Tara M. Ridge MD
Jennifer Swanson DPT
James J. Irrgang MD
The ultimate goal of rehabilitation following an athletic injury is to restore symptom-free movement and function, allowing individuals to return to their prior level of activity in the shortest possible time. Rehabilitation includes the application of therapeutic exercise and physical agents. Physical agents include various forms of heat, cold, electricity, and massage that are used to relieve pain and swelling and to aid in the healing process. Therapeutic exercise includes a variety of movements designed to restore function to the greatest possible degree in the shortest period of time and to attain high levels of physical conditioning.
Establishment of appropriate goals during rehabilitation is dependent upon the ability to assess the extent of injury and functional status of the injured athlete. Subsequently, athletic trainers and physical therapists must be able to relate the effects of the physical agents and therapeutic exercise to the rehabilitation goals of the athlete for effective outcomes. Although an understanding of the pathology and the healing process is necessary to ensure appropriate rehabilitation, sports medicine professionals must also consider anatomy, kinesiology, and biomechanics when developing the rehabilitation program.
Rehabilitation of the injured athlete is a problem-solving process that can be depicted as a feedback loop. It includes assessment of the athlete and leads to development of needs, goals, and a plan of care. As the athlete progresses, the plan of care needs to be modified to allow for continued progress. Living tissues respond and adapt to the stresses placed upon them. For example, Wolff's law states that bone adapts to stresses such as weight-bearing activities and muscular contractions that result in increased bone mass. Soft tissue structures respond in a similar manner based on the SAID (Specific Adaptations to Imposed Demands) principle. This principle implies that tissues adapt to altered patterns of use. In essence, increased use results in specific adaptations of structure and/or function that enable those tissues to withstand the stresses imposed upon them. This is an important concept in rehabilitation. It implies that the degree of functional capacity achieved is dependent upon the intensity, duration, and frequency of exercise. During rehabilitation, tissues within the body must be stressed in a positive, progressive, and appropriately planned manner with the ultimate goal being to prepare the athlete to meet the demands of his or her sport, to achieve the highest levels of structure and function. However, as the load and demands of the athlete are progressively increased, continuous reassessment is required to avoid reinjury.
Rehabilitation begins immediately after the injury and progresses through the acute and subacute phases of injury or surgery, culminating in return to sport. For an athlete, it must also include a period of reconditioning to ensure optimal levels of fitness, which are necessary to achieve maximum performance and to minimize the risk of reinjury. Specific goals of rehabilitation are dependent on the phase of injury and include, but are not limited to, reducing or limiting inflammation, decreasing pain and swelling, improving mobility and flexibility, improving muscle strength and endurance, improving cardiovascular function, and, finally, promoting coordination. The ability to accurately evaluate and identify rehabilitation goals is critical to this process.
Principles of Therapeutic Exercise
Therapeutic exercise is defined as those movements performed to restore the greatest possible degree of function in the shortest period, to attain high levels of physical fitness. Before initiating a therapeutic exercise program, the sports medicine professional must consider any precautions or contraindications to exercise as well as the nature and severity of the injury, as the intensity, frequency, and duration of the exercise must be appropriate for the stage of inflammation, healing, and conditioning. Next, the purpose of the exercise and the sequencing and progression of the program should be considered.
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Figure 11-1. Categorization of the forms of therapeutic exercise. |
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Therapeutic exercise can be categorized into static or dynamic exercise (Figure 11-1). Static exercise includes isometric exercises in which no observable movement occurs. Dynamic exercise may be either active or passive. Active exercise occurs when voluntary contraction of muscles produces movement without the application of additional external resistance. It includes range of motion and stretching exercises. Active range of motion exercises include those movements within the available range of motion. Active stretching exercises are exercises in which the athlete utilizes voluntary effort to move beyond the restricted range of motion.
Active exercise occurs when voluntary contraction of muscles produces movement without the application of additional external resistance. It includes range of motion and stretching exercises. Range of motion exercises are performed to maintain motion, whereas stretching exercises are designed to increase motion. These exercises will be discussed in greater detail later in the chapter.
Motions in accessory, passive exercises include distraction, compression, rolling, gliding, and/or spinning of the joint surfaces. Individuals are not capable of producing accessory motions with voluntary muscle activation and therefore a physical therapist or athletic trainer typically performs this manual technique to increase joint play (Figure 11-2). Joint mobilization is an example of passive, accessory motion that is performed at
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slow speeds and varying amplitudes; this will be described in further detail later in this chapter.
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Figure 11-2. Joint mobilization can be used to reduce pain and increase range of motion. A distraction technique of the glenohumeral joint in the resting position is illustrated. |
Kisner C, Colby LA: Therapeutic Exercise: Foundations and Techniques, 4th ed. F.A. Davis Company, 2002.
Rehabilitation Goals
The ultimate goal of the rehabilitation program is to restore function as efficiently as possible, allowing the athlete to safely and quickly return to athletic competition. Although we are unable to “speed up” the normal healing process following an injury, we can optimize our plan of care to minimize delayed healing by designing an appropriate and functional rehabilitation program. Such programs must take into account the normal phases of healing and must address the sport-specific demands of the individual athlete. Failure to address normal healing parameters and sport-specific requirements will delay the return to competition, increase the risk for reinjury, and reduce the performance level of the athlete.
Although the inflammatory process is part of the normal healing process, prolonged or chronic inflammation may be deleterious to athletes who are trying to rehabilitate and return to athletic competition. By controlling the pain and swelling associated with the inflammatory process, athletes may be able to progress through the clinical rehabilitation goals and advance to functional activities more quickly. The cornerstone for managing the signs and symptoms of acute inflammation are rest, ice, compression, elevation, and nonsteroidal antiinflammatory drugs (NSAIDs). Whenever possible, they should be used concurrently for maximum benefits to facilitate a quick but safe progression through rehabilitation. Pain and effusion inhibit muscle activation and decrease strength and may result in additional injuries and reduced performance. Following an injury and the subsequent management of the acute signs and symptoms, rehabilitation should involve a variety of factors designed to prepare the athlete for return to sport. These factors include clinical goals for improving range of motion and flexibility, muscular and cardiovascular endurance, and, ultimately, strength. The program should culminate with functional goals for return to sport such as increasing power, speed, and agility.
Range of Motion
The range of motion available at a particular joint is termed “joint range” and is determined by the configuration of the joint surfaces and surrounding soft tissue structures such as the capsule, ligament, muscle, tendon, fascia, and skin. When discussing the available range of motion at a particular joint, we often consider the “muscle range,” which is related to the functional excursion produced by muscles that cross the joint. It is important to note that the total joint range can be directly affected by the functional excursion and it is defined as the distance that the muscle is capable of lengthening and shortening. A one-joint muscle is expected to shorten and lengthen sufficiently to permit full active range of motion at the joint that it crosses. The functional excursion of multijoint muscles exceeds the joint range of any one of the joints that it crosses. Multijoint muscles, however, cannot lengthen or shorten sufficiently to simultaneously permit the extreme range of motion at all the joints that it crosses. For example, the hamstring muscle group cannot lengthen sufficiently to permit simultaneous full active knee flexion and hip extension. In this position, the hamstring muscle group is said to be actively insufficient. In the active insufficient position, the muscle fibers cannot shorten any further and are ineffective in generating additional tension.
To increase motion, the properties of both the noncontractile and contractile tissues that limit the motion must be considered. Noncontractile tissues include ligaments, tendons, capsule, fascia, and connective tissue components of muscle and skin. The contractile component is the muscle. The material strength of tissue is its ability to resist load or stress, and defined as tensile, compressive, and/or shearing forces. The mechanical properties of tissue are often plotted in a stress–strain curve that relates strain as a function of stress for a given tissue, whereas strain is defined as the deformation that occurs in response to stress and is typically expressed as a percentage of elongation (Figure 11-3). The toe region
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occurs at the beginning of the curve and is the region in which very little force is required to elongate the tissue. This likely represents straightening of the wavy pattern of connective tissue fibers. The elastic range represents the area in which tissue returns to its original size and shape when the stress is removed. The elastic limit is the upper end of the elastic range and is the point beyond which the tissue will not return to its original size or shape when the stress is removed. The plastic range of the stress–strain curve represents the range beyond the elastic limit that results in permanent elongation when the stress is removed.
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Figure 11-3. Stress–strain curve for connective tissue. The toe region is the area in which little stress is required to lengthen the tissue. The elastic region is the portion of the curve in which tissue returns to its original length when the stress is removed. The plastic region is that portion of the curve that results in permanent elongation when the stress is removed. (Modified from Kisner C, Colby LA: Therapeutic Exercise: Foundations and Techniques, 4th ed. F.A. Davis Company, 2002. ) |
To increase range of motion there must be lengthening of connective tissue and this requires plastic deformation that results in gradual rearrangement of the connective tissue. Adequate time must be provided for remodeling to prevent fatigue and/or rupture of the tissue. Due to the viscoelastic nature of connective tissue, it exhibits properties of creep, relaxation, and stiffness. Creep is the elongation of tissue that results from constant loading, and can be increased by raising the tissue temperature. Relaxation is the progressive decrease in stress that occurs over time. Stiffness is the ability of the tissue to resist elongation and is determined by the slope of the stress–strain curve (Figure 11-4). Because connective tissue is viscoelastic, stiffness is dependent on the rate of loading. Therefore, an increased rate of loading is associated with greater stiffness. To maximize permanent lengthening, low-magnitude forces should be applied for prolonged periods of time. This process can be facilitated by the use of heating and cooling modalities in the lengthened position (Figure 11-5).
Another way of describing the noncontractile components of muscle is as series elastic and parallel elastic components. The series elastic component includes the tissue that connects the muscle fiber to bone, whereas the parallel elastic component consists of the tissue that surrounds each muscle fiber. Lengthening of the musculotendinous unit lengthens both the series and parallel elastic components, producing a sharp rise in tension. Because muscle also consists of contractile components, as lengthening continues, mechanical disruption of the cross bridge begins as the actin and myosin filaments slide apart and an abrupt lengthening of the sarcomere occurs, known as “sarcomere give.” Sarcomeres are elastic, therefore, when the short-term stretch is removed, they return to their original length. This implies that short-term stretching is not effective in increasing length of the contractile components of the muscle.
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Figure 11-4. Tissue undergoes gradual elongation over time when subjected to constant stress. |
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Figure 11-5. To maximize permanent lengthening, low-magnitude forces can be applied for prolonged periods of time. This process can be facilitated with heating and cooling modalities. |
As mentioned earlier, plastic deformation, or permanent lengthening of contractile tissue, requires time for gradual rearrangement of connective tissue and can be achieved with prolonged immobilization. Prolonged immobilization is the lengthened position that results in the addition of sarcomeres and permanent lengthening of the contractile tissues and occurs to maintain the greatest functional overlap of the actin and myosin filaments. Conversely, prolonged immobilization in the shortened position results in a decreased number of sarcomeres and may result in contractures or a permanent loss of motion.
