Chapter 2. The Musculoskeletal System: The Skeletal Framework and Its Movements

At the conclusion of this chapter, the student should be able to:

• 1. Classify joints according to structure and explain the relationship between a joint structure and its capacity for movement.
• 2. Explain how the schedule of ossification of epiphyseal cartilage is related to the nature of activities suitable for different age groups.
• 3. Name the factors that contribute to joint range of motion and stability and explain the relationship that exists between range of motion and stability.
• 4. Assess a joint’s range of motion, evaluate the range, and describe desirable procedures for changing it when indicated.
• 5. Name and define the orientation positions and planes of the body and the axes of motion.
• 6. Demonstrate and name fundamental movement patterns using correct movement terminology.
• 7. Isolate and name single joint actions that are part of complex movements.
• 8. Perform an anatomical analysis of the joint actions and planes of motion for a selected motor skill.

It is customary—especially for students of human movement and exercise—to begin the study of anatomy with a detailed study of the bones, then to proceed to the joints, and then to the muscles. This path of investigation sometimes dampens the enthusiasm of students, whose chief focus of interest is movement. Therefore, this chapter and the next emphasize the concept of the total musculoskeletal system as a mechanism for motion. It is hoped that by using this concept the student will find the study of the structural elements of this system more meaningful.

As the phrase implies, the musculoskeletal framework is an arrangement of bones and muscles. Adjacent bones are attached to one another by joints, which provide for the motion of the articulating bones, and the muscles that span the joints provide the force for moving the bones to which they are attached. Mechanically, the total bone–joint–muscle structure is an intricate combination of levers that makes possible a great number of coordinated movements, ranging from the small hand and finger motions used in assembling a television set or playing the piano to the total body movements of a swimmer or a pole vaulter. Any single one of the levers involved in such movements is relatively simple.

In physics we are taught that a lever is defined as a rigid bar that turns about a fulcrum (fixed axis or pivot) when force is applied to it at some specific point. An anatomical lever, therefore, is simply a bone that engages in an angular or turning type of movement when a force is applied to it. This force may be the contracting force of a muscle attached to the bone, or it may be an external force such as gravity or an external weight. If the force is muscle, it is always a pulling force, because muscles, being flexible, are unable to push; they can only pull. If the force is external, such as when a weight is too heavy to be maintained, the muscle force controls the rate at which the lever is allowed to move.

Although anatomy texts give the number of bones in the human skeleton as 206, only 177 of them engage in voluntary movement. The skeleton consists of two major parts, the axial skeleton and the appendicular skeleton (Figure 2.1). The axial section comprises the skull, spinal column, sternum, and ribs, and the appendicular section includes the bones of the upper and lower extremities. The bones of the upper extremity include the scapula, clavicle, humerus, ulna, radius, carpal bones, metacarpals, and phalanges, and those of the lower extremity include the three fused bones of the pelvis, the femur, tibia, fibula, tarsal bones, metatarsals, and phalanges. Although the pelvis may be classified with either the axial or the appendicular skeleton, it is actually a link between the axial skeleton and the lower extremity branch of the appendicular skeleton and is functionally as important to one as to the other. The skeleton serves to support weight, as a place for muscles to attach, and to protect critical structures such as the spinal cord, brain, and various organs.

The process of bone development, referred to as osteogenesis, begins with the formation of chondrocytes. These cells lay down a matrix upon which osteoblasts form bone. Osteogenesis is a complex process that continues throughout the life span to some degree. Bone is resorbed by osteoclast action and reformed by osteoblasts. Bone growth takes place at the growth plates, or epiphyses, where cartilage calcifies and is converted into bone. Growth peaks after puberty, but bone regeneration continues in response to muscle activity, stress, injury, and aging. Bone growth, or demineralization, also occurs in response to the levels of a number of hormones, including, but not limited to, estrogen and testosterone. Exercise is one of the best ways to ensure bone growth and strengthening.

Bone develops into two primary types. Compact (or cortical) bone is the dense layer of bone that makes up the outer layer of bone, especially in the long bones. Cancellous (or trabecular) bone has a more open, spongy appearance. This type of bone allows for a relatively high strength-to-weight ratio. As can be seen in Figure 2.2, cancellous bone forms a series of trusses that are aligned to tolerate compressive stress. Cancellous bone is found throughout the body but is especially prevalent in the condyles and epicondyles of the long bones and in the vertebral bodies of the spine.

Types of Bones

In spite of the great variety of shapes and sizes of bones, there are only four major bone categories: long, short, flat, and irregular.

Long Bones

Characterized by a cylindrical shaft with relatively broad, knobby ends. The shaft or body has thick walls of compact bone and contains a central cavity known as a medullary canal. The bones belonging in this category are the clavicle, humerus, ulna, radius, metacarpals, and phalanges of the upper extremity, and the femur, tibia, fibula, metatarsals, and phalanges of the lower extremity.

Short Bones

Relatively small, chunky, solid bones. The carpals and tarsals (wrist and ankle bones) belong to this category.

Flat Bones

Flat, platelike bones. The sternum, scapulae, ribs, and pelvic bones are examples.

Irregular Bones

The bones of the spinal column. The twenty-four vertebrae, the sacrum, and the coccyx are found in this category.

Mechanical Axis of a Bone

It has already been stated that bones serve as levers. When muscular action is analyzed mechanically, one must know what is meant by the mechanical axis of the bone, or the body segment, that is serving as the lever. The student will find it advantageous, therefore, to learn what this term is now while studying the bones. The mechanical axis of a bone or segment is a straight line that connects the midpoint of the joint at one end with the midpoint of the joint at the other end or, in the case of a terminal segment, with the midpoint of its distal end. This axis does not necessarily pass lengthwise through the shaft of the bony lever. If the shaft is curved or if the articulating process projects at an angle from the shaft, the greater part of the axis may lie outside the shaft, as in the case of the femur (Figure 2.3).

Skeletal Changes

An epiphysis is a part of a bone separated from the main bone by a layer of cartilage whose presence in the bone is an indication that the bone has not completed its growth. In the long bones the shaft is separated from the ends and from articulating knobs by epiphyseal cartilages. This is where growth occurs. As growth ceases, the cartilages gradually become ossified (replaced by bone) and, when closure is complete, no more growth can occur. Surprisingly, several epiphyses do not completely ossify until the twentieth, or even the twenty-fifth, year of life. Hence, most high school students and many college men are engaging in vigorous sports before their bones are fully matured. Physical education instructors, trainers, and coaches need to be aware of this fact. Although this is not usually harmful in the case of noncontact sports, in violent contact sports like football and boxing the consequences may be extremely serious. Protective devices are helpful, but high standards for these devices must be insisted upon. Even more important, coaches, especially those of young adolescents, must be knowledgeable about anatomy, growth patterns, and safety precautions and conscientious in the application of this knowledge. They must accept their responsibility for the health and safety of each individual player as being of far greater importance than a winning record.

There is also danger to elementary and junior high school youngsters who are permitted to bear weights that are too heavy for them. “Husky-looking” boys and girls are often mistakenly permitted to form the base of a pyramid because they give the impression of being strong. If they are on their hands and knees supporting a second and perhaps a third row of youngsters, the weight may be too much for them. This outcome is especially likely if the individuals in the upper rows are careless about where they place their hands and knees. If they are not placed in line with the supporting individuals’ arms and thighs, the vertebrae and possibly the arms and thighs will be subjected to rotary stress, which may result in serious injury to the epiphyses of these bones. The approximate ages at which the ossification of the epiphyseal cartilages is completed are presented in Tables 2.1 and 2.2 (Goss 1980).

Consideration also must be given to the other end of the age spectrum, the elderly, and the concern for osteoporosis. Osteoporosis is the loss of calcium, other minerals, and the matrix from the bones, which causes the bones to become porous and brittle. Some of this loss is natural, resulting from the aging process. In serious cases, bones subjected to weight bearing and muscle pull may collapse. In some situations, falling may be the result of bones breaking from their brittleness. At best, osteoporosis negatively affects a person’s ability to move. Osteoporosis occurs more often in women than in men. Current research indicates that the negative effects may be reduced through good nutrition and regular exercise throughout life. Moreover, nutrition, hormone regulation, and regular exercise begun late in life also tend to control some of the adverse effects of osteoporosis.

The structure and function of joints are so interrelated that it is difficult to discuss them separately. Hence, in the discussion of structure, there is much that relates to function and, conversely, much that relates to structure when function is discussed. Careful inspection of the joints depicted in Figure 2.4 shows the relationship between the shape of the joint and the movements it permits. In much the same way that railroad tracks determine the route available to the train, the configuration of the bones that form an articulation, together with the reinforcing ligaments, determine and limit the movements that the involved segment can make.

Structural Classification

There are many different patterns of joint structure, and these form the basis for their classification. The classification found in most anatomy texts is based on the presence or absence of a joint cavity—that is, a space between the articulating surfaces of the bones. Each type of joint is further classified either according to shape or according to the nature of the tissues that connect the bones. These classifications, with their subdivisions, may be grasped more readily in outline form and in Figure 2.4:

• I. Diarthrosis (from the Greek, meaning a joint in which there is a separation or articular cavity) (Figures 2.5 and 2.6)
  • A. Characteristics
    • 1. An articular cavity is present.
    • 2. The joint is encased within a sleevelike ligamentous capsule.
    • 3. The capsule is lined with synovial membrane that secretes synovial fluid for lubricating the joint.
    • 4. The articular surfaces are smooth.
    • 5. The articular surfaces are covered with cartilage, usually hyaline, but occasionally fibrocartilage.
  • B. Classification
    • 1. Irregular (arthrodial; plane). The joint surfaces are irregularly shaped, usually flat or slightly curved. The only movement permitted is of a gliding nature; hence, it is nonaxial.
    • 2. Hinge (ginglymus). One surface is spool-like; the other is concave. The concave surface fits over the spool-like process and glides partially around it in a hinge type of movement. This constitutes movement in one plane about a single axis of motion; hence, it is uniaxial. The movements that occur are flexion and extension.
    • 3. Pivot (trochoid; screw). This kind of joint may be characterized by a peglike pivot, as in the joint between atlas and axis, or by two long bones fitting against each other near each end in such a way that one bone can roll around the other one, as do the radius and ulna of the forearm. In the latter type, a small concave notch on one bone fits against the rounded surface of the other. The rounded surface may be either the edge of a disc (like the head of the radius) or a rounded knob (like the head of the ulna). The only movement permitted in either kind of pivot joint is rotation. It is a movement in one plane about a single axis; hence, the joint is uniaxial.
    • 4. Condyloid (ovoid; ellipsoidal). An oval or egg-shaped convex surface fits into a reciprocally shaped concave surface. Movement can occur in two planes, forward and backward, and from side to side. The former movement is flexion and extension, and the latter abduction and adduction or lateral flexion. The joint is biaxial. When these movements are performed sequentially, they constitute circumduction.
    • 5. Saddle (sellar; reciprocal reception). This may be thought of as a modification of a condyloid joint. Both ends of the convex surface are tipped up, making the surface concave in the other direction, like a western saddle. Fitting over this is a reciprocally concave-convex surface. This is a biaxial joint, permitting flexion and extension, abduction and adduction, and circumduction. The saddle joint has greater freedom of motion than the condyloid joint.
    • 6. Ball-and-socket (spheroidal; enarthrodial). In this type of joint the spherical head of one bone fits into the cup or saucerlike cavity of the other bone (see Figures 5.7, 5.8, 7.8, and 7.9). It is much like the swivel joint on a trailer hitch. It permits flexion and extension, abduction and adduction, circumduction (the sequential combination of the preceding four motions), horizontal adduction and abductions, and rotation. It is a triaxial joint because it permits movements about three axes.

(See Appendix B for a more complete chart of diarthrodial joints and their motions.)

• II. Synarthrosis (from the Greek, meaning literally “with joint” or, according to our usage, a joint in which there is no separation or articular cavity)
  • A. Characteristics
    • 1. In two of the types (cartilaginous and fibrous), the two bones are united by means of an intervening substance, such as cartilage or fibrous tissue, which is continuous with the joint surfaces.
    • 2. The third type (ligamentous) is not a true joint but is a ligamentous connection between two bones, which may or may not be contiguous.
    • 3. There is no articular cavity, hence no capsule, synovial membrane, or synovial fluid.
  • B. Classification
    • 1. Cartilaginous. Only the joints that are united by fibrocartilage permit motion of a bending and twisting nature. Example of fibrocartilaginous type: articulations between the bodies of the vertebrae (see Figure 9.2). Those united by hyaline cartilage permit only a slight compression. Example of hyaline type: epiphyseal unions.
    • 2. Fibrous. The edges of bone are united by means of a thin layer of fibrous tissue that is continuous with the periosteum. No movements are permitted. Only example: the sutures of the skull.
    • 3. Ligamentous. Two bodies, which may be adjacent or may be quite widely separated, are tied together by one or more ligaments. These ligaments may be in the form of cords, bands, or flat sheets. The movement that occurs is usually limited and of no specific type. Examples: coracoacromial union (Figure 5.9); midunion of radius and ulna (see Figures 6.2 and 6.3).
  • C. Summary
    • The synarthrodial joints of greatest concern to the kinesiologist are those of the vertebral bodies. The thickness of the intervertebral discs permits a moderate amount of motion simulating that of ball-and-socket joints. The movements are flexion and extension, lateral flexion, circumduction, and rotation.

Suggestions for Studying Joint Structure

To understand thoroughly the structure of a joint and especially the relation of structure to function, the student should supplement book study with firsthand study of a skeleton or of the disarticulated bones that enter into the formation of each major joint.

The movements of each joint should be studied both on the skeleton and on the living subject. When using the latter method, it is important to consider all the joints involved in the movement in question. For instance, in studying the movements of the elbow joint, the articulation between the humerus and radius must not be overlooked. The close relationship that exists between certain joints should be noted, for example, the relationship between the elbow joint and proximal radioulnar articulation. The tilting of the pelvis that accompanies many movements of the lower extremity and lumbar spine should be recognized, as should the movements of the shoulder girdle that accompany those of the shoulder joint. A particular pitfall awaits those who study the movements of the shoulder joint only by observing the living subject. If not forewarned, students may overlook the part played by the shoulder girdle. Its movements may be detected by palpating the scapula and clavicle in all movements of the upper arm. To follow the movements of the scapula, the thumb should be placed at the inferior angle, one finger on the root of the scapular spine and another on the acromion process. Firm contact should be maintained as the scapula moves.

A student of movement must always be observant of motion possibilities in all the joints and not limit attention to just one joint. Only through the cooperative involvement of various parts can the total range of movement potential be experienced and understood. Therefore, focusing on one joint would be misleading and would result in misinterpretations of what was being observed.

Joint Stability

The function of the joints is obviously to provide the bones with a means of moving or, rather, of being moved. But because such provisions bring with them a threat of instability, the joints have what might be called a secondary function of providing for stability without interfering with the desired motions.

All the joints of the body do not have the same degree of strength or stability. Some, such as the hip or elbow, are fairly stable. Others, such as the shoulder or knee, are less stable and therefore more easily injured. The strength or degree of freedom of joints follows Emerson’s law: “For everything that is given, something is taken.” In the shoulder, movement is gained at the expense of stability, whereas in the hip, movement is sacrificed for stability.

By joint stability we mean resistance to displacement. Some widely accepted factors in joint stability are the joint ligaments such as the lateral ligaments of hinge joints, the shape of the bony structure, muscle tension (see stabilizing components of muscular force), fascia, and atmospheric pressure. The latter is particularly effective at the hip joint. Proprioception and neuromuscular control are also important considerations in stability.

Shape of Bony Structure

Shape may refer to the kind of joint, such as hinge, condyloid, or ball-and-socket, but it is even more likely to refer to specific characteristics of the particular joint. Both the shoulder and the hip joints, for instance, are ball-and-socket joints, yet they differ markedly in their stability. The depth of the cuplike acetabulum of the hip joint, in contrast to the small size and shallowness of the glenoid fossa of the shoulder joint, is a case in point. The bony structure of the hip joint obviously gives greater protection against displacement. This difference is reflective of the weight-bearing function of the lower extremity and manipulation function of the upper extremity.

Ligamentous Arrangements

Ligaments are strong, flexible, stress-resistant, somewhat elastic, fibrous tissues that may be in the form of straplike bands or round cords. They attach the ends of the bones that form a movable joint and help maintain them in the right relationship to each other. They also check the movement when it reaches its normal limits, and they resist movements for which the joint is not constructed. For instance, the collateral ligaments of the knee help prevent any tendency there might be for this joint to abduct or adduct. Likewise, the ulnar and radial collateral ligaments of the elbow prevent abduction and adduction. The ligaments do not always succeed in preventing abnormal or excessive movements because collisions and violent motions may cause them to tear. Also, if they are subject to prolonged periods of stress (a force that deforms), they become abnormally stretched. Because they are not very elastic (i.e., they do not have the capability to return quickly to normal shape) following deformation, ligaments take a long time to recover from a stretch. If overstretched, they may never regain their normal length. Ligaments stretched and damaged in joint injuries should be given plenty of time to heal before being subjected to strenuous activity and strain (deformation). As long as the ligaments remain undamaged, they are an important factor in contributing to joint stability, but once stretched, their usefulness is permanently affected and joint stability is diminished.

Muscular Arrangement

The muscles and muscle tendons that span the joint also play a part in the stability of joints, especially in those joints whose bony structure contributes little to stability. The shoulder joint is a notable example, getting its greatest strength from the shoulder and arm muscles that cross it. (See Chapter 5 for an in-depth discussion of the stability of the shoulder joint.)

Of the muscles that act on the shoulder joint, four of them, known as the rotator cuff (subscapularis, supraspinatus, infraspinatus, and teres minor), are particularly important as stabilizers of this joint. One of their chief functions is protection of the shoulder joint and prevention of displacement of the humeral head. Figure 5.13 shows that all four of these muscles have a strong inward pull on the humeral head toward the glenoid fossa. Likewise, the knee joint depends greatly on the tendons of the quadriceps femoris and hamstring muscles for its strength. A most important defense against joint injury is an increase in the strength of the muscles that support the joint. The role of muscles as stabilizers, as opposed to that of movers or neutralizers, is included in greater detail in Chapter 3.

Fascia and Skin

Fasciae consist of fibrous connective tissue that forms sheaths for individual muscles, partitions that lie between muscles, and smaller partitions that separate bundles of muscle fibers within a single muscle. According to their location and function, they may vary in structure from thin membranes to tough, fibrous sheets. In composition they are similar to ligaments in that they are flexible and elastic, within limits, but are susceptible to permanent stretch if subjected to stress that is too intense or too prolonged. The iliotibial tract of the fascia lata and the thick skin covering the knee joint are examples of fascia and skin serving to help stabilize a joint.

Atmospheric Pressure

Atmospheric pressure plays a key role in stability of both the hip and the shoulder (glenohumeral) joints. The slight negative pressure that exists within the joint capsules forms a vacuum that holds the head of the long bone into the socket. This pressure has been found to serve a role in joint stability that may be equal to that of muscle. The absence of this vacuum, as might happen in an injury, may disrupt joint mechanics (Habermeyer et al. 1992; Wingstrand et al. 1990).

Factors Affecting the Range of Motion

The simplest way to limit motion is to put an obstacle in the path of the moving object. An obstacle limiting joint motion may be of soft or rigid tissue or both. Consequently, all joints in the same individual do not have the same amount of movement, and the same joints in different individuals do not have the same amount or range of motion (ROM). The ROM depends on several factors. Three factors that affect the stability of a joint are also related to its range of motion. These are the shape of the articular surfaces, the restraining effect of the ligaments, and the controlling action of the muscles.

Muscles and their tendons are undoubtedly the single most important factor in maintaining both the stability and degree of movement in joints. The tightness of the hamstring tendons (behind the knees) is often felt when someone attempts to touch the toes without bending the knees. Many have also discovered that continued practice will stretch these tendons and improve the joint’s range of motion appreciably. It is important to remember that flexibility should not exceed the muscles’ ability to maintain the integrity of the joints. Exercising the muscles on all sides of a joint can contribute to both flexibility and strength. The apposition of bulky tissue also affects the degree of movement in a joint. Well-developed musculature or excessive fatty tissue will restrict motion. Bulky arm muscles restrict flexion of the forearm at the elbow, and large deposits of abdominal fat limit trunk flexion. Additional factors in the range of motion include gender, body build, heredity (in addition to body build), occupation, personal exercise habits, state of physical fitness, injury, and age.

Any conditions or diseases resulting in a decrease in range of motion may limit one’s ability in activities of daily living or participation in physical activity. Reduction in range of motion may result in loss of ability to withstand normal stresses and may result in pain. Therefore, movement opportunities are avoided, resulting in further decreases in joint mobility. This negative cycle often typifies joint disease and injury. One such condition is arthritis, which can affect a person of any age. Regardless of age, maintaining range of movement without pain must be a goal in order to control for other complications. Exercises done in water have been found to be beneficial, as have exercises using elastic bands and other light resistance devices.

Methods of Assessing a Joint’s Range of Motion

The usual way to assess a joint’s range of motion is to measure the number of degrees from the starting position of the segment to its position at the end of its maximal movement. This is the way to measure flexion; extension is usually measured as the return movement from flexion. If the movement continues beyond the starting position, that constitutes hyperextension. Abduction and adduction are either measured separately from the starting position or, if desired, the total range from maximal abduction to maximal adduction is measured.

Measuring ROM can be done in various ways, depending on the joint that is measured. The instrument most commonly used is the double-armed goniometer, with one arm stationary and the other movable (Figure 2.7). The pin or axis of the movable arm is placed directly over the center of the joint at which the motion occurs. The stationary arm is held in line with the stationary segment, and the movable arm is either held against the segment as it moves or placed in line with the segment after its limit of motion has been reached. At the completion of the movement, the indicator shows the number of degrees through which the segment has moved. When the anatomical landmarks are well defined, and the examiner has identified the joint center properly, the use of the goniometer may be considered accurate, but when the bony landmarks are not well defined because of excess soft tissue coverage or other causes, or the goniometer axis is not properly placed over the joint axis, the goniometer may provide inaccurate information. For enhanced accuracy of the joint being measured, all other joints and segments should be stabilized. Digital methods of measuring joint angles are often used in laboratory settings and will be discussed more completely in Chapter 23.

A more recent method makes use of videotape. Before filming the subject, joint centers are marked so as to be visible in the projected image. Joint angles can then be obtained from the projected images. The range of motion is the difference between the joint angles of two images, one at the start of the movement and the other at its completion (Figure 2.8). When this method is used, the segment action must occur in the picture plane (i.e., at a right angle to the camera).

Average Ranges of Joint Motion

Because of the many factors that affect range of motion, including differences in measuring techniques, ranges vary and it is difficult to establish norms. Age, gender, body build, and level of activity may all be factors. Where the extremities are concerned, the individual’s opposite is perhaps the best norm. Some averages that may be used as a guide are presented in Table 2.3. Illustrations showing joint range of motion for most fundamental movements are found in Appendix B.

In preparation for defining the fundamental movements of the major segments of the body and for the analysis of these movements, certain orientation concepts and points of reference need to be established. The essential ones are the center of gravity, the line of gravity, the orientation planes of the body and axes of motion, and the standard starting positions from which the fundamental movements are made.

The Center of Gravity

The center of gravity is defined as a point representing the weight center of an object; it is also “that point in a body about which all the parts exactly balance each other” and can be viewed as “the point at which the entire weight of the body may be considered concentrated.” In a perfect sphere or cube, the weight center coincides with the geometric center. Its precise location in the human body depends on the individual’s anatomical structure, habitual standing posture, current position, and whether external weights are being supported. In a person of average build, standing erect with the arms hanging at the sides, the center of gravity is located in the pelvis in front of the upper part of the sacrum. It is usually lower in women than in men because of women’s heavier pelvises and thighs and shorter legs. (See Chapter 14 for a more detailed discussion of the center of gravity and related concepts.)

The Line of Gravity

The line of gravity is a vertical line that, by definition, passes through the center of gravity. Hence, its location depends on the position of the center of gravity, which changes with every shift of the body’s position.

Orientation Planes of the Body and Axes of Motion

Three traditional planes correspond to the three dimensions of space. Each plane is perpendicular to each of the other two. There are likewise three axes of motion, each perpendicular to the plane in which the motion occurs. The planes and axes of the body are defined as follows.

• 1. The sagittal, or median, plane is a vertical plane passing through the body from front to back, dividing it into right and left halves (Figure 2.9a)
• 2. The frontal, or coronal, plane is a vertical plane passing through the body from side to side, dividing it into anterior and posterior halves (Figure 2.9b)
• 3. The transverse, or horizontal, plane is a plane that passes horizontally through the body, dividing it into upper and lower halves (Figure 2.9c)

Because each plane bisects the body, it follows that each plane must pass through the center of gravity. