After reading this chapter, the student will be able to:
To study kinesiology and biomechanics using this textbook requires a fresh mind. Remember that human movement is the theme and the focus of study in both disciplines. A thorough understanding of various aspects of human movement may facilitate better teaching, successful coaching, more observant therapy, knowledgeable exercise prescription, and new research ideas. Movement is the means by which we interact with our environment, whether we are simply taking a walk in a park, strengthening muscles in a bench press, competing in the high jump at a collegiate track meet, or stretching or rehabilitating an injured joint. Movement, or motion, involves a change in place, position, or posture relative to some point in the environment.
This textbook focuses on developing knowledge in the area of human movement in such a manner that you will feel comfortable observing human movement and solving movement problems. Many approaches can be taken to the study of movement, such as observing movement using only the human eye or collecting data on movement parameters using laboratory equipment. Observers of activities also have different concerns: A coach may be interested in the outcome of a tennis serve, but a therapist may be interested in identifying where in the serve an athlete with tendinitis is placing the stress on the elbow. Some applications of biomechanics and kinesiology require only a cursory view of a movement such as visual inspection of the forearm position in the jump shot. Other applications, such as evaluating the forces applied by a hand on a basketball during a shot, require some advanced knowledge and the use of sophisticated equipment and techniques.
Elaborate equipment is not needed to apply the material in this text but is necessary to understand and interpret numerical examples from data collected using such intricate instruments. Qualitative examples in this text describe the characteristics of movement. A qualitative analysis is a nonnumeric evaluation of motion based on direct observation. These examples can be applied directly to a particular movement situation using visual observation or video.
This text also presents quantitative information. A quantitative analysis is a numeric evaluation of the motion based on data collected during the performance. For example, movement characteristics can be presented to describe the forces or the temporal and spatial components of the activity. The application of this material to a practical setting, such as teaching a sport skill, is more difficult because it is more abstract and often cannot be visually observed. Quantitative information can be important, however, because it often substantiates what is seen visually in a qualitative analysis. It also directs the instructional technique because a quantitative analysis identifies the source of a movement. For example, a front handspring can be qualitatively evaluated through visual observation by focusing on such things as whether the legs are together and straight, the back is arched, and the landing is stable and whether the handspring was too fast or slow. But it is through the quantitative analysis that the source of the movement, the magnitude of the forces generated, can be identified. A force cannot be observed qualitatively, but knowing it is the source of the movement helps with qualitative assessment of its effects, that is, the success of the handspring.
This chapter introduces terminology that will be used throughout the remainder of the text. The chapter begins by defining and introducing the various areas of study for movement analysis. This will be the first exposure to the areas presented in much greater depth later in the text. Then the chapter discusses methods and terminology describing how we arrive at the basic mechanical properties of various structures. Finally, the chapter establishes a working vocabulary for movement description at both structural and whole-body levels.
Core Areas of Study
Biomechanics Versus Kinesiology
Those who study human movement often disagree over the use of the terms kinesiology and biomechanics. Kinesiology can be used in one of two ways. First, kinesiology as the scientific study of human movement can be an umbrella term used to describe any form of anatomical, physiological, psychological, or mechanical human movement evaluation. Consequently, kinesiology has been used by several disciplines to describe many different content areas. Some departments of physical education and movement science have gone so far as to adopt kinesiology as their department name. Second, kinesiology describes the content of a class in which human movement is evaluated by examination of its source and characteristics. However, a class in kinesiology may consist primarily of functional anatomy at one university and strictly biomechanics at another.
Historically, a kinesiology course has been part of college curricula as long as there have been physical education and movement science programs. The course originally focused on the musculoskeletal system, movement efficiency from the anatomical standpoint, and joint and muscular actions during simple and complex movements. A typical student activity in the kinesiology course was to identify discrete phases in an activity, describe the segmental movements occurring in each phase, and identify the major muscular contributors to each joint movement. Thus, if one were completing a kinesiological analysis of the act of rising from a chair, the movements would be hip extension, knee extension, and plantarflex-ion via the hamstrings, quadriceps femoris, and triceps surae muscle groups, respectively. Most kinesiological analyses are considered qualitative because they involve observing a movement and providing a breakdown of the skills and identification of the muscular contributions to the movement.
The content of the study of kinesiology is incorporated into many biomechanics courses and is used as a precursor to the introduction of the more quantitative biomechanical content. In this text, biomechanics will be used as an umbrella term to describe content previously covered in courses in kinesiology as well as content developed as a result of growth of the area of biomechanics.
In the 1960s and 1970s, biomechanics was developed as an area of study in the undergraduate and graduate curricula across North America. The content of biomechanics was extracted from mechanics, an area of physics that consists of the study of motion and the effect of forces on an object. Mechanics is used by engineers to design and
build structures and machines because it provides the tools for analyzing the strength of structures and ways of predicting and measuring the movement of a machine. It was a natural transition to take the tools of mechanics and apply them to living organisms. Biomechanics was defined by the American Society of Biomechanics (1) as “the application of the laws of mechanics to animate motion.” Another definition proposed by the European Society of Biomechanics (2) is “the study of forces acting on and generated within a body and the effects of these forces on the tissues, fluid, or materials used for the diagnosis, treatment, or research purposes.”
A biomechanical analysis evaluates the motion of a living organism and the effect of forces on the living organism. The biomechanical approach to movement analysis can be qualitative, with movement observed and described, or quantitative, meaning that some aspect of the movement will be measured. The use of the term biomechanics in this text incorporates qualitative components with a more specific quantitative approach. In such an approach, the motion characteristics of a human or an object are described using such parameters as speed and direction; how the motion is created through application of forces, both inside and outside the body; and the optimal body positions and actions for efficient, effective motion. For example, to biomechanically evaluate the motion of rising from a chair, one attempts to measure and identify joint forces acting at the hip, knee, and ankle along with the force between the foot and the floor, all of which act together to produce the movement up out of the chair. The components of a biomechanical and kinesiologic movement analysis are presented in Figure 1-1. We now examine some of these components individually.