The neurophysiologic properties of contractile tissue must be considered when attempting to increase range of motion limited by musculotendinous structures. The muscle spindle is a sensory organ sensitive to muscle lengthening. Sudden stretching of the muscle results in lengthening of the muscle spindle and initiation of the monosynaptic stretch reflex. Consequently, sudden or ballistic stretching of the musculotendinous units may cause the muscle to contract as it is being lengthened, thus resulting in increased soreness or no appreciable change in length.
Another sensory organ, the Golgi tendon organ (GTO), is found in the musculotendinous junction and is sensitive to tension caused by passive stretching or active contraction of the musculotendinous unit.
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Excessive musculotendinous tension causes the GTO to discharge, inhibiting the muscular contraction. Stretching techniques such as contract/relax utilize the GTO to inhibit the muscle contraction, allowing the muscle to lengthen. Similarly, reciprocal inhibition occurs when the antagonist muscle is inhibited as an agonist muscle contracts. This principle can also be incorporated into stretching techniques, such as contract/ relax/contract and agonist contraction to facilitate muscle lengthening; this will be discussed in greater detail later in the chapter.
Dynamic exercise can involve passive or active range of motion (Figure 11-1). Passive range of motion occurs without voluntary muscular effort on the part of the athlete and is the result of forces external to the body. Passive range of motion is indicated when the athlete is not able to move the body segment voluntarily or when voluntary muscle activity would be detrimental to the healing process. It limits the adverse effects of immobility and is used to maintain the available joint range of motion. However, passive range of motion will not prevent muscle atrophy or affect muscle strength or endurance, nor will it improve circulation to the same extent as voluntary, active exercise.
Active range of motion occurs when voluntary contraction of muscles produces movement without the application of additional external resistance. It includes range of motion and stretching exercises. Active range of motion exercises include those movements within the available range of motion. Active stretching exercises are exercises in which the athlete utilizes voluntary effort to move beyond the restricted range of motion. There is more information on active exercises later in this chapter in the section on strengthening.
Neither purely active nor purely passive range of motion, active assistive range of motion combines active voluntary contraction with an outside force to complete motion within the unrestricted range (Figure 11-6). Such exercises can be used when the athlete is able to actively contract muscles to move the segment and when there are no contraindications for active voluntary muscle contractions. Extremely useful during the early stages of rehabilitation, they can be used to limit the adverse effects of immobility and maintain contractility of muscles. In addition, these exercises provide sensory feedback and a stimulus for maintaining integrity of bone. Lastly, they can be used to improve coordination and motor skills necessary for functional activities.
Range of motion exercise may be performed in anatomic planes, combined patterns, and sport-specific functional patterns incorporating movement in several planes simultaneously. Except when stretching is indicated, range of motion should be pain free and motion beyond the available range should not be forced. Generally 5–10 repetitions several times per day are adequate to limit the adverse effects of immobility. The athlete's response to range of motion exercises should be closely monitored and documented. Treatment must be modified as the athlete progresses and it is important to recognize signs of excessive exercise if range of motion exercises are performed acutely after injury. These signs include increased pain, swelling, warmth, redness, and loss of motion that persists for more than 1–2 hours after the exercise is completed.
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Figure 11-6. The athlete uses a cane to assist thevoluntary contraction of the involved upper extremity within the unrestricted range of motion. |
Flexibility
Some definitions need to be clarified before continuing. Flexibility is the ability of a muscle to relax and yield to a stretching force. Tightness is a nonspecific term used to describe mild shortness of the musculotendinous unit that does not result in a significant loss of joint motion. Common in multijoint muscles such as the hamstring, rectus femoris, and gastrocnemius muscle groups, tightness can be improved by self-stretching or flexibility exercises. A joint contracture is a significant loss of motion from any cause. Contractures are described by identifying the involved joint and the direction of the contracture. For example, a lack of full knee extension would be termed a knee flexion contracture whereas a lack of full knee flexion would be termed a knee extension contracture.
Flexibility exercises are used to increase the length of the musculotendinous unit and the term flexibility exercise is often used synonymously with stretching exercise. Stretching exercises are designed to increase range of motion and lengthen pathologically shortened soft
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tissue structures. In active stretching, the stretching force is created by voluntary contraction of the athlete's muscles and allows for incorporation of the neurophysiological principles of stretching. Passive stretching movements are movements beyond the restricted range performed in an attempt to increase motion. External force may be applied by an athlete's own body, a machine, gravity, or another individual. The external force can be applied manually or mechanically. Manual passive stretching exercises are generally of short duration lasting 15–30 seconds per repetition. Passive mechanical stretching is performed by applying a low (5–10 lb) external load to the shortened tissues for a prolonged period of time (15–30 minutes). Passive mechanical stretching may be performed with the use of ankle weights (Figure 11-7) or other mechanical equipment. Prolonged mechanical stretch often results in greater permanent lengthening of contractile and noncontractile tissues, based on the TERT (Total End Range Time) Principle. This is the amount of time that the tissue is engaged into the restricted range. Increased time at end range facilitates remodeling of the connective tissue and plastic deformation occurs, permanently lengthening the tissue.
Neurophysiologic principles can be incorporated to relax muscles prior to elongation. This allows the contractile component to be lengthened more easily. These techniques can be used to stretch tight contractile structures more comfortably, such as those associated with muscle spasm; however, they do not generally result in a permanent increase in length. Examples of neurophysiologic stretching techniques include contract/relax and contract/relax/contract. Contract/relax stretching techniques involve isometric contraction of the tight muscle followed by lengthening of the muscle. The prestretch contraction of the short muscle results in stimulation of the GTO. A contract/relax stretching technique to address a tight hamstring muscle would incorporate contraction of the hamstring with simultaneous hip extension and knee flexion, followed by passive relaxation of the hamstring muscle and contraction of the quadriceps and hip flexor muscles as the hamstring is lengthened.
