and locomotion. In N. L. Jones et al. [Eds.]. Human Muscle Power. Champaign, IL: Human Kinetics, 111-129). B. Similar recruitment pattern, with slow-twitch fibers recruited for up to 40% of the exercise intensity, at which point the fast-twitch oxidative fibers are recruited. It is not until 80% of exercise intensity is reached that the fast-twitch glycolytic fibers are recruited (Reprinted with permission from
Sale, D. G. . Influence of exercise and training on motor unit activation. Exercise and Sport Science Reviews, 16:95-151, 1987.
The motor units are recruited asynchronously, whereby the activation of a motor unit is temporally spaced but is summed with the preceding motor unit activity. If the tension is held isometrically over a long time, some of the larger motoneurons are activated. Likewise, in vigorous, rapid movements, both small and large motoneurons are activated.
The motor unit recruitment pattern proceeds from small to large motoneurons, slow to fast, small force to large force, and fatigue-resistant to fatigable muscles. After a motor unit is recruited, it will remain active until the force declines, and when the force declines, the motor units are deactivated in reverse order of activation, with the large motoneurons going first. Also, the motor unit recruitment pattern is established in the muscle for a specific movement pattern (58). If the joint position changes and a new pattern of movement is required, the recruitment pattern changes because different motor units are recruited, although the order of recruitment from small to large remains the same. The force developed during recruitment does not increase in a jerky manner because the larger motoneurons are not brought into action until the muscle is already developing a large amount of force. In fact, the fractional increase in force is constant such that the larger the tension already in the muscle, the larger the size of motor units recruited.
The frequency of motor unit firing can also influence the amount of force or tension developed by the muscle. This is known as frequency coding or rate coding and involves intermittent high-frequency bursts of action potentials or impulses ranging from 3 to 120 impulses per second (53). With constant tension or slow increases in tension, the firing frequency is in the range of 15 to 50 impulses per second. This frequency rate can increase to a range of 80 to 120 impulses per second during fast contraction velocities. With increased rate coding, the rate of impulses increases in a linear fashion and only after all of the motor units are recruited (7).
In the small muscles, all of the motor units are usually recruited and activated when the external force of the muscle is at levels of only 30% to 50% of the maximum voluntary contraction level. Beyond this level, the force output in the muscle is increased through increases in rate coding, allowing for the production of a smooth, precise contraction.
In the large muscles, recruitment of motor units takes place all through the total force range, so that some muscles are still recruiting more motor units at 100% of maximum voluntary contraction. The deltoid and the biceps brachii are examples of muscles still recruiting motor units at 80% to 100% of maximum output of the muscle.
The rate coding also varies with fiber type and changes with the type of movement. Examples of the rate coding of both high- and low-threshold fibers in two muscle
contractions is illustrated in Figure 4-9. In ballistic movements, the higher threshold fast-twitch motor units fire at higher rates than the slow-twitch units. To produce rapid accelerations of the segments, the fast-twitch motor units increase the firing rates more than the slow-twitch motor units (25). The high-threshold fast-twitch fibers cannot be driven for any considerable length of time, but it is believed that trained athletes can drive the high-threshold units longer by maintaining the firing rates, resulting in the ability to produce a vigorous contraction for a limited time. Eventually, the frequency of motor unit firing decreases during any continuous muscular contraction, whether vigorous or mild.
FIGURE 4-9 A. Tension development in the muscle is influenced by the frequency at which a motor unit is activated, termed rate coding. In a sub-maximal muscle contract and hold, the high-threshold fast-twitch fibers increase firing rates in the ramp phase more than the low-threshold units. The frequency of motor unit firing drops off during the hold phase, and the high-threshold units cease firing. B. In a more vigorous contract and hold, the rate coding increases and is maintained further into the contraction by both the high and low threshold motor units (Reprinted with permission from
Sale, D. G. . Influence of exercise and training on motor unit activation. Exercise and Sport Science Reviews, 16:95-151, 1987.
The action potential in a motor unit can be facilitated or inhibited by the input it receives from the many neurons that are connecting to it within the spinal cord. As shown in Figure 4-10, a motor unit receives synaptic input from other neurons and from interneurons, which are connecting branches that can be both excitatory and inhibitory. The input is in the form of a local graded potential that, unlike the action potential, does not maintain its amplitude as it travels along. Thus, the stimulus has to be sufficient to reach its destination on another neuron and be large enough to generate a response in the neuron with which it has interfaced.
FIGURE 4-10 A. The action potential traveling through a motor unit can be altered by input from interneurons, which are small connecting nerve branches that generate a local graded potential; the potential may or may not institute a change in the connecting neuron. The interneurons may produce an excitatory local graded potential, which facilitates the action potential, or an inhibitory local graded potential sufficient to inhibit the action potential. B. A special interneuron, the Renshaw cell, receives excitatory information from a collateral branch of another neuron, stimulating an inhibitory local graded potential.
The alpha motoneuron has many collateral branches interacting with other neurons, and the number of collateral branches is highest in the distal muscles. An inhibitory interneuron receiving input from these collaterals is the Renshaw cell, also in the spinal cord. The Renshaw cell is considered one of the key elements in organizing muscular response in the agonists, antagonists, and synergists when it is stimulated sufficiently by a collateral branch (29,30).
Some evidence suggests that alternative recruitment patterns may be initiated by input from the excitatory and inhibitory pathways. This is done through interneurons that alter the threshold response of the slow- and fast-twitch units. The threshold level of the fast-twitch motor unit can be lowered via excitatory interneurons.
In ballistic movements involving rapid alternating movements, there appears to be synchronous or concurrent activation of the motor unit pool whereby large motor units are recruited along with the small motoneurons. This synchronous firing has also been shown to
occur as a result of weight training. It is believed that in athletic performance requiring a wide range of muscular output, the neuromuscular sequence may actually be reversed, with the fast-twitch fibers recruited first in vigorous muscle actions (8,14).
Sensory Receptors and Reflexes
The body requires an input system to provide feedback on the condition and changing characteristics of the musculoskeletal system and other body tissues, such as the skin. Sensors collect information on such events as stretch in the muscle, heat or pressure on the muscle, tension in the muscle, and pain in the extremity. These sensors send information to the spinal cord, where the information is processed and used by the central nervous system in the adjustment or initiation of motor output to the muscles. These sensors are connected to the spinal cord via sensory neurons.
When the sensory information from one of these receptors brings information into the cord, triggering a predictable motor response, it is termed a reflex. A reflex is an involuntary neural response to a specific sensory stimulus and is a stereotypical behavior in both time and space. A simple reflex arc is shown in Figure 4-11. In the case of tendon jerk reflex at the knee joint (stretch reflex), the magnitude of the reflex contraction of the quadriceps muscle resulting in a sudden involuntary extension of the leg is proportional to the intensity of the tap stimulus applied to the patellar tendon. Most reflexes are not that simple and can be modified with input from different areas. For example, a flexor reflex initiates a quick withdrawal response after receiving sensory information indicating pain such as from touching something hot. Compare this reflex with an anticipatory situation in which you are told that you are going to be jerked by the hand and need to maintain your balance by resisting with arm flexion. When the jerk is applied, there is a reflex response, but it is different from the flexor reflex because the context is different, and even though the response occurs at the spinal cord level, the circuitry has been reset to respond differently. Each reflex is influenced by the state of many interneurons, which receive input both from segmental and descending systems (47).