Anatomy Versus Functional Anatomy
Anatomy, the science of the structure of the body, is the base of the pyramid from which expertise about human movement is developed. It is helpful to develop a strong understanding of regional anatomy so that for a specific region such as the shoulder, the bones, arrangement of muscles, nerve innervation of those muscles, and blood supply to those muscles and other significant structures (e.g., ligaments) can be identified. A knowledge of anatomy can be put to good use if, for example, one is trying to assess an injury. Assume a patient has a pain on the inside of the elbow. Knowledge of anatomy allows one to recognize the medial epicondyle of the humerus as the prominent bony structure of the medial elbow. It also indicates that the muscles that pull the hand and fingers toward the forearm in a flexion motion attach to the epicondyle. Thus, familiarity with anatomy may lead to a diagnosis of medial epicondylitis, possibly caused by overuse of the hand flexor muscles.
FIGURE 1-1 Types of movement analysis. Movement can be analyzed by assessing the anatomical contributions to the movement (functional anatomy), describing the motion characteristics (kinematics), or determining the cause of the motion (kinetics).
Functional anatomy is the study of the body components needed to achieve or perform a human movement or function. Using a functional anatomy approach to analyze a lateral arm raise with a dumbbell, one should identify the deltoid, trapezius, levator scapulae, rhomboid, and supraspinatus muscles as contributors to upward rotation and elevation of the shoulder girdle and abduction of the arm. Knowledge of functional anatomy is useful in a variety of situations, for example, to set up an exercise or weight training program and to assess the injury potential in a movement or sport or when establishing training techniques and drills for athletes. The prime consideration of functional anatomy is not the muscle's location but the movement produced by the muscle or muscle group.
Linear Versus Angular Motion
Movement or motion is a change in place, position, or posture occurring over time and relative to some point in the environment. Two types of motion are present in a human movement or an object propelled by a human. First is linear motion, often termed translation or translational motion. Linear motion is movement along a straight or curved pathway in which all points on a body or an object move the same distance in the same amount of time. Examples are the path of a sprinter, the trajectory of a baseball, the bar movement in a bench press, and the movement of the foot during a football punt. The focus in these
activities is on the direction, path, and speed of the movement of the body or object. Figure 1-2 illustrates two focal points for linear movement analysis.
FIGURE 1-2 Examples of linear motion. Ways to apply linear motion analysis include examination of the motion of the center of gravity or the path of a projected object.
The center of mass of the body, of a segment, or of an object is usually the point monitored in a linear analysis (Fig. 1-2). The center of mass is the point at which the mass of the object appears to be concentrated, and it represents the point at which the total effect of gravity acts on the object. However, any point can be selected and evaluated for linear motion. In skill analysis, for example, it is often helpful to monitor the motion of the top of the head to gain an indication of certain trunk motions. An examination of the head in running is a prime example. Does the head move up and down? Side to side? If so, it is an indication that the central mass of the body is also moving in those directions. The path of the hand or racquet is important in throwing and racquet sports, so visually monitoring the linear movement of the hand or racquet throughout the execution of the motion is beneficial. In an activity such as sprinting, the linear movement of the whole body is the most important component to analyze because the object of the sprint is to move the body quickly from one point to another.
The second type of motion is angular motion, which is motion around some point so that different regions of the same body segment or object do not move through the same distance in a given amount of time. As illustrated in Figure 1-3, swinging around a high bar represents angular motion because the whole body rotates around the contact point with the bar. To make one full revolution around the bar, the feet travel through a much greater distance than the arms because they are farther from the point of turning. It is typical in biomechanics to examine the linear motion characteristics of an activity and then follow up with a closer look at the angular motions that create and contribute to the linear motion.
All linear movements of the human body and objects propelled by humans occur as a consequence of angular contributions. There are exceptions to this rule such as skydiving or free falling, in which the body is held in a position to let gravity create the linear movement downward, and when an external pull or push moves the body or an object. It is important to identify the angular motions and their sequence that make up a skill or human movement because the angular motions determine the success or failure of the linear movement.
Angular motions occur about an imaginary line called the axis of rotation. Angular motion of a segment, such as the arm, occurs about an axis running through the joint. For example, lowering the body into a deep squat entails angular motion of the thigh about the hip joint, angular motion of the leg about the knee joint, and angular motion of the foot about the ankle joint. Angular motion can also occur about an axis through the center of mass. Examples of this type of angular motion are a somersault in the air and a figure skater's vertical spin. Finally, angular motion can occur about a fixed external axis. For example, the body follows an angular motion path when swinging around a high bar, with the high bar acting as the axis of rotation.
For proficiency in human movement analysis, it is necessary to identify the angular motion contributions to the linear motion of the body or an object. This is apparent in a simple activity such as kicking a ball for maximum distance. The intent of the kick is to make contact between a foot traveling at a high linear speed and moving in the proper direction to send the ball in the desired direction. The linear motion of interest is the path and velocity of the ball after it leaves the foot. To create the high speeds and the correct path, the angular motions of the segments of the kicking leg are sequential, drawing speed from each
other so that the velocity of the foot is determined by the summation of the individual velocities of the connecting segments. The kicking leg moves into a preparatory phase, drawing back through angular motions of the thigh, leg, and foot. The leg whips back underneath the thigh very quickly as the thigh starts to move forward to initiate the kick. In the power phase of the kick, the thigh moves vigorously forward and rapidly extends the leg and foot forward at very fast angular speeds. As contact is made with the ball, the foot is moving very fast because the velocities of the thigh and leg have been transferred to the foot. Skilled observation of human movement allows the relationship between angular and linear motion shown in this kicking example to serve as a foundation for techniques used to correct or facilitate a movement pattern or skill.
FIGURE 1-3 Examples of angular motion. Angular motion of the body, an object, or segment can take place around an axis running through a joint (A), through the center of gravity (B), or about an external axis (C).
Kinematics Versus Kinetics
A biomechanical analysis can be conducted from either of two perspectives. The first, kinematics, is concerned with the characteristics of motion from a spatial and temporal perspective without reference to the forces causing the motion. A kinematic analysis involves the description of movement to determine how fast an object is moving, how high it goes, or how far it travels. Thus, position, velocity, and acceleration are the components of interest in a kinematic analysis. Examples of linear kinematic analysis are the examination of the projectile characteristics of a high jumper or a study of the performance of elite swimmers. Examples of angular kinematic analysis are an observation of the joint movement sequence for a tennis serve or an examination of the segmental velocities and accelerations in a vertical jump. Figure 1-4 presents both an angular (top) and linear (bottom) example of the kinematics of the golf swing. By examining an angular or linear movement kinematically, we can identify the segments involved in that movement that require improvement or obtain ideas and technique enhancements from elite performers or break a skill down into its component parts. By each of these, we can further our understanding of human movement.