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Figure 11-7. Passive mechanical stretching can be performed with the use of ankle weights or othersimilar equipment. |
The athlete should be taught self-stretching techniques that incorporate the use of the athlete's body weight with active inhibition to stretch tight muscles. These should be performed following the passive stretching and active inhibition techniques described above. The athlete should also be instructed to perform self-stretching exercises several times daily to continue to make gains in motion.
Stretching exercises are indicated when the athlete demonstrates limited range of motion in the subacute or chronic phases of healing. Generally, stretches are held in the restricted range for approximately 15–30 seconds and should cause only mild discomfort. If stretching exercises cause a persistent increase in pain that lasts longer than 1–2 hours, the athlete should be instructed to decrease the intensity of the stretch by staying within less restricted ranges. In addition, aggressive stretching exercises during the acute stages of healing may jeopardize the healing tissue and aggravate inflammation and, therefore, should be avoided in the acute stage of soft tissue healing. Stretching exercises can also be used to correct muscle imbalances that result when an opposing muscle group is weak. Generally, the athlete should address the tight muscle group first with stretching exercises and then progress to a strengthening program of the opposite muscle. Stretching exercises may also be indicated before and after activity as a warm-up and after cool-down, respectively, to minimize the risk of musculotendinous injuries. Athletes should avoid stretching or forcing the joint beyond the normal range of motion. In essence, care should be taken to avoid creating a hypermobile joint.
Prior to stretching, local application of heat or active exercise is used to elevate body temperature and to increase soft tissue extensibility. In addition, massage may be employed to promote relaxation and decrease muscle spasm, making it easier to stretch tight muscles. Lastly, if mobility of a joint surface is limited, mobilization techniques should be utilized prior to stretching exercises to increase the accessory range of motion.
Joint Mobilization
Movement of the joint surfaces may include distraction, compression, rolling, gliding, and/or spinning. Rolling occurs when new points on one joint surface meet new points on an opposing joint surface, similar to a tire rolling along a road. Gliding of joint surfaces involves the same point on one surface, similar to a locked tire sliding over a road. Normal joint motion may combine both rolling and gliding of the joint surfaces to maintain congruency as the limb moves through the range of motion.
Joint mobilization techniques, designed to restore the normal gliding of joint surfaces necessary for physiologic
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motion, are contraindicated during periods of active inflammation and hypermobility. In addition, the use of mobilization techniques following fractures should be delayed until there is radiographic evidence of union.
Joint mobilization can be performed using oscillatory or sustained movements. The Maitland or Australian system uses oscillatory techniques and are graded I through IV. Grade I and II oscillations are large-amplitude motions used to stimulate mechanoreceptors to decrease pain. Grade III and IV oscillatory movements are used to stretch tight structures in order to increase joint mobility and range of motion. The Kaltenborn or Norwegian system, which utilizes sustained mobilization techniques, has three grades of motion. Grade I, or piccolo motion, separates the joint surfaces just enough to equalize intraarticular and atmospheric pressure and is typically utilized to decrease pain. A grade II, or slack technique, removes the slack from the capsule and surrounding ligaments and can be utilized to increase range of motion. Grade III, or stretch techniques, utilize sufficient force to stretch joint structures to improve mobility. In general, the oscillatory motions of the Australian system are utilized for pain modulation whereas the sustained movements in the Kaltenborn system are utilized to improve joint mobility and range of motion.
Proper application of joint mobilization techniques requires a thorough examination of the involved joint to determine the tissues limiting motion as well as the stage of pathology. The mobilizing force should be correlated with the pain/restriction sequence. Pain occurring before resistance to motion is reached indicates an acute condition. Mobilization for acute conditions should consist of grade I and II oscillating techniques to decrease pain and maintain joint play. Pain synchronous with resistance to motion indicates a subacute condition. Grade III oscillatory or grade II (slack) mobilization techniques are appropriate for subacute conditions. A trial of gentle stretching should be utilized for sub-acute conditions. Pain engaged after the resistance to motion is indicative of a chronic condition and vigorous stretching is indicated. Joint mobilization techniques for chronic conditions include grade III and IV oscillatory or grade III (stretch) sustained techniques.
Joint mobilization techniques should be utilized only when mobility testing reveals decreased joint play. Hypermobile joints should not be mobilized. Generally, mobilization techniques are utilized when passive range of motion is limited from capsular contracture and joint play is limited in the direction of the restricted motion.
When performing joint mobilization techniques, the athlete should be positioned to promote relaxation and stabilization of the part to be mobilized. Initially, mobilization should be performed with the joint in the position at which the capsule has the greatest amount of laxity. This position generally occurs in the middle of the available range of motion and as range of motion improves, joint mobilization techniques can be performed in the restricted position. Forces should be applied as close to the opposing joint surfaces as possible. The area of contact with the hand should be graded according to the stage of the condition and the intended goals of treatment as described above.
The direction of movement is dictated by the direction of the restricted motion and the shape of the joint surface. The treatment plane is a plane perpendicular to a line from the axis of rotation to the center of the concave articulating surface (Figure 11-8). When joint surfaces are distracted, the force should be applied perpendicular to the treatment plane. When gliding joint surfaces, force should be applied parallel to the treatment plane, utilizing the following convex/concave rule: Concave joint surfaces should be glided in the direction of the limited swing of the bone, whereas convex surfaces should be glided in the direction opposite to the limited swing of the bone. When performing joint mobilization techniques, angular motion of the bone should be minimized to reduce compression of the joint surfaces, which may damage the articular surface.