FIGURE 4-11 A simple reflex or monosynaptic arc. Sensory information from receptors is brought into the cord, where it initiates a motor response sent back out to the extremities. The stretch reflex is a reflex arc that sends sensory information into the cord in response to stretch of the muscle; the cord sends back motor stimulation to the same muscle, causing a contraction.
Reflexes that bring information into the spinal cord and are processed through both sides and different levels of the spinal cord are termed propriospinal. An example of this type of reflex is the crossed extensor reflex, which is initiated by receiving or expecting to receive a painful stimulus, such as stepping on a nail. This sensory information is processed in the spinal cord by creating a flexor and withdrawal response in the pained limb and an increase or excitation in the extension muscles of the other limb.
Another propriospinal reflex is the tonic neck reflex, which is stimulated by movements of the head that create a motor response in the arms. When the head is rotated to the left, this reflex stimulates an asymmetric response of extension of the same-side arm (left) and a flexion of the opposite arm (right). Also, when the head flexes or extends, this reflex initiates flexion or extension of the arms, respectively.
Another type of reflex is the supraspinal reflex, which brings information into the spinal cord and processes it in the brain. The result is a motor response. The labyrinthine righting reflex is an example of this type of reflex. This reflex is stimulated by leaning, being upside down, or falling out of an upright posture. The response from the upper centers is to stimulate a motor response from the neck and limbs to maintain or move to an upright position. This complex reflex involves many levels of the spinal cord as well as the upper centers of the nervous system. Examples of these various reflex actions are presented in Figure 4-12.
Proprioceptors are sensory receptors in the musculoskeletal system that transform mechanical distortion in the muscle or joint, such as any change in joint position, muscle length, or muscle tension, into nerve impulses that enter the spinal cord and stimulate a motor response (63). The muscle spindle is a proprioceptor found in higher abundance in the belly of the muscle lying parallel to the muscle fibers and actually connecting into the fascicles via connective tissue (Fig. 4-13). The fibers of the muscle spindle are termed intrafusal compared with muscle fibers that are termed extrafusal. The intrafusal fibers of the spindle are contained within a capsule, forming a spindle shape, hence the name muscle spindle. Some muscles, such as those of the eye, hand, and upper back, have hundreds of spindles; other muscles, such as the latissimus dorsi and other shoulder muscles, may have only a handful (63). Every muscle has some spindles. However, the muscle spindle is absent from some of the type IIb fast-twitch glycolytic muscle fibers within some muscles.
Each spindle capsule may contain as many as 12 intrafusal fibers, which can be either of two types: nuclear bag or nuclear chain (63). Both types of fibers have noncontractile centers that contain the nuclei of the fiber in addition to
sensory nerve fibers that take information into the system through the dorsal root of the spinal cord. The spindle also has contractile ends that can be innervated by a gamma motoneuron, creating shortening upon receipt of motor input. The gamma or fusimotor motoneuron is intermingled with the alpha motor neuron in the ventral horn of the spinal column. Smaller than the alpha motoneuron, each gamma motoneuron innervates multiple muscle spindles.
FIGURE 4-12 Examples of reflex actions (a reflex is a motor response developed in the central nervous system after sensory input is received). A. The flexor reflex is triggered by sensory information registering pain, which facilitates a quick flexor withdrawal from the pain source. B. The crossed extensor reflex is also initiated by pain; it works with the flexor reflex to create flexion on the stimulated limb and extension on the contralateral limb. C. The tonic neck reflex is stimulated by head movements; it creates flexion or extension of the arms, depending on the direction of the neck movement. D. The labyrinthine righting reflex is stimulated by body positioning; it causes movements of the limbs and neck to maintain a balanced, upright posture.
FIGURE 4-13 The muscle spindle lies parallel with the muscle fibers. Within each spindle capsule are the spindle fibers, which can be either of two types: nuclear chain or nuclear bag fibers. Both types have contractile ends that are innervated by gamma motoneurons. Sensory information responding to stretch leaves the middle portion of both the chain and bag fibers through the type la sensory neuron and from the ends of the nuclear chain fibers via the type II sensory neuron.
The nuclear bag fiber has a large cluster of nuclei in its center. It is also thicker, and its fibers connect to the capsule and to the actual connective tissue of the muscle fiber itself. The nuclear chain fiber is smaller, with the nuclei arranged in rows in the equatorial region. The nuclear chain fiber does not connect to the actual muscle fiber but only makes connection with the spindle capsule.
Exiting from the equatorial region of both types of spindle fibers, the type Ia primary afferent neuron is stimulated by a change in length of the middle of the spindle. Information from the sensory endings sends information into the dorsal horn and makes a monosynaptic, or direct, connection with a motor neuron, resulting in a contraction of the same muscle. Because the spindle lies parallel to the muscle fibers, it is subject to the same stretch as the muscle. The other mechanism of “stretching” the middle portion of the spindle is through contraction of the ends of the spindle via gamma motoneuron innervation. Both the nuclear bag and nuclear chain fibers are innervated by their own gamma motoneuron, the dynamic and static gamma efferents, respectively. The shortening of the ends of the spindle fibers through gamma innervation allows tuning of the muscle spindle to meet the needs of the movement parameters (Fig. 4-14).
FIGURE 4-14 Two afferent pathways bring information from the spindle into the spinal cord. The type Ia primary afferent pathway exiting from the equatorial regions of both the nuclear chain and bag fibers provides sensory information about muscle length and velocity of stretch. The type II secondary afferent pathway exiting from the ends of the nuclear chain fibers provides information about muscle length. Both fiber types receive motor innervation of the contractile ends via gamma efferent neurons.
From the polar ends of the nuclear chain fiber, additional sensory information is transmitted via the type II secondary afferent sensory neuron. This sensory neuron is medium sized and is stimulated by stretch in the muscle, responding at a higher threshold of stretch than the type I sensory neuron. There are generally one or two type II sensory neurons per muscle spindle, although some muscle spindles and even some muscles (10% to 20%) have none (63).
The primary afferents are sensitive to the rate of change of the stretch of the muscle and act as velocity sensors. The sensitivity of the primary afferents is nonlinear and is very sensitive to small changes in length and rate of change (short range stretch), but falls off with slower or larger changes in length. The secondary afferents are muscle length sensors with some sensitivity to rate of change in length. Figure 4-15 illustrates the response of the primary and secondary afferents in the absence of any gamma innervation with stretch of the muscle, a quick tap of the muscle, a cyclic stretch and release, and with the release of the stretch.
When a stretch is imposed on the muscle, the equatorial region of the intrafusal fibers deforms the nerve endings and the type I sensory neuron sends impulses into the spinal cord. Sensory action potentials connect with
interneurons, generating an excitatory local graded potential that is sent back to the muscle being stretched. If the stretch is vigorous enough, a local graded impulse is sent back to the same muscle with sufficient magnitude to initiate a contraction via the alpha motoneurons. Sensory information enters and motor information leaves the spinal cord at the same level, creating a monosynaptic reflex arc in which the sensory input connects directly on the motor neuron. An example of this reflex is the stretch or myotatic reflex, which is stimulated by sensory neurons responding to stretch in the muscle, which in turn, initiates an increase in the motor input to the same muscle (63). The type Ia loop is illustrated inFigure 4-16. It is also termed autogenic facilitation because of the facilitation of the alpha motoneurons of the same muscle. The stretch reflex primarily recruits slow-twitch muscle fibers.