Pushing on a table may or may not move the table, depending on the direction and strength of the push. A push or pull between two objects that may or may not result in motion is termed a force. Kinetics is the area of study that examines the forces acting on a system, such as the human body, or any object. A kinetic movement analysis examines the forces causing a movement. A kinetic movement analysis is more difficult than a kinematic analysis both to comprehend and to evaluate because forces cannot be seen (Fig. 1-5). Only the effects of forces can be observed. Watch someone lift a 200-lb barbell in a squat. How much force has been applied? Because the force cannot be seen, there is no way of accurately evaluating the force unless it can be measured with recording instruments. A likely estimate of the force is at least 200 lb because that is the weight of the bar. The estimate may be inaccurate by a significant amount if the
weight of the body lifted and the speed of the bar were not considered.
FIGURE 1-4 Examples of kinematic movement analysis. Kinematic analysis focuses on the amount and type of movement, the direction of the movement, and the speed or change in speed of the body or an object. The golf shot is presented from two of these perspectives: the angular components of the golf swing (top) and the direction and speed of the club and ball (bottom).
The forces produced during human movement are important because they are responsible for creating all of our movements and for maintaining positions or postures having no movement. The assessment of these forces represents the greatest technical challenge in biomechanics because it requires sophisticated equipment and considerable expertise. Thus, for the novice movement analyst, concepts relating to maximizing or minimizing force production in the body will be more important than evaluating the actual forces themselves.
FIGURE 1-5 Examples of kinetic movement analysis. Kinetic analysis focuses on the cause of movement. The weight lifter demonstrates how lifting can be analyzed by looking at the vertical forces on the ground that produce the lift (linear) and the torques produced at the three lower extremity joints that generate the muscular force required for the lift (Redrawn from
Lander, J. et al. . Biomechanics of the squat exercise using a modified center of mass bar. Medicine and Science in Sports and Exercise, 18:469-478
A kinetic analysis can provide the teacher, therapist, coach, or researcher with valuable information about how the movement is produced or how a position is maintained. This information can direct conditioning and
training for a sport or movement. For example, kinetic analyses performed by researchers have identified weak and strong positions in various joint positions and movements. Thus, we know that the weakest position for starting an arm curl is with the weights hanging down and the forearm straight. If the same exercise is started with the elbow slightly bent, more weight can be lifted.
Kinetic analyses also identify the important parts of a skill in terms of movement production. For example, what is the best technique for maximizing a vertical jump? After measuring the forces produced against the ground that are used to propel the body upward, researchers have concluded that the vertical jump incorporating a very quick drop downward and stop-and-pop upward action (often called a countermovement action) produces more effective forces at the ground than a slow, deep gather jump.
Lastly, kinetics has played a crucial role in identifying aspects of a skill or movement that make the performer prone to injury. Why do 43% of participants and 76% of instructors of high-impact aerobics incur an injury (3)? The answer was clearly identified through a kinetic analysis that found forces in typical high-impact aerobic exercises to be in the magnitude of 4 to 5 times body weight (4). For an individual weighing 667.5 N (newtons) or 150 lb, repeated exposure to forces in the range of 2670 to 3337.5 N (600-750 lb) partially contributes to injury of the musculoskeletal system.
Examination of both the kinematic and kinetic components is essential to full understanding of all aspects of a movement. It is also important to study the kinematic and kinetic relationships because any acceleration of a limb, of an object, or of the human body is a result of a force applied at some point, at a particular time, of a given magnitude, and for a particular duration. Although it is of some use to merely describe the motion characteristics kinematically, one must also explore the kinetic sources before a thorough comprehension of a movement or skill is possible.
Statics Versus Dynamics
Examine the posture used to sit at a desk and work at a computer. Are forces being exerted? Yes. Even though there is no movement, there are forces between the back and the chair and the foot and the ground. In addition, muscular forces are acting throughout the body to counteract gravity and keep the head and trunk erect. Forces are present without motion and are produced continuously to maintain positions and postures that do not involve movement. Principles of statics are used to evaluate the sitting posture. Statics is a branch of mechanics that examines systems that are not moving or are moving at a constant speed. Static systems are considered to be in equilibrium. Equilibrium is a balanced state in which there is no acceleration because the forces causing a person or object to begin moving, to speed up, or to slow down are neutralized by opposite forces that cancel them out.
Statics is also useful for determining stresses on anatomical structures in the body, identifying the magnitude of muscular forces, and identifying the magnitude of force that would result in the loss of equilibrium. How much force generated by the deltoid muscle is required to hold the arm out to the side? Why is it easier to hold an arm at the side if you lower the arm so that it is no longer perpendicular to the body? What is the effect of a lordosis (increased curvature of the back, or swayback) on forces coming through the lumbar vertebrae? These are the types of questions static analysis may answer. Because the static case involves no change in the kinematics of the system, a static analysis is usually performed using kinetic techniques to identify the forces and the site of the force applications responsible for maintaining a posture, position, or constant speed. Kinematic analyses, however, can be applied in statics to substantiate whether there is equilibrium through the absence of acceleration.
To leave the computer workstation and get up out of the chair, it is necessary to produce forces in the lower extremity and on the ground. Dynamics is the branch of mechanics used to evaluate this type of movement because it examines systems that are being accelerated. Dynamics uses a kinematic or kinetic approach or both to analyze movement. An analysis of the dynamics of an activity such as running may incorporate a kinematic analysis in which the linear motion of the total body and the angular motion of the segments are described. The kinematic analysis may be related to a kinetic analysis that describes forces applied to the ground and across the joints as the person runs. Because this textbook deals with numerous examples involving motion of the human or a human-propelled object, dynamics is addressed in detail in specific chapters on linear and angular kinematics and kinetics.
Anatomical Movement Descriptors
It is important to identify segment names correctly and use them consistently when analyzing movement. To flex the shoulder, does one lift the arm with weights in the hand or raise the whole arm in front? Whatever interpretation is placed on the segment name, the term arm will determine the type of movement performed. The correct interpretation of flexing at the shoulder is to raise the whole arm because the arm is the segment between the shoulder and the elbow, not the segment between the elbow and the wrist or the hand segment. A review of segment names is worthwhile preparation for more extensive use of them in the study of biomechanics.