Oscillatory joint mobilization techniques should be performed at a rate of 1–2 cycles per second for 1–2 minutes. Sustained joint mobilization techniques should be performed for 5–15 seconds and repeated 10 times. Joint mobility and range of motion should be reassessed at the completion of joint mobilization and the athlete should perform range of motion and stretching exercises as a follow-up treatment to joint mobilization. The athlete may experience some increase in soreness; however, these symptoms typically subside within several hours.
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Figure 11-8. The solid line represents the treatment plane and is defined as the line perpendicular to the line drawn from the axis of rotation to the center of the concave joint surface. When joint surfaces aredistracted, the force should be applied perpendicular to the treatment plane. When gliding joint surfacesthe forces should be applied parallel to the treatment plane. |
Moore KL: Clinically Oriented Anatomy, 5th ed. Williams & Wilkins, 2002.
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Principles of Muscle Performance: Strength & Endurance
The development of strength and endurance is a key component related to overall muscle performance and must be addressed during the rehabilitation of athletes. Strength refers to the amount of force a muscle or muscle group is able to generate during a maximal contraction at a constant velocity. Force is defined as a linear measurement referring to an action that changes the state or motion of a body to which it is applied and is measured in newtons. Forces can be divided into two categories, internal and external, with the latter a result of gravity. Internal forces are generated by muscle, bone, and soft tissue deformation, and when applied to the musculoskeletal system, a muscle force produces a rotation of a joint about its axis. Torque is force applied at a distance from the axis of rotation. Muscle endurance is closely associated with muscle strength and refers to the ability to perform multiple contractions against a set resistance for an extended period of time. All of these variables play a key role in muscle performance and can be manipulated during rehabilitation to maximize improvement.
Strength
Strength is defined as the maximum amount of force a muscle or muscle group can generate at a specified velocity. Muscle weakness or imbalance can result in abnormalities that can impair normal functional movement and must be addressed during rehabilitation from injury. Strength is mediated by a number of physiologic, biomechanical, and neuromuscular factors. Various forms of strength training are used to meet different goals and functional outcomes at each stage of tissue healing after injury or following surgical repair.
There are multiple factors that influence the strength of a normal muscle. There is a direct relationship between the physiologic cross-sectional area of the muscle fibers and the maximum amount of force that a muscle can generate, such that a larger muscle diameter correlates with greater strength. Force generation is also influenced by the length of the muscle at the time of contraction. According to the length–tension relationship, a muscle can generate maximal force at its resting length, defined as the position in which there is a maximum number of cross-bridges between the actin and myosin filaments. As the muscle shortens, the contractile force that a muscle can generate decreases due to the overlapping of myofilaments. Although the contractile force generated by a muscle decreases as the muscle lengthens, it is counterbalanced by an increase in noncontractile tension. Thus, the passive lengthening of the connective tissue results in a net increase in force. Therefore, the total force produced by the musculotendinous unit (including both contractile and noncontractile forces) increases as the muscle lengthens.
A number of contractile properties of muscle fibers contribute to production of force. Strength, endurance, power, speed, and resistance to fatigue vary based on the characteristics of different types of muscle fibers. Type I, slow twitch muscle fibers generate low levels of force and are resistant to fatigue. In contrast, type IIA and B, fast twitch muscle fibers have the ability to generate a large amount of tension but fatigue rapidly. Fiber-type distribution thus plays a large role in the ability of a muscle to generate force.
The order of muscle fiber recruitment is dependent upon the type of activity, the amount of force required, and the pattern of movement desired. Small motor neurons innervate type I, slow twitch muscle fibers and are initially recruited during low-intensity, long-duration endurance activities. As force requirements increase, large motor neurons innervating type II, fast-twitch muscle fibers are progressively recruited.
In addition to muscle fiber type, force generation is influenced by the speed and type of muscle contraction being performed. Greater torque is produced at lower speeds, and is related to the increased opportunity for recruitment of motor units. Eccentric contractions, in which the muscle is lengthened against resistance, produce the greatest force output. Tension increases as the speed of motion increases due in part to the facilitation of the stretch reflex and stretching of the series elastic component in muscle. In contrast, lower levels of force are generated during concentric muscle contractions. As the muscle shortens and the speed of contraction increases, there is an overall decrease in tension, as the muscle lacks adequate time to develop force. There is an inverse relationship between speed and force production during concentric muscle contractions. Adequate stores of energy and blood flow are also necessary to allow the muscle to contract efficiently, generate appropriate levels of tension, and resist fatigue. The amount of force generated by a muscle is also influenced by characteristics unique to the athlete, as the degree of motivation and willingness to put forth a maximal effort to generate maximum forces are dependent on the individual.
Neuromuscular changes that lead to increased strength include hypertrophy and hyperplasia. Hypertrophy refers to an increase in the size of the individual skeletal muscle fibers and is related to the increased contractile protein and the number of fibrils within the muscle fiber, as well as an increased density of the capillary bed surrounding individual muscle fibers. Increases in the connective tissue component of muscle may contribute to hypertrophy as well. Heavy resistance strength training has been shown to cause selective hypertrophy of type II fast-twitch muscle fibers. Rapid gains in strength during the early phases of resistance training are most likely attributed to recruitment rather than hypertrophy, largely due to motor
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learning that results in neural adaptations such as greater recruitment and synchronization of motor units. Hyperplasia refers to an increase in the number of muscle fibers resulting from the longitudinal splitting of muscle fibers. Although hyperplasia is controversial in humans, its presence has been demonstrated in laboratory animals exposed to heavy resistance training.
Strength is directly related to the amount of tension a contracting muscle can produce. To increase strength, the muscle must be progressively overloaded, such that it exceeds the metabolic capacity of the muscle. Overload is created by increasing either the resistance or the speed of the muscle action, or through a combination of both. Increasing levels of tension will develop in response to these loads, leading to hypertrophy and recruitment of motor units in the muscle.
The purpose of strengthening exercises is to increase the maximum force a muscle can generate. Strengthening is highly specific to the type of exercise and can be categorized into static or dynamic exercise (Figure 11-1). Static exercise includes isometric exercises in which no observable movement occurs. The length of the muscle appears constant, however, there is shortening at the sarcomere level. Isometric contractions occur when torque produced by muscle tension is equal to external resistance and no movement occurs about the joint. Isometric exercises may be initiated early in the rehabilitation program to help regain baseline strength lost to injury or disuse. These exercises can be used even when motion is contraindicated. One of the limiting factors involves the concept of joint angle specificity. Multiple-angle isometrics are necessary to develop strength throughout the entire range of motion whereas isometric exercises develop strength only at the position in which the exercise is performed.
Dynamic exercise can be passive, discussed above in the range of motion section of this chapter, or active. Active exercise can be resistive and includes exercises in which the individual utilizes voluntary muscle contraction to move against an applied resistance, such as isotonic and isokinetic exercises.
Isotonic exercises, by definition, should result in constant muscle tension through the range of shortening. However, this rarely occurs because motion against a fixed external resistance results in variable muscle tension due to the length–tension relationship of the muscle fiber and the changing mechanical advantage that the line of muscle action has on the skeletal system. Therefore, the term isotonic exercise has come to imply movement against a fixed resistance. Isotonic exercises are one of the most popular forms of strength training used in rehabilitative programs.
Isokinetic exercise involves movement at a constant speed. External resistance is variable and accommodating and is proportional to the effort put forth by the athlete. Isokinetic training encourages the muscle to generate maximum force throughout the full range of joint motion at different angular velocities. During the earlier phases of rehabilitation, isokinetic training should be performed at submaximal levels with maximal levels of training reserved for the final stages of rehabilitation as the individual is progressing back to sport or other functional activities. Resistance machines have been developed to provide variable resistance that matches the torque curves produced by a particular muscle or muscle group. In variable resistance exercises, the resistance is not accommodating and the speed is not controlled.
Resistance exercise programs can be designed to selectively recruit different muscle fiber types by controlling the intensity, duration, and speed of exercise. Specificity of training refers to the principle that the adaptive effects of training, including strength, power, and endurance, are highly specific to the type of training utilized and thus, whenever possible, exercises incorporated in the training program should mimic the desired function.
Isotonic, isokinetic, and variable resistance exercises can be performed concentrically and eccentrically. Concentric contraction implies that the muscle shortens as it contracts, whereas an eccentric contraction implies lengthening as the muscle contracts. Concentric contractions are necessary to accelerate the body and eccentric contractions are necessary for deceleration. This information should be considered when choosing the most appropriate types of exercise and subsequent muscle contraction when attempting to match the demands of the sport. It is important to note that the force–velocity relationship is different for each type of contraction. During a concentric contraction, the force created by the muscle decreases as the speed of contraction increases. This type of contraction occurs when the internal force created by the muscle is greater than the external resistance. During an eccentric contraction, however, the force created by the muscle increases as the speed of lengthening increases. This type of contraction occurs when the external resistance overcomes the internal resistance created by the muscle and is often associated with increased muscle soreness and increased injury. The differing relationships related to force and velocity are believed to result partly from stretching of the connective tissue component within the musculotendinous unit and facilitation of the stretch reflex, both of which give rise to increased muscle tension with increased speed of lengthening.
The force in resistive exercise can be applied either manually or mechanically. Manual resistance is useful during
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the early stages of rehabilitation when the affected muscle is weak and can overcome only minimal to moderate resistance. It is also useful when working in a limited range of motion. In contrast, mechanical resistance is applied through the use of equipment or a motorized device. The amount of resistance can be quantitatively measured and progressed over time.
The type of exercise should be carefully chosen when developing the strengthening program. Open and closed kinetic chain exercises play an important role in both clinical and functional rehabilitations. Open-chain exercises occur when the distal segment moves freely in space. During closed-chain activities, the distal aspect of the extremity is fixed and thus motion occurs simultaneously at all joints that comprise the kinetic chain. The variables must match the requirements and demands placed upon the athlete during functional exercises in order for the exercises to be specific. Generally, muscle groups may be isolated during open-chain exercises, and more cocontraction of various muscle groups occurs during closed-chain exercises. Most activities incorporate some combination of both open and closed kinetic chain activities, with the latter particularly important during functional weight-bearing activities, and include exercises such as partial squats, step-ups, and lunges.
During the initial phases of recovery, rehabilitation focuses on pain modulation and restoration of joint range of motion and flexibility of the individual muscles. Next, strength training is introduced into the program. As the athlete progresses, the emphasis of the program shifts to functional training to develop balance, proprioception, and synergistic muscle activity required for sport-specific activities. Typically, strength exercises utilizing functional movement patterns that incorporate multiplane and multijoint movements are integrated into the program during the latter phase of the rehabilitation program.
It is essential to incorporate exercises performed at rapid speeds during later phases of rehabilitation and reconditioning, as it is reflective of the demands related to athletics. Thus, strengthening programs must progressively integrate individual muscle actions into functional muscle group actions, essentially transitioning from general exercises to sport-specific exercises designed to replicate movements common in given sports.
Well-designed rehabilitative programs are structured with specific objectives and consider variables such as frequency, intensity, volume, progression, and recovery. According to the principle of specificity, training should include therapeutic exercises designed to target the desired muscle with the level of force production, velocity, and type of muscle contraction required by the sport. Strengthening should occur throughout the entire range of motion and reflect the pattern of movement (open versus closed chain) to match the desired activity.
The amount of resistance used generally reflects the intensity of the exercise and is based on a percentage of a one-repetition maximum (1 RM). This is the maximum amount of weight that can be lifted one time before fatigue prevents the completion of an additional repetition and is a function of the amount of resistance used.
Several specific exercise regimes have been proposed to improve strength. One technique of progressive resistive exercises (PREs) begins by establishing a repetition maximum. This is defined as the maximum amount of weight that can be lifted 10 times with proper technique. In this training program, three sets of 10 repetitions are performed, with the first set against one-half of the 10 RM weight. Resistance is increased during both the second and third sets, with three-quarters 10 RM and full 10 RM used in each set, respectively.
Another technique attempts to accommodate for the effects of fatigue such that the first set is performed against the full 10 RM weight and the third set is performed against one-half of the 10 RM.
Knight proposed a program consisting of daily adjustable progressive resistive exercises (DAPRE) in an attempt to utilize objective measurements to determine the amount and frequency for progressing the resis-tance. Initially a 6 RM weight was proposed, with the first set of 10 repetitions performed at 50% of this weight and the second set at 75% of this weight. The third set is performed for as many repetitions as possible at the full 6 RM weight. The number of repetitions performed during the third set is used to determine the resistance for the fourth set. If more than six repetitions are performed during the third set, the weight is increased. If less than six repetitions are performed then the weight is decreased. The number of repetitions during the fourth set determines the amount of weight to be used for the next session (Table 11-1).
Knight KL: Knee rehabilitation by the daily adjustable progressive resistive exercise technique. Am J Sports Med 1979;7:336.
Endurance
The term endurance refers to the ability of a muscle or muscle group to generate or sustain low-intensity repetitive forces until the onset of muscle fatigue or decline of proper technique. The effects of endurance training on muscle are different relative to those evoked when training for strength or power, as they result in a combination of central and peripheral adaptations, enhancing an individual's ability to sustain a given workload over an extended period of time.
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Table 11-1. Knight's Daily Adjusted Progressive Resistive Exercise program.1 |
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Endurance training consists of high repetitions of moderate-resistance exercises. The development of muscle endurance is speed specific and, thus, rehabilitation activities must simulate the speed of athletic performance required for the return to competition.
Peripheral adaptations are localized to the muscle or muscle groups involved in the endurance training exercise, usually resulting in improvements in the oxidative capacity of muscle fibers. Physiologic responses to endurance training include adaptations to the respiratory, cardiovascular, and musculoskeletal systems. In terms of the respiratory system, adaptations include enhanced oxygen exchange and improved blood flow in the lungs and decreased submaximal respiratory and pulmonary ventilation rates. Adaptations to the cardiovascular system include increased cardiac output, blood volume, red blood cell numbers, and hemoglobin concentration, as well as enhanced blood flow to the skeletal muscle. Resting heart rate and heart rate at a given load both decrease in response to endurance training. A reduction in submaximal heart rate and improvements in thermoregulation also occur. In addition, the musculoskeletal system undergoes positive changes, reflected in increased mitochondrial size and density, increased oxidative enzyme and myoglobin concentrations, increased muscle bed capillarization, and increased atriovenous oxygen difference.
Many cardiovascular adaptations are associated with resistance training including a decrease in heart rate and in systolic and diastolic blood pressure, and a reduction in total cholesterol. A sufficient training stimulus is necessary for cardiovascular changes to occur in response to strength and endurance training. As the strength of a muscle increases, the cardiovascular response of the muscle improves, and results in an increase in endurance and power.
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Table 11-2. Motion at the shoulder joint. |
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Heart rate can be used to measure training over a broad range of intensities as well as set upper limits on training intensity to allow for recovery. Maximum heart rate (HRmax) can be estimated by using 220 minus the individual's age. To increase aerobic capacity for example, the training level or target heart rate (THR) can be established at 70% of HRmax. The equation used to determine target heart rate is THR = (HRmax–RHR) × % of desired training intensity + RHR (resting heart rate) (Table 11-2).
Exercise Contraindications & Precautions
The primary contraindications to exercise include active inflammation and pain. Use of resistive exercise in the presence of active inflammation can lead to further tissue trauma and aggravate pain and swelling. Resistive
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exercises should be modified or discontinued if they produce an increase in pain persisting more than several hours following exercise.
Given the potential intensity of resistance training, cardiovascular precautions should be noted and include avoidance of the valsalva maneuver, which may cause a transient, but marked increase in blood pressure, during resistive training. Careful attention must be given to ensuring that patients do not hold their breath during exercise, which may be facilitated by having patients count out loud or exhaling during exercise. In addition, athletes must be observed carefully when performing resistance exercises to detect substitute motions when the prime movers are weak or fatigued. Often, alternate muscles or compensatory motions are responsible for completion of the desired movement, thus further perpetuating muscle weakness or resulting in secondary tissue damage. Appropriate amounts of resistance, stabilization, and instruction regarding proper technique can reduce muscle spasm and prevent subsequent injury.
Muscle Soreness
Resistance exercise can potentially result in the development of muscle soreness immediately after exercise or 24–72 hours following exercise. Immediate onset of muscle soreness develops during or directly following intense activity and generally subsides quickly with rest. It is thought to be related to muscle injury, ischemia, or the build-up of metabolites in the muscle itself, secondary to inadequate blood flow during exercise.
Delayed-onset muscle soreness develops 12–48 hours following vigorous exercise. Numerous theories have been proposed to explain this phenomenon, including lactic acid accumulation, Devries' pain–spasm theory, and microscopic tearing of muscle and/or connective tissue during strenuous activity, inducing an inflammatory response. Local muscular fatigue may also result from pain, discomfort, and inhibitory influences from the central nervous system. Decreased blood glucose or depletion of muscle and/or liver glycogen may be responsible for total body fatigue following prolonged resistance exercises.
According to the pain–spasm theory, a feedback cycle of a pain-induced reflex muscle spasm develops in response to ischemia and the build-up of waste products in the muscle. Prevention of exercise-induced muscle soreness includes a gradual increase in both the intensity and duration of exercise. In addition, adequate recovery time must be incorporated into the training program to avoid fatigue and promote the removal of lactic acid and the replenishment of energy and oxygen stores and is ultimately required to improve prolonged performance. Multiple studies have demonstrated the effectiveness of light exercise in the facilitation of recovery, justifying a gradual warm-up and cool-down following vigorous exercise.
Functional Rehabilitation Goals
Power, speed, and agility are incorporated in the final phase of rehabilitation, with an emphasis on sport-specific skills. Prior to initiation of these activities, it is essential that the athlete have adequate strength, mobility, and endurance to safely and effectively execute speed and agility drills.
Power
Power is an important measure of muscle performance, requiring a combination of strength, speed, and skill. Power is defined as the rate of work performed per unit of time and is expressed in watts. Improvements in power occur by reducing the amount of time required to produce a given force, by increasing the distance over which the force is applied, or by increasing the amount of work a muscle is able to perform during a specified period of time. Although power is primarily a function of both strength and speed, it is the latter that is most often manipulated during training programs.
Maximum power occurs at intermediate velocities for concentric contractions. When training specifically for the development of power, exercises should be performed with lighter weights and at higher speeds relative to more traditional strength training programs.
Power is necessary to maximally accelerate the body and is an important component in a wide variety of sports skills. Effective use of power requires not only superior neuromuscular control and the ability to rapidly generate force, but also baseline strength at two speeds–slow and fast. Training for power must be performed at both the velocity and force specific to the demands of the desired activity.
Several different ways to improve power in the athlete are utilized in training programs. Plyometric exercises, one of the most effective interventions for developing power, facilitate the stretch-shortening cycle to (1) elicit a more forceful concentric muscle contraction and (2) increase the reactivity of the nervous system. Plyometric training consists of isotonic exercises that combine speed, strength, and functional activities. These exercises are incorporated during the final phase of rehabilitation, and should closely mimic both the movement pattern and the speed of execution of actual sports performance (Figure 11-9).
Speed
Speed, utilized to some degree in every sport or functional activity, must be addressed during the rehabilitation of athletes. The development of speed depends on the ability to rapidly generate force and optimize motor unit recruitment, and is often compromised after an injury. Activities emphasizing neural activation, motor unit synchronization, strength, and the development of a motor program for ballistic movements should be
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incorporated into the training program to facilitate the return to sport. These interventions should match the speed of the functional movement or sport-specific activity as the speed of training is directly related to the speed at which strength gains occur.
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Figure 11-9. Plyometric exercises, such as box jumping, can be an effective way to increase power. These exercises combine speed, strength, and function. |
Agility
Agility, which refers to the ability to abruptly change the direction of the body or to quickly shift the direction of movement in a controlled manner without losing balance, is imperative in most sports and is often more important than simply achieving or maintaining a maximum velocity. It is dependent on a combination of factors including strength, speed, dynamic balance, and coordination.
These skills require rapid force development and high power output, as well as the ability to efficiently couple concentric and eccentric actions into dynamic, explosive movements. Agility training emphasizes rapid decelerations, directional changes, and subsequent reaccelerations with the primary objective aimed at making these movements as automatic and efficient as possible.
Agility exercises should preferentially incorporate the movements and demands of the individual's sport. However, the ability to safely decelerate from a given velocity is a prerequisite before combining this skill with rapid changes in direction. Thus, it is essential to establish each athlete's ability to decelerate from varying speeds before changing directions and progressively advance accordingly.
Agility can be developed in many ways, initially incorporating activities such as shuttle runs, side-stepping, crossovers, and change in direction drills. At first, these exercises should be performed at submaximal speeds to allow the athlete to learn the appropriate body mechanics and ensure proper technique. The speed is progressively increased as these techniques are mastered to simulate game situations.
Once athletes have successfully completed a functional rehabilitation program, they must meet a series of clinical guidelines before returning to unrestricted sport. These criteria may vary slightly, depending on the athlete, the sport, and the recommendation of the sports medicine team. However, the premature return to sport places the athlete at risk for reinjury and will likely reduce the athlete's performance (Table 11-3).
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Table 11-3. Criteria for full return to sports activities. |
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Rehabilitation of an injured athlete requires the restoration of function in the shortest time possible, allowing the athlete to safely and quickly return to competition. Although the normal healing process cannot be facilitated, the sports medicine team, when designing a therapeutic exercise program, can optimize the conditions for injury repair by taking into account the pathology of the injury and the normal phases of healing. A successful and efficient rehabilitation program should incorporate basic principles related to anatomy, kinesiology, and biomechanics to address the underlying impairments, while utilizing therapeutic modalities and exercise to ultimately promote functional speed, power, agility, and sport-specific activities.