FIGURE 4-15 The primary and secondary afferent firing rates differ in response to an imposed stretch or relaxation of the muscle. The responses of both the primary and secondary afferent are shown for three different stretch conditions with the influence of any gamma innervation removed. The primary afferent responds to a stretch imposed on the muscle and fires at higher rates when a rapid stretch is imposed on the muscle in the case of a tap. When the stretch is removed, the primary afferent ceases firing. The secondary afferent fires at a more consistent rate to reflect the length of the muscle.
The information coming into the spinal cord via the type I sensory neuron is also sent to the cerebellum and cerebral sensory areas to be used as feedback on muscle length and velocity. Additional connections are made in the spinal cord with inhibitory interneurons, creating a reciprocal inhibition, or relaxation of the antagonistic muscles (29). Other excitatory interneuron connections are made with the alpha motoneurons of synergistic muscles to facilitate their muscle activity along with the agonist.
When the type II, or secondary, afferent neuron is stimulated, it has a different response from that of the type I sensory neuron. It produces a sensory input in response to stretch or change in length in the muscle, and it is a good feedback indicator of the actual length in the muscle because its sensory impulses do not diminish when the muscle is held in a stationary position.
FIGURE 4-16 The type Ia loop initiated by a stretch of the muscle. Responding proportionally to the rate of stretch, the muscle spindle sends impulses to the spinal cord via the type Ia sensory neuron. Within the cord, connections with interneurons produce a local, graded potential that inhibits the antagonistic muscles and excites the synergists and the muscle in which the stretch occurred. This is the typical stretch reflex response, also termed autogenic facilitation.
The innervation of the ends of the spindle fibers by the gamma motoneuron alters the response of the muscle spindle considerably. The first important effect of gamma innervation of the spindle is that it does not allow spindle discharge to cease when a muscle is shortened. If the muscle shortened with no alpha-gamma coactivation, the spindle activity would be silenced by the removal of the external stretch on the muscle. The alpha-gamma coactivation keeps the spindle taut and allows it to continue to provide position and length information despite shortening of the muscle (63). There is some indication that this is only true for slow movements and for movements under load but is not true for fast movements. In fast movements, the stretching activity in the spindles of the antagonistic muscle may provide the length and position information.
The second major input from gamma motoneuron innervation of the muscle spindle is an indirect enhancement of the motor impulses being sent to the muscle via the alpha neuron pathways. This adds to the impulses coming down through the system, alters the gain, and increases the potential for full activation via the alpha pathways. It is a main contributor to coordinating the output and patterning of the alpha motoneurons.
In anticipation of lifting something heavy, the alpha and gamma motoneurons establish a certain level of excitability in the system for accommodating the heavy resistance. If the object lifted is much lighter than anticipated, the gamma system acts to reduce the output of the type I afferent, making a quick adjustment in the alpha motoneuron output to the muscle and reduce the number of motor units activated.
Finally, the gamma motoneuron is activated at a lower threshold than the alpha motoneuron and can therefore
initiate responses to postural changes by resetting the spindle and activating the alpha output (27). The afferent pathways, gamma pathways, and alpha pathways are all part of the gamma loop, which is shown in Figure 4-17.
FIGURE 4-17 The type Ia loop in which information is sent from the spindle (4), causing inhibition (3) and excitation of synergists and agonists (2,1). It is facilitated by input from the gamma motoneuron (5), which initiates a contraction of the ends of the spindle fibers, creating an internal stretch of the spindle fibers. The gamma motoneuron receives input via the upper centers or other interneurons in the spinal cord (6,7,8).
Golgi Tendon Organ
Another important proprioceptor significantly influencing muscular action is the Golgi tendon organ (GTO). This structure monitors force or tension in the muscle. As illustrated inFigure 4-18, the GTO lies at the musculoskeletal junction. It is a spindle-shaped collection of collagen fascicles surrounded by a capsule that continues inside the fascicles to create compartments. The collagen fibers of the GTO are connected directly to extrafusal fibers from the muscles (63).
Two sensory neurons exit from a site between the collagen fascicles. When the collagen is compressed through a stretch or contraction of the muscle fibers, the type Ib nerve endings of the GTO generate a sensory impulse proportional to the amount of deformation created in them. The response to the load and the rate of change in the load are linear. Several muscle fibers insert in one GTO, and any tension generated in any of the muscles will generate a response in the GTO.
In a stretch of the muscle, the tension in the individual GTO is generated along with all other GTOs in the tendon. Consequently, the GTO response is more sensitive in tension than in stretch. This is because the GTO measures load bearing in series with the muscle fibers but is parallel to the tension developed in the passive elements during stretch (33). Thus, contraction has a lower threshold than stretch.
The GTO generates an inhibitory local graded potential in the spinal cord known as the inverse stretch reflex. If the graded potential is sufficient, relaxation or autogenic inhibition is produced in the muscle fibers connected in series with the GTO stimulated. The alpha motoneuron output to muscles undergoing a high-velocity stretch or producing a high-resistance output is reduced.
The GTO is very sensitive to small changes in tension, so it is used to modulate changes in force. It assists with providing information on force so that the individual applies just the right amount of force to overcome a load. The GTO is reliable in signaling whole-muscle tension whether it is active or passive tension, even after a fatiguing routine (24). The GTO can generate an inhibitory response via the type Ib pathway to reduce contraction strength in a muscle experiencing a rapid increase in force. Alternately, the GTO can actually provide excitatory input in an activity such as walking, during which the GTO detects tension in the support muscles and stimulates an extensor reflex. Again, with input from upper neural centers, the context changes and circuits are adjusted accordingly.
Tactile and Joint Sensory Receptors
There is limited information on the sensory neuron input from the tactile and joint receptors placed in and around the synovial joints (Fig. 4-19). One such tactile receptor, the Ruffini ending, lies in the joint capsule and responds to change in joint position and velocity of movement of
the joint (54). The pacinian corpuscle is another tactile receptor in the capsule and connective tissue that responds to pressure created by the muscles and to pain within the joint (54). These joint receptors, as well as other receptors in the ligaments and tendons, provide continuous input to the nervous system about the conditions in and around the joint.
FIGURE 4-18 A. The Golgi tendon organ (GTO) is at the muscle-tendon junction. B. When tension is at this site, the GTO sends information into the spinal cord via type Ib sensory neurons. The sensory input from the GTO facilitates relaxation of the muscle via stimulation of inhibitory interneu-rons. This response is as the inverse stretch reflex, or autogenic inhibition.
FIGURE 4-19 A number of other sensory receptors send information into the central nervous system. In the joint capsules and connective tissue are found the pacinian corpuscle, which responds to pressure, and the Ruffini endings, which respond to changes in joint position. Also, free nerve endings around the joints create pain sensations.
Effect of Training and Exercise
During training of the muscular system, a neural adaptation modifies the activation levels and patterns of the neural input to the muscle. In strength training, for example, significant strength gains can be demonstrated after approximately 4 weeks of training. This strength gain is not attributable to an increase in muscle fiber size but is rather a learning effect in which neural adaptation has occurred (59), resulting in increases in factors such as firing rates, motor neuron output, motor unit synchronization, and motorneuron excitability (1).
The effect of the neural adaptation is an improved muscular contraction of higher quality through coordination
of motor unit activation. The neural input to the muscle, as a consequence of maximal voluntary contractions, is increased to the agonists and synergists, and inhibition of the antagonists is greater. This neural adaptation, or learning effect, levels off after about 4 to 5 weeks of training and is typically the result of an increase in the frequency of motor unit activation. Increases in strength beyond this point are usually attributable to structural changes and physical increases in the cross-section of the muscle. The influence of training on both the electromechanical delay and the amount of electromyographic activity is presented from the work of Hakkinen and Komi (26) in Figure 4-20. Specificity of training is important for enhancement of neural input to the muscles. If one limb is trained at a time, greater force production can be attained with more neural input to the muscles of that limb than if two limbs are trained at once. The loss of both force and neural input to the muscles through bilateral training is termed bilateral deficit (5,14). In fact, training of one limb neu-rologically enhances the activity and increases the voluntary strength in the other limb.