The head, neck, and trunk are segments comprising the main part of the body, or the axial portion of the skeleton. This portion of the body accounts for more than 50% of a person's weight, and it usually moves much more
slowly than the other parts of the body. Because of its large size and slow speed, the trunk is a good segment to observe visually when one is learning to analyze movement or following the total body activity.
The upper and lower extremities are termed the appendicular portion of the skeleton. Generally speaking, as one moves away from or distal to the trunk, the segments become smaller, move faster, and are more difficult to observe because of their size and speed. Thus, whereas shoulder flexion is raising the upper extremity in front, forearm flexion describes a movement at the elbow. The movements of the arm are typically described as they occur in the shoulder joint, forearm movements are described in relation to elbow joint activity, and hand movements are described relative to wrist joint activity. Figure 1-6 illustrates the axial and appendicular regions of the body with the correct segment names.
In the lower extremity, the thigh is the region between the hip and knee joints, the leg is the region between the knee and ankle joints, and the foot is the region distal to the ankle joint. The movement of the thigh is typically described as it occurs at the hip joint, leg movement is described by actions at the knee joint, and foot movements are determined by ankle joint activity.
The description of a segmental position or joint movement is typically expressed relative to a designated starting position. This reference position, or the anatomical position, has been a standard reference point used for many years by anatomists, biomechanists, and the medical profession. In this position, the body is in an erect stance with the head facing forward, arms at the side of the trunk with palms facing forward, and the legs together with the feet pointing forward. Some biomechanists prefer to use what is called thefundamental position as the reference position. This reference position is similar to the anatomical position except that the arms are in a more relaxed posture at the sides with the palms facing in toward the trunk. Whatever starting position is used, all segmental movement descriptions are made relative to some reference position. Both of these reference positions are illustrated in Figure 1-6.
FIGURE 1-6 Anatomical vs fundamental starting position. The anatomical and fundamental starting positions serve as a reference point for the description of joint movements.
To discuss joint position, we must define the joint angle, or more correctly, the relative angle between two segments. A relative angle is the included angle between the two segments (Fig. 1-7). The calculation of the relative angle is illustrated in a later chapter in this book.
The starting position is also called the zero position for description of most joint movements. For example, when a person is standing, there is zero movement at the hip joint. If the thigh is flexed or rotated internally or externally (in or out), the amount of movement is described relative to the fundamental or anatomical starting position. Most zero positions appear to be quite obvious because there is usually a straight line between the two segments so that no relative angle is formed between them. Zero position in the trunk occurs when the trunk is vertical and in line with the lower extremity. The zero position at the knee is found in the standing posture when there is no angle between the thigh and the leg. One not so obvious zero position is at the ankle joint. For this joint, the zero position is assumed in stance with the sole of the foot perpendicular to the leg.
Movement description or anatomical location can best be presented using terminology that is universally accepted and understood. Movement terms should become a part
of a working vocabulary, regardless of the level of application of kinesiology required. Development of solid knowledge of the movement characteristics of the various phases of a human movement or sport skill can improve the effectiveness of teaching a skill, assist in correcting flaws in a performance, identify important movements and segments for emphasis in conditioning, and identify aspects of the skill that may be associated with injury. The experienced researcher, coach, or teacher can determine the most relevant movements in a skill and will use a specific vocabulary of terms to instruct students or athletes. A standardized set of terms is most helpful in this situation.
FIGURE 1-7 Relative angles of the elbow (A) and knee (B).
The anatomical terms describing the relative position or direction are illustrated in Figure 1-8. The term medial refers to a position relatively close to the midline of the body or object or a movement that moves toward the midline. In the anatomical position, the little finger and the big toe are on the medial side of the extremity because they are on the side of the limb closest to the midline of the body. Also, pointing the toes toward the midline of the body is considered a movement in a medial direction. The opposite of medial is lateral, that is, a position relatively far from the midline or a movement away from the midline. In the anatomical position, the thumb and the little toe are on the lateral side of the hand and foot, respectively, because they are farther from midline. Likewise, pointing the toes out is a lateral movement. Landmarks are also commonly designated as medial or lateral based on their relative position to the midline, such as medial and lateral condyles, epicondyles, and malleoli.
Proximal and distal are used to describe the relative position with respect to a designated reference point, with proximal representing a position closer to the reference point and distal being a point further from the reference. The elbow joint is proximal, and the wrist joint is distal relative to the shoulder joint. The ankle joint is proximal, and the knee joint is distal relative to the point of heel contact with the ground. Both proximal and distal must be expressed relative to some reference point.
FIGURE 1-8 Anatomical terms used to describe relative position or direction.
A segment or anatomical landmark may lie on the superior aspect of the body, placing it above a particular reference point or closer to the top of the head. It may lie on an inferior aspect, that is, lower than a reference segment or landmark. For example, the head is positioned superior to the trunk, the trunk is superior to the thigh, and so on. The greater trochanter is located on the superior aspect of the femur, and the medial epicondyle of the humerus is located on the inferior end of the humerus.
The location of an object or a movement relative to the front or back is anterior or posterior, respectively. Thus, whereas the quadriceps muscle group is located on the anterior region of the thigh, the hamstrings muscle group is located on the posterior region of the thigh. Anterior is also synonymous with ventral for a location on the human body, andposterior refers to the dorsal surface or position on the human.
The term ipsilateral describes activity or location of a segment or landmark positioned on the same side as a particular reference point. Actions, positions, and landmark locations on the opposite side can be designated as contralateral. Thus, when a person lifts the right leg forward, there is extensive muscular activity in the iliopsoas muscle of that leg, the ipsilateral leg, and extensive activity in the gluteus medius of the contralateral leg to maintain balance and support. In walking, as the ipsilateral lower limb
swings forward, the other limb, the contralateral limb, pushes on the ground to propel the walker forward.
Six basic movements occur in varying combinations in the joints of the body. The first two movements, flexion and extension, are movements found in almost all of the freely movable joints in the body, including the toe, ankle, knee, hip, trunk, shoulder, elbow, wrist, and finger. Flexion is a bending movement in which the relative angle of the joint between two adjacent segments decreases. Extension is a straightening movement in which the relative angle of the joint between two adjacent segments increases as the joint returns to the zero or reference position. Numerous examples of both flexion and extension are provided in Figure 1-9. A person can also perform hyperflexion if the flexion movement goes beyond the normal range of flexion. For example, this can happen at the shoulder only when the arm moves forward and up in flexion through 180° until it is at the side of the head, and then hyper-flexes as it continues to move past the head toward the back. Hyperextension can occur in many joints as the extension movement continues past the original zero position. It is common to see hyperextension movements in the trunk, arm, thigh, and hand.