FIGURE 4-20 A. Explosive strength training has been shown to decrease the electromechanical delay (EMD) in the muscle contraction after 12 weeks of training. The EMD increases again, however, if training continues to 24 weeks and drops off slightly with detraining. The influence of heavy resistance training on EMD is negligible. B. The IEMG increases in the early weeks during heavy resistance training but not with explosive training. It is believed that some neural adaptation occurs in the early stages of specific types of resistance training, which facilitates an early increase in force production. (Reprinted with permission from
Hakkinen, K., and Komi, P. V. . Training-induced changes in neuromuscular performance under voluntary and reflex conditions. European Journal of Applied Physiology, 55:147-155.
When working with athletes who use the limbs asymmetrically, as in running or throwing, the trainer should incorporate some unilateral limb movements into the conditioning program. Participants in sports or activities that use both limbs together, such as weight lifting, should train bilaterally.
Specificity of training also determines the fiber type that is enhanced and developed. Through resistive training, type II fibers can be enhanced through reduction in central inhibition and increased neural facilitation. This may serve to resist fatigue in short-term, high-intensity exercise in which the fatigue is brought on by the inability to maintain optimal nerve activation.
Even a short warmup (5 to 10 minutes) preceding an event or performance influences neural input by increasing the motor unit activity (38). Another factor that enhances the neural input to the muscle is the use of an antagonistic muscle contraction that precedes the contraction of the agonist, such as seen in preparatory movements in a skill (e.g., backswing, lowering). This diminishes the inhibitory input to the agonist and allows for more neural input and activation in the agonist contraction.
A stretch of a muscle before it contracts produces some neural stimulation of the muscle via the stretch reflex arc. Athletes who must produce power, such as jumpers and sprinters, have been shown to have excitable systems in which the reflex potentiation is high (38).
When fatigue occurs during exercise, a reduction occurs in the maximal force capacity of the muscle, caused by impairment of both muscular and neural mechanisms (31). A decline in force output involves multiple neural mechanisms, including the influence of afferent feedback, descending inputs, and spinal circuitry influence on the output of the motor pool. The mechanism contributing to fatigue depends on the task, and variations in contraction intensity create differences in the balance of excitatory and inhibitory inputs to the pool.
In summary, the neural input to muscle can be enhanced through training to increase the number of active motor units contributing, alter the pattern of firing, and increase the reflex potentiation of the system (52). Likewise, immobilization of the muscle can create the opposite response by lowering neural input to the muscle and decreasing reflex potentiation.
Flexibility is viewed by many to be an essential component of physical fitness and is seen as an important component of performance in sports such as gymnastics and dance. Flexibility can be increased with a stretching program.
No solid evidence demonstrates that increased flexibility is important for injury reduction or that it is a protection against injury (67). In fact, stretching before a sport performance may even have a negative influence by reducing force production and power output (46) and is only recommended for activities that require high levels of flexibility (37). Nevertheless, to maintain a functional range of motion, a regular stretching routine integrated into a conditioning program is recommended. A flexibility conditioning program should be undertaken daily or at least three times a week and should preferably take place after exercise.
Flexibility, as it is used in this section, is defined as the terminal range of motion of a segment. This can be obtained actively through some voluntary contraction of an agonist creating the joint movement (active range of motion) or passively, as when the agonist muscles are relaxed as the segment is moved through a range of motion by an external force, such as another person or object (passive range of motion) (60,68).
Many components contribute to one's flexibility or lack thereof. First, joint structure is a determinant of flexibility because it limits the range of motion in some joints and produces the termination or end point of the movement. This is true in a joint such as the elbow, in which the movement of extension is terminated by bony contact between the olecranon process and fossa on the back of the joint. A person who can hyperextend the forearm at the elbow is not one who is exceptionally flexible but is someone who has either a deep olecranon fossa or a small olecranon process. Bony restrictions to range of motion are present in a variety of joints in the body, but this type of restriction is not the main mechanism that limits or enhances joint flexibility.
Soft tissue around the joint is another factor contributing to flexibility. As a joint nears the ends of the range of motion, the soft tissue of one segment is compressed by the soft tissue of the adjacent segment. This compression between adjacent tissue components eventually contributes to the termination of the range of motion. This means that obese individuals and individuals with large amounts of muscle mass or hypertrophy usually demonstrate lower levels of flexibility. An individual with hypertrophied muscles, however, can obtain good flexibility in a joint by applying a greater force at the end of the range of motion, which compresses the restrictive soft tissue to a greater degree. An obese individual who lacks strength is definitely limited in flexibility because of an inability to produce the force necessary to achieve the greater range of motion.
Ligaments restrict range of motion and flexibility by offering maximal support at the end of the range of motion. For example, the ligaments of the knee terminate the extension of the leg. An individual who can hyperextend the knees is commonly called double jointed but actually has slightly long ligaments that allow more than the usual joint motion.
The main factors that influence flexibility are the actual physical length of the antagonistic muscle or muscles; the viscoelastic characteristics of the muscles, ligaments, and other connective tissues; and the level of neurological innervation in a muscle being stretched. Soft tissue extensibility is related to the resistance of the tissue when it lengthens, and stretching overcomes the passive resistance in the tissue (57). All of these factors can be influenced by specific types of flexibility training.
When a muscle is stretched, neurological mechanisms can influence the range of motion. In a rapid stretch, the type Ia primary afferent sensory neuron in the muscle spindle initiates the stretch reflex, creating increased muscular activity through alpha motoneuron innervation. This response is proportional to the rate of stretch. Thus, the faster the stretch, the more the same muscle contracts. After the stretch is completed, the type Ia sensory neurons decrease to a lower firing level, reducing the level of motoneuron activation or resistance in the muscle. A flexibility technique that enhances this response is ballistic stretching, in which the segments are bounced to achieve the terminal range of motion. This type of stretching is not recommended for the improvement of flexibility because
of the stimulation of the type Ia neurons and the increase in the resistance in the muscle. Slow rates of elongation permit greater stress relaxation and generate lower tissue forces. However, ballistic stretching is a component of many common movements, such as a preparatory windup in baseball or the end of the follow-through in a kick.
A better stretching technique for the improvement of range of motion is static stretching, in which the limb is moved slightly beyond the terminal position slowly and then maintained in that position for 10 to 30 seconds (9). Moving the limb slowly decreases the response of the type Ia sensory neuron, and holding the position at the end reduces the type Ia input, allowing minimal interference to the joint movement.
The primary restriction to the stretch of a muscle is found in the connective tissue and tendons in and around the muscle (23,50). This includes the fascia, epimysium, perimysium, endomysium, and tendons. The actual muscle fibers do not play a significant role in the elongation of a muscle through flexibility training. To understand how the connective tissue responds to a stretch, it is necessary to examine the stress-strain characteristics of the muscle unit.
When a stretch is first imposed, the muscle creates a linear response to the load through elongation in all parts of the muscle. This is the elastic phase of external stretch. If the external load is removed from the muscle during this phase of stretching, it will return to its original length within a few hours, and no residual or long-term increase in muscle length will remain. The stretching techniques working the elastic response of the muscle are common; they include short-duration, repetitive joint movements. These stretches, usually preceding an activity, produce some increase in muscle length for use in the practice or game but do not produce any long-term improvement in flexibility.
If a muscle is put in a terminal position and maintained in the position for an extended period, the tissue enters the plastic region of response to the load, elongating and undergoing plastic deformation (66). This plastic deformation is a long-term increase in the length of the muscle and carries over from day to day (41). A model describing the behavior of the elastic and plastic elements acting in a stretch is presented in Figure 4-21.
To create increases in length through plastic or long-term elongation, the muscle should be stretched while it is warm, and the stretch should be maintained for a long time under a low load (54,56). Thus, to gain long-term benefits from stretching, the stretch should occur after a practice or workout, and individual stretches should be held in the terminal joint positions for an extended period.
Cooling of a warm muscle enhances the permanent elongation of the tissues in that muscle. The joint positions should be held for at least 30 seconds and ideally up to 1 minute. However, in muscles that are inflexible and require extra attention, however, stretching should occur for longer times, 6 to 10 minutes (61). To avoid any significant tissue damage, stretching with pain should not take place.
FIGURE 4-21 When a repetitive stretch of short duration is applied to the muscle, the connective tissue and muscle respond like a spring, with a short-term elongation of the tissue but a return to the original length after a short time. In a long-term sustained stretch, especially while the muscle is warm, the tissues behave more hydraulically, as a long-term deformation of the tissues takes place. (Reprinted with permission from
Sapega, A. A., Quedenfeld, T. C, Moyer, R. A., and Butler, R. A. . Biophysical factors in range of motion exercises. Physician and Sports Medicine, 9:57-64.
Proprioceptive Neuromuscular Facilitation
Proprioceptive neuromuscular facilitation (PNF) is a technique used to stimulate relaxation of the stretched muscle so that the joint can be moved through a greater range of motion (36). This technique, used in rehabilitation settings, can also be put to good use with athletes and individuals who have limited flexibility in certain muscle groups, such as the hamstrings (56).
PNF incorporates various combination sequences using relaxation and contraction of the muscles being stretched. A simple PNF exercise is passive movement of an individual's limb into the terminal range of motion, have him or her contract back isometrically against the manual resistance applied by a partner, and then relax and move further into the stretch (contract-relax). Repeating this cycle can achieve a significant increase in the terminal range of motion (20). This procedure increases the range of motion because the input from the type Ia afferent from the muscle spindle is reduced by the resetting of the spindle (27). PNF exercises are usually diagonal and in line with the fiber direction of the muscle. Stretching in an oblique pattern is closer to the actions found in common movements (39).
The process can be enhanced even more if a contraction of the agonist occurs at the end of the range of motion. This sets up an increase in the relaxation of the antagonist or the muscle being stretched. For example, passively move the foot into plantarflexion to stretch the dorsiflexors. Contract the dorsiflexors isometrically against resistance applied by a partner on the top of the foot. Move the foot farther into plantarflexion and then
contract the plantarflexors. Both of these techniques produce the greatest increase in the range of motion. In a hold-relax PNF exercise, the GTO is stimulated so that reflex inhibition is produced, making the subsequent passive stretch easier. In a slow-reversal hold-relax PNF exercise, the GTO is also stimulated in the isometric “hold” phase. The antagonists generate a slow reversal movement to elongate the target muscle, activating the muscle spindles and desensitizing the spindle during the follow-up passive elongation. Examples of PNF exercises for the muscles of the hip and shoulder joint are presented in Figure 4-22.
FIGURE 4-22 Examples of PNF exercises. A. At the hip, the thigh moves through a diagonal pattern, with manual resistance applied at the foot and thigh. B. In the shoulder, the arm moves into flexion, with manual resistance offered at the hand.
The purpose of plyometric training is to improve the velocity and power output in a performance. Plyometric training has been effective in increasing power output in athletes in sports such as volleyball, basketball, high jumping, long jumping, throwing, and sprinting. Plyometrics builds on the idea of specificity of training, whereby a muscle trained at higher velocities will function better at those velocities.
A plyometric exercise consists of rapidly stretching a muscle and immediately following with a contraction of the same muscle (5). The stretch-contract principle behind plyometric exercise was discussed in the previous chapter and shown to be an effective stimulator of force output. For example, a countermovement jump can make a 2- to 4-cm difference in the height of a vertical jump compared with a squat jump that does not include the stretch-contract sequence (10). Plyometric exercises improve power output in the muscle through facilitation of the neurological input to the muscle and through increased muscle tension generated in the elastic components of the muscle.
The neurological basis for plyometrics is the input from the stretch reflex via the type Ia sensory neuron. Rapid stretching of the muscle produces excitation of the alpha motoneurons contracting that muscle. This excitation is increased with the velocity of the stretch and is at its maximum level at the conclusion of a rapid stretch, after which the excitation levels decrease. Thus, if a muscle can be rapidly stretched and immediately contracted with no pause at the end of the stretch, this reflex loop produces maximum facilitation. If an individual pauses at the end of the stretch, this myoneural input is greatly diminished. The myoelectric enhancement of the muscle being stretched accounts for approximately 25% to 30% of the increase of the force output in the plyometric stretch-contract sequence (40).
The factor accounting for most of the increases in output (70% to 75%) as a consequence of plyometric exercise is the restitution of elastic energy in the muscle (40). At the end of the stretch phase in a plyometric exercise, the muscle initiates an eccentric muscle action that increases the force and stiffness in the musculotendinous unit, resulting in storage of elastic energy. When a muscle is stretched, elastic potential energy is stored in the connective tissue and tendon and in the cross-bridges as they are rotated back with the stretch (2). With a vigorous short-term stretch, maximal recovery of the elastic potential energy is returned to the succeeding contraction of that same muscle. The net result of this short-range prestretch with a small time period between the stretch and the contraction is that larger forces can be produced for any given velocity, enhancing the power output of the system (12). Implementation of this technique suggests that a quick stretch through a limited range of motion should be followed immediately by a vigorous contraction of the same muscle.
A plyometric exercise program includes a series of exercises imposing a rapid stretch followed by a vigorous contraction. Because the muscle is undergoing a vigorous eccentric contraction, attention should be given to the number of exercises and the load imposed through the eccentric contraction (16,45). It is suggested that plyometric exercises be done on yielding surfaces and not more than 2 days a week. Injury rates are higher in the use of plyometric training if these factors are not taken into account. Furthermore, plyometric training should be used very conservatively when the participants lack strength in the muscles being trained. A strength base should be developed first. It is suggested that an individual be able to squat 60% of body weight five times in 5 seconds before beginning plyometrics (15). This is done to see if eccentric and concentric muscle actions can be reversed quickly.
Lower extremity plyometric exercises include activities such as single-leg bounds, depth jumps from various heights, stair hopping, double-leg speed hops, split jumps, bench jumps, and quick countermovement jumping. The height from which the plyometric jump is performed is an important consideration. Heights can range from 0.25 to 1.5 m and should be based on the fitness level of the participant. A height is too high if a quick, vigorous rebound cannot be achieved shortly after landing.
Plyometric exercises can be done one to two times per week by a conditioned athlete. A sample plyometric workout may include three to five low-intensity exercises (10 to 20 repetitions), such as jumping in place or double-leg hops; three to four moderate-intensity exercises (5 to 10 repetitions) including single-leg hops, double-leg hops over a hurdle, or bounding; and two to three high-intensity exercises, including depth jumping (5 to 10 repetitions). In the beginning, the height of the box for depth jumping should be limited to avoid injury because the amount of force to be absorbed and controlled will increase with each height increase.
Upper extremity activities can best be implemented with surgical tubing or material that can be stretched. The muscle can be pulled into a stretch by the surgical tubing, after which the muscle can contract against the resistance offered by the tubing. For example, hold surgical tubing in a diagonal position across the back and simulate a throwing motion with the right hand while holding the left hand in place. The arm will generate a movement against the surgical tube resistance and then be drawn back into a quick stretch by the tension generated in the tubing. These resistive tubes or straps can be purchased in varying resistances offering compatibility with a variety of different strength levels.
Other forms of upper extremity plyometrics include catching a medicine ball and immediately throwing it. This puts a rapid stretch on the muscle in the catch which is followed by a concentric contraction of the same muscles in the throw. Figure 4-23 shows specific plyometric exercises.
FIGURE 4-23 Plyometric exercises can be developed for any sport or region of the body by use of a stretch-contract cycle in exercise. Examples for the lower extremity include bounding (A) and depth (B) jumps. For the upper extremity, the use of surgical tubing (C) and medicine ball throws (D) are good exercises.
A form of combined training is called complex training in which strength is combined with speed work to enhance multiple components of the muscle. For example, a squat exercise could be paired with depth jumps. The squat will facilitate concentric performance via strength training, and the depth jump will facilitate eccentric performance and rate of force development through plyometrics (18).
The electrical activity in the muscle can be measured with electromyography (EMG). This allows for the measurement of the change in the membrane potential as the action potentials are transmitted along the fiber. The study
of muscle from this perspective can be valuable in providing information concerning the control of voluntary and reflexive movements. The study of muscle activity during a particular task can yield insight into which muscles are active and when the muscles initiate and cease their activity. In addition, the magnitude of the electrical response of the muscles during the task can be quantified. EMG has limitations, however, and these must be clearly understood if it is to be used correctly.
The electromyogram is the profile of the electrical signal detected by an electrode on a muscle, that is, it is the measure of the action potential of the sarcolemma. The EMG signal is complex and is the composite of multiple action potentials of all active motor units superimposed on each other. Figure 4-24 illustrates the complexity of the signal. Note that the raw signal has both positive and negative components.
The amplitude of the EMG signal varies with a number of factors (discussed in a later section). Although the amplitude increases as the intensity of the muscular contraction increases, this does not mean that a linear relationship exists between EMG amplitude and muscle force. In fact, increases in EMG activity do not necessarily indicate an increase in muscle force (65). Only in isometric contractions are muscle electrical activity and muscle force closely associated (22).
FIGURE 4-24 Electromyography (EMG) records. A. A single action potential. B. A single EMG record containing many action potentials. The duration of the single action potential is much shorter than that of the EMG signal in B.
Recording an Electromyographic Signal
The EMG signal is recorded using an electrode. An electrode, which acts like an antenna, may be either indwelling or on the surface. The indwelling electrode, which may be either a needle or fine wire, is placed directly in the muscle. These electrodes are used for deep or small muscles. Surface electrodes are placed on the skin over a muscle and thus are mainly used for superficial muscles; they should not be used for deep muscles. The surface electrode is most often used in biomechanics, so most of the following discussion addresses surface electrodes.
Surface electrodes can be placed in either a monopolar or bipolar arrangement (Fig. 4-25). In a monopolar mode, one electrode is placed directly over the muscle in question, and a second electrode goes over an electrically neutral site, such as a bony prominence. Monopolar recordings are nonselective relative to bipolar recordings, and although they are used in certain situations, such as static contractions, they are poor in nonisometric movements. Bipolar electrodes are much more commonly used in biomechanics. In this case, two electrodes with a diameter of about 8 mm are placed over the muscle about 1.5 to 2.0 cm apart, and a third electrode is placed at an electrically neutral site. This arrangement uses a differential amplifier, which records the difference between the two recording electrodes. This differential technique removes any signal that is common to the inputs from the two recording electrodes.
The correct placement of electrodes is critical to a good recording. It is obvious that the electrodes must be placed so that the action potentials from the underlying muscle can be recorded. Therefore, electrodes should not be placed over tendinous areas of the muscle or over the motor point, that is, the point at which the nerve enters the muscle. Because action potentials propagate in both directions along the muscle from the motor point, signals recorded above the motor point have the potential to be attenuated because of cancellation of signals from both electrodes. Various sources describe the standard locations for electrode placement (42).
Electrodes must also be oriented correctly, that is, parallel to the muscle fiber. The EMG signal is greatly affected when the electrodes are perpendicular rather than parallel to the fiber.
FIGURE 4-25 Electromyography electrodes can have either monopolar (A) or bipolar (B) configuration.
When using surface electrodes, the resistance of the skin must be taken into consideration. For an electrical signal to be detected, this resistance should be very low. To obtain a low skin resistance, the skin must be thoroughly prepared by shaving the site, abrading the skin, and cleaning the skin with alcohol. When this is done, the electrodes can be placed properly.
Amplification of the Signal
The EMG signal is relatively small, varying from 10 to 5 mV. It is therefore imperative that the signal be amplified, generally up to a level of 1 V. The usual type is the differential amplifier, which can amplify the EMG signal linearly without amplifying the noise or error in the signal. The noise in the EMG signal can be from sources other than the muscle, such as power line hum, machinery, or the amplifier itself. In addition, the amplifier must have high input impedance (resistance) and good frequency response and must be able to eliminate common noise from the signal.
Factors Affecting the Electromyogram
Any of a number of factors, both physiological and technical, can influence the interpretation of an EMG signal (35) (Figure 4-26). It is essential to fully understand these factors before a knowledgeable interpretation of the EMG signal can be made. Some, such as muscle fiber diameter, number of fibers, number of active motor units, muscle fiber conduction velocity, muscle fiber type and location, motor unit firing rate, muscle blood flow, distance from the skin surface to the muscle fiber, and tissue surrounding the muscle, may appear obvious because they all relate to the muscle itself. Others, including electrode-skin interface, signal conditioning, and electrode spacing, essentially relate to how the data are collected. These factors are amplified when measuring a dynamic contraction, and additional factors, such as a nonstationary EMG signal, shifting of the electrodes relative to the action potential origins, and changes in tissue conductivity characteristics, also become concerns (21).
FIGURE 4-26 Some of the influences on the electromyographic signal. 1. Muscle fiber diameter. 2. Number of muscle fibers. 3. Electrode-skin interface. 4. Signal condition-ing.5. Number of active motor units. 6. Tissue. 7. Distance from skin surface to muscle fiber. 8. Muscle fiber conduction velocity. 9. Muscle blood flow. 10. Interelectrode spac-ing.11. Fiber type and location. 12. Motor unit firing rate. (Adapted with permission from
Kamen, G., Caldwell, G. E.  Physiology and interpretation of the elec-tromyogram.Journal of Clinical Neuro-physi-ology, 13:366-384.
Analyzing the Signal
Except under special circumstances, it is difficult to record a single action potential. Thus, we are left with a signal made up of numerous action potentials from many motor units. Researchers are often interested in quantifying the EMG signal, and employ several procedures to do so (44). Most often, biomechanists first rectify the signal. Rectification involves taking the absolute value of the raw signal, that is, making all values in the signal positive. At this point, a linear envelope may be determined. This involves filtering out the high-frequency content of the signal to produce a smooth pattern that represents the volume of the activity. An alternative technique to the linear envelope is to integrate the rectified signal. When the signal is integrated, the EMG activity is summed over time so that the total accumulated activity can be determined over the chosen time period. Rectification, linear enveloping, and integration can be accomplished using electronic hardware, although they can also be done by computer. Figure 4-27 illustrates the results of these procedures.
In the procedures just described, the EMG signal was presented as a function of time or in the time domain. The EMG signal has also been analyzed in the frequency domain so that the frequency content of the signal can be determined. In this case, the power of the signal is plotted as a function of the frequency of the signal (Fig. 4-28). This profile is referred to as a frequency spectrum.
When a muscle is activated by a signal from the nervous system, the action potential must travel the length of the muscle before tension can be developed in the muscle. Thus, a temporal disassociation or delay is seen between the onset of the EMG signal and the onset of the development of force in the muscle. This is referred to as the electromechanical delay (EMD). Tension develops at some time after the signal is detected because chemical events need to occur before the contraction takes place. The EMD portion of the EMG signal represents the activation of the motor units and the
shortening of the series elastic component of the muscle and can be affected by mechanical factors that change the rate of series elastic shortening. These factors include the initial muscle length and muscle loading. It has been reported that athletes who have a high percentage of fast-twitch muscle fibers exhibit a short EMD (34). The actual duration of this delay is not known, and values in the literature range from 50 to 200 ms. Figure 4-29 illustrates this concept.
FIGURE 4-27 A raw electromyography (EMG) signal, full-wave rectified EMG signal, linear envelope, and integrated EMG signal.
FIGURE 4-28 A raw electromyography signal in the time domain (A) and frequency domain (B).
FIGURE 4-29 The electromechanical delay (EMD) of the biceps brachi during elbow flexion. Top. The electromyography activity of the biceps brachii. Bottom. The elbow angular velocity profile (Reprinted with permission from
Gabriel, D. A., Boucher, J. P. . Effects of repetitive dynamic contractions upon electromechanical delay. European Journal of Physiology, 79:37-40.
Application of Electromyography
Muscle Force-Electromyography Relationship
In isometric conditions, the relationship between muscle force and EMG activity is relatively linear (32,43). That is, for a given increment in muscle force, there is a concomitant increase in EMG amplitude. These increases in EMG amplitude are probably produced by a combination of motor unit recruitment and an increase in motor unit firing rate. Many relationships, however, including both linear and curvilinear, between EMG and force have been suggested for different muscles (6) (Fig. 4-30).
In terms of concentric or eccentric contractions, descriptions of EMG-force relationships are controversial. The methodologies of the studies that report such relationships are often questioned because of the predominant use of isokinetic dynamometers that constrain the joint velocity. In the literature, only a few studies have attempted to relate EMG and force during unconstrained movements (28,55).
FIGURE 4-30 A linear relationship between electromyography (EMG) amplitude and external muscle force is frequently observed (dotted line) Numerous exceptions often result in a curvilinear relationship. (Adapted from a drawing by
G. Kamen, University of Massachusetts at Amherst.
EMG has greatly enhanced the study of muscle fatigue. Fatigue can result from either peripheral (muscular) or central (neural) mechanisms, although EMG cannot directly determine the exact site of the fatigue. This section briefly discusses local muscle fatigue. When a motor unit fatigues, the frequency content and the amplitude of the EMG signal change (3). The signal in the frequency domain shifts toward the low end of the frequency scale, and the amplitude increases (Fig. 4-31). A number of physiological explanations have been proposed for these changes, including motor unit recruitment, motor unit synchronization, firing rate, and motor unit action potential rate. Basically, the force capacity in the muscles diminishes because of the impairment in both the neural and muscular mechanisms (31). The shifts in the frequency domain are recoverable after sufficient rest, with the amount of rest dependent on the type and duration of loading. The recovery in the frequency spectrum of the signal, however, does not appear to correspond to mechanical or physiological recovery of the muscle (51).
FIGURE 4-31 The frequency and amplitude changes during a sustained isometric contraction of the first dorsal interosseous muscle. (Adapted from
Basmajian, J. V., DeLuca, C. J. . Muscles Alive: Their Functions Revealed by Electromyography [5th ed.]. Baltimore: Williams & Wilkins, 205
Clinical Gait Analysis
In the clinical setting, a gait analysis often involves EMG to determine which muscle group is used at a particular phase of the gait cycle (55). Generally, the raw or rectified EMG signal is used to determine when the muscles are active and when they are inactive, that is, to determine the activation order. Onsets and offsets of muscles should not be evaluated from any type of signal other than the raw or rectified signal because further processing, such as filtering the data, distorts the onset or offset. More often, however, linear envelopes of EMG signals are used after appropriate scaling to determine amplitudes. Figure 4-32 illustrates typical EMG activity of lower extremity muscle groups during walking.
FIGURE 4-32 Typical EMG activity of the major lower extremity muscle groups during a stride cycle of walking. FA, feet adjacent; HR, heel raise; IC, initial contact; OI, opposite initial contact; OT, opposite toe-off; TO, toe-off; TV, tibial vertical. (Reprinted with permission from
Whittle, M. W. . Gait Analysis [2nd ed.]. Oxford, UK: ButterworthHeinemann, 68.
Electromyography has been used in ergonomics for many applications. For example, studies have used EMG to investigate the effects of sitting posture, hand and arm movement, and armrests on the activity of neck and shoulder muscles of electronics assembly-line workers (62); to investigate the shoulder, back, and leg muscles during load carrying by varying load magnitude and the duration of load carrying (10); to study the erector spinae muscles of persons sitting in chairs with inclined seat pans (64); and to study postal letter sorting (17).
A particularly interesting use of EMG in ergonomics has been in the study of the low back in industry (48,49). This research has focused on proper lifting techniques and rehabilitation of workers who have low-back problems. In addition, EMG has been used to study low-back mechanics in exercise and during weight lifting.
Limitations of Electromyography
At best, EMG is a semiquantitative technique because it gives only indirect information regarding the strength of the contraction of muscles. Although many attempts have been made to quantify EMG, they have been largely unsuccessful. A second limitation is that it is difficult to obtain satisfactory recordings of dynamic EMG during movements such as walking and running. The EMG recording, therefore, is an indication of muscle activity only. One positive aspect of EMG recordings, however, is that they do reveal when a muscle is active and when it is not.
The nervous system controls and monitors human movement by transmitting and receiving signals through an extensive neural network. The central nervous system, consisting of the brain and the spinal cord, works with the peripheral nervous system via 31 pairs of spinal nerves that lie outside the spinal cord. The main signal transmitter of the nervous system is the motoneuron, which carries the impulse to the muscle.
The nerve impulse travels to the muscle as an action potential, and when it reaches the muscle, a similar action potential develops in the muscle, eventually initiating a muscle contraction. The actual tension in the muscle is determined by the number of motor units actively stimulated at one time.
Sensory neurons play an important role in the nervous system by providing feedback on the characteristics of the muscle or other tissues. When a sensory neuron brings information into the spinal cord and initiates a motor response, it is termed a reflex. The main sensory neurons for the musculoskeletal system are the proprioceptors. One proprioceptor, the muscle spindle, brings information into the spinal cord about any change in the muscle length or velocity of a muscle stretch. Another important proprioceptor is the GTO, which responds to tension in the muscle.
Flexibility, an important component of fitness, is influenced by a neurological restriction to stretching that is produced by the proprioceptive input from the muscle spindle. Another area of training that uses the neurological input from the sensory neurons is plyometrics. A plyo-metric exercise is one that involves a rapid stretch of a muscle that is immediately followed by a contraction of the same muscle.
Electromyography is a technique whereby the electrical activity of muscle can be recorded. From EMG, we can gain insight into which muscles are active and when the muscles initiate and cease their activity. A number of concepts must be understood, however, if one is to clearly interpret EMG signals.
True or False
An electrical current that travels through the nerve or muscle as the membrane potential changes because of the exchange of ions.
Active Range of Motion
The range of motion achieved through some voluntary contraction of an agonist, creating the joint movement.
The nerve pathway carrying sensory information into the spinal cord.
The stimulation of a muscle fiber that causes the action potential to travel over either the whole muscle fiber (activation threshold) or none of the muscle fiber.
An afferent neuron with a large cell body in or near the spinal cord from which a long axon projects from the spinal cord to the muscle fibers that it innervates.
Describing events that do not occur at the same time. In skeletal muscle contraction, the spacing of the activation of the motor unit.
Internally generated excitation of the alpha motoneurons through stretch or some other input.
Neuron process carrying nerve impulses away from the cell body of the neuron. The pathway through which the nerve impulse travels.
Moving a limb to the terminal range of motion through rapid movements initiated by strong muscular contractions and continued by momentum.
The loss of both force and neural input to the muscles through bilateral training of both limbs.
The portion of the neuron that contains the nucleus and a well-marked nucleolus. The cell body receives information through the dendrites and sends information through the axon. Also called the soma.
Central Nervous System
The brain and the spinal cord.
Crossed Extensor Reflex
Reflex causing extension of a flexed limb when stimulated by rapid flexion or withdrawal by the contralateral limb.
Reflex that causes relaxation of the muscle upon receiving stimuli in the form of heat or massage.
Processes on the neuron that receive information and transmit information to the cell body of the neuron.
The nerve pathway carrying motor information from the spinal cord.
The measurement of electrical activity of muscle.
The recorded signal of the electrical activity of muscle.
The temporal disassociation or delay between the onset of the EMG signal and the onset of the development of force in the muscle.
Fibers outside the muscle spindle; muscle fibers.
Reflex initiated by a painful stimulus that causes a withdrawal or flexion of the limb away from the stimulus.
See Rate Coding.
An analysis technique whereby the power of the signal is plotted as a function of the frequency of the signal.
Readjustment of the muscle spindle length by contracting the ends of the intrafusal fiber. Initiated by voluntary control, such as when anticipating the receipt of a heavy weight.
A reflex arc that works with the stretch reflex, in which descending motor pathways synapse with both alpha and gamma motoneurons of the muscle fiber and the muscle spindle.
A neuron that innervates the contractile ends of the muscle spindle.
Nerve cell bodies outside the central nervous system.
Golgi Tendon Organ
A sensory receptor located at the muscle-tendon junction that responds to tension generated during both stretch and contraction of the muscle. Initiates the inverse stretch reflex if the activation threshold is reached.
An EMG electrode that is placed directly in the muscle.
The number of fibers controlled by one neuron.
Small, connecting neuron in the spinal cord; can be excitatory or inhibitory.
Fibers that are inside the muscle spindle.
Inverse Stretch Reflex
Reflex initiated by high tension in the muscle, which inhibits contraction of the muscle through the Golgi tendon organ, causing relaxation of a vigorously contracting muscle.
Labyrinthine Righting Reflex
Reflex stimulated by tilting or spinning of the body, which alters the fluid in the inner ear. The body responds to restore balance by
bringing the head to the neutral position or thrusting arms and legs out for balance.
Local Graded Potential
An excitatory or inhibitory signal in the nerve or muscle that is not propagated.
Monosynaptic Reflex Arc
The reflex arc whereby a sensory neuron is stimulated and facilitates the stimulation of a spinal motoneuron.
A flattened expansion in the sarcolemma of the muscle that contains receptors to receive the expansions from the axonal terminals; also called the neuromuscular junction.
Neurons that carry impulses from the brain and spinal cord to the muscle receptors.
Groups of neurons in the spinal cord that innervate a single muscle.
A motoneuron and all of the muscle cells it stimulates.
An encapsulated sensory receptor that lies parallel to muscle fibers and responds to stretch of the muscle.
Nerve fibers that have a myelin sheath composed of a fatty insulated lipid substance.
Reflex initiated by stretching the muscle, which facilitates a contraction of the same muscle via muscle spindle stimulation; also called the stretch reflex.
Region where the motoneuron comes into close contact with the skeletal muscle; also called the motor endplate.
A conducting cell in the nervous system that specializes in generating and transmitting nerve impulses.
Node of Ranvier
Gaps in the myelinated axon where the axon is enclosed only by processes of the Schwann cells.
Nuclear Bag Fiber
An intrafusal fiber within the muscle spindle that has a large clustering of nuclei in the center. The type Ia afferent neurons exit from the middle portion of this fiber.
Nuclear Chain Fiber
An intrafusal fiber within the muscle spindle with nuclei arranged in rows. Both type Ia and the type II sensory neurons exit from this fiber.
Sensory receptor in the skin that is stimulated by pressure.
Passive Range of Motion
The degree of motion that occurs between two adjacent segments through external manipulation, such as gravity or manual manipulation.
Peripheral Nerve System
All nerve branches lying outside the brain and spinal cord.
Exercise that uses the stretch-contract sequence of muscle activity.
Sensory nerve fibers from the muscle spindle that are sensitive to stretch and respond to stretch by initiating the stretch reflex.
Proprioceptive Neuromuscular Facilitation
Rehabilitation technique that enhances the response from a muscle through a series of contract-relax exercises.
A sensory receptor in the joint, muscle, or tendon that can detect stimuli.
Reflex processed on both sides and at different levels of the spinal cord; an example is the crossed extensor reflex.
The frequency of the discharge of the action potentials. Also referred to as frequency coding.
Relaxation of the antagonistic mus-cle(s) while the agonist muscles produce a joint action.
A system of motor unit activation.
Involuntary response to stimuli.
Interneuron that receives excitatory input from collateral branches of other neurons and then produces an inhibitory effect on other neurons.
Movement coordination via spindle activity that causes the antagonistic muscle to relax while the agonist muscle is contracting.
Sensory receptors in the joint capsule that respond to change in joint position.
Cells that cover the axon and produce myelination, which are numerous concentric layers of the Schwann cell plasma membrane.
Sensory nerve fibers from the muscle spindle that are sensitive to stretch and that facilitate flexors and inhibit extensor activity.
Neuron that carries impulses from the receptors in the body into the central nervous system.
The principle that describes the order of motor unit recruitment as a function of size.
The portion of the nerve cell that contains the nucleus and well-marked nucleolus. The soma receives information from the dendrites and sends information through the axon; also called the cell body.
The 31 pairs of nerves that arise from the various levels of the spinal cord.
Moving a limb to the terminal range of motion slowly and then holding the final position.
Reflex initiated by stretching the muscle, which facilitates a contraction of the same muscle via muscle spindle stimulation; also called the myotatic reflex.
Reflex brought into the spinal cord but processed in the brain; an example is the labyrinthine righting reflex.
An electromyography electrode that is placed directly on the skin above the muscle that is being recorded.
The junction or point of close contact between two neurons or between a neuron and a target cell.
Describes events occurring at the same time. In muscular contraction, the concurrent activation of motor units.
Tonic Neck Reflex
Reflex stimulated by head movements, which stimulates flexion and extension of the limbs. The arms flex with head flexion and extend with neck extension.