A toe-touch movement entails flexion at the vertebral, shoulder, and hip joints. The return to the standing position involves the opposite movements: of vertebral extension, hip extension, and shoulder extension. The power phase of the jump shot is produced via smooth timing of lower extremity hip extension, knee extension, and ankle extension coordinated with shoulder flexion, elbow extension, and wrist flexion in the shooting limb. This example illustrates the importance of the lower extremity extension movements to the production of power. Lower extremity extension often serves to produce upward propulsion that works against the pull of gravity. It is opposite in the shoulder joint, where flexion movements are primarily used to develop propulsion upward against gravity to raise the limb.
Abduction and adduction is another pair of movements that is not as commonly known as flexion and extension, occurring only in particular joints, such as the metatarsophalangeal (foot), hip, shoulder, wrist, and metacarpophalangeal (hand) joints. Many of these movements are presented in Figure 1-10. Abduction is a movement away
from the midline of the body or the segment. Raising an arm or leg out to the side or the spreading of the fingers or toes is an example of abduction. Hyperabduction can occur in the shoulder joint as the arm moves more than 180° from the side all the way up past the head. Adduction is the return movement of the segment back toward the midline of the body or segment. Bringing the arms back to the trunk, bringing the legs together, and closing the toes or fingers are examples of adduction. Hyperadduction occurs frequently in the arm and thigh as the adduction continues past the zero position so that the limb crosses the body. These side-to-side movements are commonly used to maintain balance and stability during the performance of both upper and lower extremity sport skills. Controlling or preventing abduction and adduction movements of the thigh is especially crucial to the maintenance of pelvic and limb stability during walking and running.
FIGURE 1-9 Flexion and extension. These movements occur in many joints in the body, including the vertebra, shoulder, elbow, wrist, metacarpophalanx, interphalanx, hip, knee, and metatarsophalanx.
FIGURE 1-10 Abduction and adduction. These movements occur in the sternoclavicular, shoulder, wrist, metacarpophalangeal, hip, intertarsal, and metatarsophalangeal joints.
The last two basic movements involve rotations, illustrated in Figure 1-11. A rotation can be either medial (also known as internal) or lateral (also known as external). Rotations are designated as right and left for the head and trunk only. When in the fundamental starting position, medial or internal rotation refers to the movement of a segment about a vertical axis running through the segment so that the anterior surface of the segment moves toward the midline of the body while the posterior surface moves away from the midline. Lateral or external rotation is the opposite movement in which the anterior surface moves away from the midline and the posterior surface of the segment moves toward the midline. Because the midline runs through the trunk and head segments, the rotations in these segments are described as left or right from the perspective of the performer. Right rotation is the movement of the anterior surface of the trunk so that it faces right while the posterior surface faces left, and left rotation is the opposite movement so that the anterior trunk faces left and the posterior trunk faces right. Rotations occur in the vertebrae, shoulder, hip, and knee joints. Rotation movements are important in the power phase of sport skills involving the trunk, arm, or thigh. For throwing, the throwing arm laterally rotates in the preparatory phase and medially rotates in the power and follow-through phases. The trunk complements the arm action with right rotation in the preparatory phase (right-handed thrower) and left rotation in the power and follow-through phase. Likewise,
the right thigh laterally rotates in the preparatory phase and medially rotates until the lower extremity comes off the ground in the power phase.
FIGURE 1-11 Rotation. Rotation occurs in the vertebral, shoulder, hip, and knee joints.
Specialized Movement Descriptors
Specialized movement names are assigned to a variety of segmental movements (Fig. 1-12). Although most of these segmental movements are technically among the six basic movements, the specialized movement name is the terminology commonly used by movement professionals. Right and left lateral flexion applies only to movement of the head or trunk. When the trunk or head is tilted sideways, the movement is termed lateral flexion. If the right side of the trunk or head moves so that it faces down, the movement is termedright lateral flexion and vice versa.
The shoulder girdle has specialized movement names that can best be described by observing the movements of the scapula. Whereas raising the scapula, as in a shoulder shrug, is termed elevation, the opposite lowering movement is called depression. If the two scapulae move apart, as in rounding the shoulders, the movement is termed protraction. The return movement, in which the scapulae move toward each other with the shoulders back, is called retraction. Finally, the scapulae can swing out such that the bottom of the scapula moves away from the trunk and the top of the scapula moves toward the trunk. This movement is termed upward rotation, and the opposite movement, when the scapula swings back down into the resting position, is downward rotation.
In the arm and the thigh segments, a combination of flexion and adduction is termed horizontal adduction, and a combination of extension and abduction is called horizontal abduction. Horizontal adduction, sometimes called horizontal flexion, is the movement of the arm or thigh across the body toward the midline using a movement horizontal to the ground. Horizontal abduction, or horizontal extension, is a horizontal movement of the arm or thigh away from the midline of the body. These movements are used in a wide variety of sport skills. The arm action of the discus throw is a good example of the use of horizontal abduction in the preparatory phase and horizontal adduction in the power and follow-through phase. Many soccer skills use horizontal adduction of the thigh to bring the leg up and across the body for a shot or pass.
In the forearm, pronation and supination occur as the distal end of the radius rotates over and back on the ulna at the radioulnar joints. Supination is the movement of the forearm in which the palm rotates to face forward from the fundamental starting position. Pronation is the movement in which the palms face backward. Supination and pronation joint movements have also been referred to as external and internal rotation, respectively. As the forearm moves from a supinated position to a pronated position, the forearm passes through the semiprone position, in which the palms face the midline of the body with the thumbs forward. The actions of forearm pronation and supination are used with arm rotation movements to increase the range of motion, add spin, enhance power,
and change direction during the force application phases in racquet sports, volleyball, and throwing.
FIGURE 1-12 Examples of specialized movements. Some joint movements are designated with specialized names, even though they may technically be one of the six basic movements.
At the wrist joint, whereas the movement of the hand toward the thumb is called radial flexion, the opposite movement of the hand toward the little finger is called ulnar flexion. These specialized movement names are easier to remember because they do not depend on forearm or arm position, as do the interpretation of abduction and adduction, and they can easily be interpreted if the locations of the radius (thumb side) and the ulna (little finger side) are known. Ulnar and radial flexion are important in racquet sports for control and stabilization of the racquet. Also, in volleyball, ulnar flexion is a valuable component of the forearm pass because it helps to maintain the extended arm position and increases the contact area of the forearms.
In the foot, plantar flexion and dorsiflexion are specialized names for foot extension and flexion, respectively. Plantarflexion is the movement in which the bottom of the foot moves down and the angle formed between the foot and the leg increases. This movement can be created by raising the heel so the weight is shifted up on the toes or by placing the foot flat on the ground in front and moving the leg backward so that the body weight is behind the foot. Dorsiflexion is the movement of the foot up toward the leg that decreases the relative angle between the leg and the foot. This movement may be created by putting weight on the heels and raising the toes or by keeping the feet flat on the floor and lowering with weight centered over the foot. Any foot-leg angle greater than 90° is termed a plantar flexed position, and any foot-leg angle less than 90° is termed dorsiflexion.
The foot has another set of specialized movements, called inversion and eversion, that occur in the intertarsal and metatarsal articulations. Inversion of the foot takes place when the medial border of the foot lifts so that the sole of the foot faces medially toward the other foot. Eversion is the opposite movement of the foot: The lateral aspect of the foot lifts so that the sole of the foot faces away from the other foot.
Often confusion exists over the use of the terms inversion and eversion and the popularized use of pronation and
supination as descriptors of foot motion. Inversion and eversion are not the same as pronation and supination; in fact, they are only a part of pronation and supination. Pronation of the foot is actually a set of movements consisting of dorsiflexion at the ankle joint, eversion, and abduction of the forefoot. Supination is created through ankle plantarflexion, inversion, and forefoot adduction. Pronation and supination are dynamic movements of the foot and ankle that particularly occur when the foot is on the ground during a run or walk. These two movements are determined by structure and laxity of the foot, body weight, playing surfaces, and footwear.
TABLE 1-1 Movement Review
The final specialized movement, circumduction, can be created in any joint or segment that has the potential to move in two directions, such that the segment can be moved in a conic fashion as the end of the segment moves in a circular path. An example of circumduction is placing the arm out in front and drawing an imaginary circle in the air. Circumduction is not a simple rotation; rather, it is four movements in sequence. The movement of the arm in the creation of the imaginary O is actually a combination of flexion, adduction, extension, and abduction. Circumduction movements are also possible in the foot, thigh, trunk, head, and hand. The movements of all of the major segments are reviewed in Table 1-1.
Relative Versus Absolute
A reference system is essential for accurate observation and description of any type of motion. The use of joint movements relative to the fundamental or anatomical starting position is an example of a simple reference system. This system was previously used in this chapter to describe movement of the segments. To improve on the precision of a movement analysis, a movement can be evaluated with respect to a different starting point or position.
A reference system is necessary to specify position of the body, segment, or object so as to describe motion or identify whether any motion has occurred. The reference frame or system is arbitrary and may be within or outside of the body. The reference frame consists of imaginary lines called axes that intersect at right angles at a common point termed the origin. The origin of the reference frame is placed at a designated location such as a joint center.
The axes are generally given letter representations to differentiate the direction in which they are pointing. Any position can be described by identifying the distance of the object from each of the axes. In two-dimensional or planar movement, there are two axes, a horizontal axis and a vertical axis. In a three-dimensional movement, there are three axes, two horizontal axes that form a plane and a vertical axis. It is important to identify the frame of reference used in the description of motion.
An example of a reference system placed outside the body is the starting line in a race. The center of an anatomical joint, such as the shoulder, can be used as a reference system within the body. The arm can be described as moving through a 90° angle if abducted until perpendicular to the trunk. If the ground is used as a frame of reference, the same arm abduction movement can be described with respect to the ground, such as movement to a height of 1.6 m from the ground.
When angular motion is described, the joint positions, velocities, and accelerations can be described using either an absolute or a relative frame of reference. An absolute reference frame is one in which the axes intersect in the center of the joint and movement of a segment is described with respect to that joint. The axes are generally oriented horizontally and vertically. The horizontal axis is generally called the x-axis and the vertical axis the y-axis, although these axes may be called by any name as long as they are defined and consistent. A segment angle is measured from the right horizontal axes (Fig. 1-13A) and defines the orientation of the segment in space. The absolute positioning of an abducted arm perpendicular to the trunk is 0° or 360° when described relative to the axes running through the shoulder joint. A relative reference frame is one in which the movement of a segment is described relative to the adjacent segment. This type of reference frame is often used to describe a joint angle.
The axes in this reference frame are not horizontal and vertical. Figure 1-13B shows the y-axis placed along one segment, the leg, and the x-axis perpendicular to the y-axis. The knee angle can then be determined from the lower portion of the y-axis to the dotted line describing the thigh segment.
FIGURE 1-13 Absolute vs relative reference frame. Left, An absolute reference frame measures the segment angle (A) with respect to the distal joint. Right, A relative reference frame measures the relative angle (B) formed by the two segments. It is important to desgnate the reference frame in movement description.
In the previously described example of the arm, with abduction perpendicular to the trunk, the relative positioning of the arm with respect to the trunk is 90°. The reference frame should be clearly identified so that the results can be interpreted accordingly and, because reference systems vary among researchers, the reference system and reference point must be identified before comparing and contrasting results between studies. For example, some researchers label a fully extended forearm as a 180° position, and others label the position 0°. After 30° of flexion at the elbow joint, the final position is 150° or 30°, respectively, for the two systems described above. Considerable confusion can arise when trying to interpret an article using a different reference system from that of the authors.
Planes and Axes
The universally used method of describing human movements is based on a system of planes and axes. A plane is a flat, two-dimensional surface. Three imaginary planes are positioned through the body at right angles to each other so they intersect at the center of mass of the body. These are the cardinal planes of the body. Movement is said to occur in a specific plane if it is actually along that plane or parallel to it. Movement in a plane always occurs about an axis of rotation perpendicular to the plane (Fig. 1-14). If you stick a pin through a piece of cardboard and spin the paper around the pin, the movement of the cardboard takes place in the plane, and the pin
represents the axis of rotation. The cardboard can spin around the pin while the pin is front to back horizontal, vertical, or sideways, for movement of the cardboard in all three of the planes. This example can be applied to describe imaginary lines running through the total body center of mass in the same three pin directions. These planes allow full description of a motion and contrast of an arm movement straight out in front of the body with one straight out to the side of the body. The planes and axes of the human body for motion description are presented in Figure 1-15.
FIGURE 1-14 The plane and axis. Movement takes place in a plane about an axis perpendicular to the plane.
FIGURE 1-15 Planes and axes on the human body. The three cardinal planes that originate at the center of gravity are the sagittal plane, which divides the body into right and left; the frontal plane, dividing the body into front and back; and the transverse plane, dividing the body into top and bottom. Movement takes place in or parallel to the planes about a mediolateral axis (sagittal plane), an anteroposterior axis (frontal plane), or a longitudinal axis (transverse plane).
The sagittal plane bisects the body into right and left halves. Movements in the sagittal plane occur about a mediolateral axis running side to side through the center of mass of the body. Sagittal plane movements involving the whole body rotating around the center of mass include somersaults, backward and forward handsprings, and flexing to a pike position in a dive. The frontal or coronal plane bisects the body to create front and back halves. The axis about which frontal plane movements occur is the anteroposterior axis that runs anterior and posterior from the plane. Frontal plane motions of the whole body about the center of mass are not as common as movements in the other planes. The transverse or horizontal plane bisects the body to create upper and lower halves. Movements occurring in this plane are primarily rotations about a longitudinal axis. Spinning vertically around the body, as in a figure skating spin, is an example of transverse plane movement about the body's center of mass.
Although we have described the sagittal, transverse, and frontal cardinal planes, actually any number of other planes can pass through the body. For example, we can define many sagittal planes that do not pass through the center of mass of the body. The only requirement for defining such a plane is that it is parallel to the cardinal sagittal plane. Likewise, we can have multiple transverse or frontal planes. Defining these noncardinal planes is useful for describing joint or limb movements. The intersection of the three planes is placed at the joint center so that joint actions can be described in a sagittal, transverse, or frontal plane (Fig. 1-16). Noncardinal planes can also be used in examining movements that take place about an external axis.
FIGURE 1-16 Planes and axes for the knee.
FIGURE 1-17 Movements in the sagittal plane. Sagittal plane movements are typically flexions and extensions or some forward or backward turning exercise. The movements can take place about a joint axis, the center of gravity, or an external axis.
Most planar or two-dimensional analyses in biomechanics are concerned with motion in the sagittal plane through a joint center. Examples of sagittal plane movements at a joint can be demonstrated by performing flexion and extension movements, such as raising the arm in front, bending the trunk forward and back, lifting and lowering the leg in front, and rising on the toes. Examples of sagittal plane movements of the body about an external support point include rotating the body over the planted foot and running and rotating the body over the hands in a vault. The most accurate view of any motion in a plane is obtained from a position perpendicular to the plane of movement to allow viewing along the axis of rotation. Therefore, sagittal plane movements are best viewed from the side of the body to allow focus on a frontal axis of rotation (Fig. 1-17).
Similar to sagittal plane movements, frontal plane movements can occur about a joint. Characteristic joint movements in the frontal plane include thigh abduction and adduction, finger and hand abduction and adduction, lateral flexion of the head and trunk, and inversion and eversion of the foot. Frontal plane motion about an external point of contact can especially be seen often in dance and ballet as the dancers move laterally from a pivot point and in gymnastics with the body rotating sideways over the hands, such as when doing a cartwheel. The best position to view frontal plane movements is in front or in back of the body to focus on the joint or the point about which the whole body rotates (Fig. 1-18).
Examples of movements in the transverse plane about longitudinal joint axes are rotations at the vertebrae, shoulder, and hip joints. Pronation and supination of the forearm at the radioulnar joints is also a transverse plane movement. The axis for all of these movements is an imaginary line running longitudinally through the vertebrae, shoulder, radioulnar, or the hip joints. This is a common movement in gymnastics, dance, and ice skating. Additionally, numerous examples can be found in dance, skating, and gymnastics in which the athlete performs transverse plane movements about an external axis running through a pivot point between the foot and the ground. All spinning movements that have the whole body turning about the ground or the ice are examples. Although transverse plane motions are vital aspects of most successful sport skills, these movements are difficult to follow visually because the best viewing position is either above or below the movement, perpendicular to the plane of motion. Consequently, rotation motions are evaluated by following the linear movement of some point on the body if vertical positioning cannot be achieved. Examples of movements in the transverse plane are presented in Figure 1-19.
FIGURE 1-18 Movements in the frontal plane. Segmental movements in the frontal plane about anteroposterior joint axes are abduction and adduction or some specialized side-to-side movement. Frontal plane movements about the center of gravity or an external point involve sideways movement of the body, which is more difficult than movement to the front or back.
Most human movements take place in multiple planes at the various joints. In running, for example, the lower extremity appears to move predominantly in the sagittal plane as the limbs swing forward and back throughout the gait cycle. Upon closer examination of the limbs and joints, one finds movements in all of the planes. At the hip joint, for example, the thigh performs flexion and extension in the sagittal plane, abduction and adduction in the frontal plane, and internal and external rotation in the transverse plane. If human movements were confined to single-plane motion, we would look like robots as we performed our skills or joint motions. Examine the three-dimensional motion for an overhand throw presented in Figure 1-20. Note the positioning for viewing motion in each of the three planes.
The movement in a plane can also be described as a single degree of freedom (df). This terminology is commonly used to describe the type and amount of motion structurally allowed by the anatomical joints. A joint with I df indicates that the joint allows the segment to move through one plane of motion. A joint with 1 df is also termed uniaxial because one axis is perpendicular to the plane of motion about which movement occurs. A 1-df joint, the elbow, allows only flexion and extension in the sagittal plane.
Conventionally, most joints are considered to have 1, 2, or 3 df, offering movement potential that is uniaxial, biaxial, or triaxial, respectively. The shoulder is an example of a 3-df, or triaxial, joint because it allows the arm to move in the frontal plane via abduction and adduction, in the sagittal plane via flexion and extension, and in the transverse plane via rotation.
Joints with 3 df include the vertebrae, shoulder, and hip; 2-df joints include the knee, metacarpophalangeal (hand), wrist, and thumb carpometacarpal joints; and 1-df joints include the atlantoaxial (neck), interphalangeal (hand and foot), radioulnar (elbow), and ankle joints. Three degrees of freedom does not always imply great mobility, but it does indicate that the joint allows movement in all three planes of motion. The shoulder is much more mobile than the hip, even though they both are triaxial joints and are capable of performing the same movements. The trunk movements, although classified as having 3 df, are quite restricted if one evaluates movement at a single vertebral level. For example, the lumbar and cervical areas of the vertebrae allow the trunk to flex and extend, but this plane of movement is limited in the middle thoracic portion of the vertebrae. Likewise, the rotation actions of the trunk occur primarily in the thoracic and cervical regions because the lumbar region has limited movement potential in the horizontal plane. It is only the combination of all of the vertebral segments that allows the 3-df motion produced by the spine.
Also, gliding movements occur across the joint surfaces. Gliding movements may be interpreted as adding more degrees of freedom to those defined in the literature. For example, the knee joint is considered to have 2 df for flexion and extension in the sagittal plane and rotation in the transverse plane. The knee joint also demonstrates linear translation, however, and it is well known that there is movement in the joint in the frontal plane as the joint surfaces glide over one another to create side-to-side translation movements. Although these movements have been measured and are relatively significant, they have not been established as an additional degree of freedom for the joint. The degrees of freedom for most of the joints in the body are shown in Table 1-1.
A kinematic chain is derived from combining degrees of freedom at various joints to produce a skill or movement. The chain is the summation of the degrees of freedom in adjacent joints that identifies the total degrees of freedom available or necessary for the performance of a movement. For example, kicking a ball might involve an 11-df system relative to the trunk. This would include perhaps 3 df at the hip, 2 df at the knee, 1 df at the ankle, 3 df in the tarsals (foot), and 2 df in the toes.
FIGURE 1-19 Movements in the transverse plane. Most transverse plane movements are rotations about a longitudinal axis running through a joint, the center of gravity, or an external contact point.
Biomechanics, the application of the laws of physics to the study of motion, is an essential discipline for studying human movement. From a biomechanical point of view, human motion can be qualitatively or quantitatively assessed. A qualitative analysis is a nonnumeric assessment of the movement. A quantitative analysis uses kinematic or kinetic applications that analyze a skill or movement by identifying its components or by assessing the forces creating the motion, respectively.
FIGURE 1-20 Movements in all three planes. Most human movements use movement in all three planes. The release phase of the overhand throw illustrates movements in all three planes. The sagittal plane movements are viewed from the side; the frontal plane movements, from the rear; and the transverse plane movements, from above.
To provide a specific description of a movement, it is helpful to define movements with respect to a starting point or to one of the three planes of motion: sagittal, frontal, or transverse.
Anatomical movement descriptors should be used to describe segmental movements. This requires acknowledgment of the starting position (fundamental or anatomical), standardized use of segment names (arm, forearm, hand, thigh, leg, foot), and the correct use of movement descriptors (flexion, extension, abduction, adduction, rotation).
True or False
A movement away from the midline of the body.
Absolute Reference Frame
A reference frame in which the origin is at the joint center.
A movement toward the midline of the body.
The standardized reference position used in the medical profession.
The science of the structure of the body.
Motion around an axis of rotation in which different regions of the same object do not move through the same distance.
A position in front of a designated reference point.
The axis through the center of mass of the body running from posterior to anterior.
The bones of the extremities.
The imaginary line of a reference system along which position is measured.
The bones of the head, neck, and trunk.
Axis of Rotation
The imaginary line about which an object rotates.
The study of motion and the effect of forces on biological systems.
The planes of the body that intersect at the total body center of mass.
A movement that is a combination of flexion, adduction, extension, and abduction.
On the opposite side.
Degree of Freedom
The movement of a joint in a plane.
The lowering movement of a body part such as the scapula.
A position relatively far from a designated reference point.
The motion in which the relative angle between the foot and the leg decreases.
The action whereby the scapula swings toward the midline of the body.
The branch of mechanics in which the system being studied undergoes acceleration.
The movement in which the lateral border of the foot lifts so that the sole of the foot faces away from the midline of the body.
The action in which the relative angle between two adjacent segments gets larger.
Frontal (Coronal) Plane
The plane that bisects the body into front and back halves.
A standardized reference position similar to the anatomical position.
The study of the body components needed to achieve a human movement or function.
A combination of extension and abduction of the arm or thigh.
A combination of flexion and adduction of the arm or thigh.
Abduction movement beyond the normal range of abduction.
Adduction movement beyond the normal range of adduction.
Extension movement beyond the normal range of extension.
Flexion movement goes beyond the normal range of flexion.
A position below a designated reference point.
The movement in which the medial border of the foot lifts so that the sole of the foot faces away from the midline of the body.
On the same side.
Area of study that examines the spatial and temporal components of motion (position, velocity, acceleration).
Study of human movement.
Study of the forces that act on a system.
A position relatively far from the midline of the body.
A flexion movement of the head or trunk.
Motion in a straight or curved line in which different regions of the same object move the same distance.
The axis through the center of mass of the body running from top to bottom.
A position relatively closer to the midline of the body.
The axis through the center of mass of the body running from right to left.
Movement or Motion
A change in place, position, or posture occurring over time and relative to some point in the environment.
The intersection of the axes of a reference system and the reference point from which measures are taken.
Plane of Motion
A two-dimensional surface running through an object. Motion occurs in the plane or parallel to the plane.
The motion in which the relative angle between the foot and the leg increases.
A position behind a designated reference point.
Movement in which the front or ventral surface rotates to face downward, as seen in the forearm and foot.
The motion describing the separating action of the scapula.
A position relatively closer to a designated reference point.
A nonnumeric description or evaluation of movement based on direct observation.
A numeric description or evaluation of movement based on data collected during the performance of the movement.
The flexion movement of the hand toward the forearm on the thumb side of the hand.
A system to locate a point in space.
Relative Angle (Joint Angle)
The included angle between two adjacent segments.
Relative Reference Frame
A reference frame in which the origin is at the joint center and one of the axes is placed along one of the segments.
The motion describing the coming together action of the scapula.
A movement about an axis of rotation in which not every point of the segment or body covers the same distance in the same time.
The plane that bisects the body into right and left sides.
A branch of mechanics in which the system being studied undergoes no acceleration.
Transverse (Horizontal) Plane
The plane that bisects the body into top and bottom halves.
A position above a designated reference point.
Movement in which the back or dorsal surface rotates upward, as seen in the forearm and foot.
The flexion movement of the hand toward the forearm on the little finger side of the hand.
The action whereby the scapula swings out from the midline of the body.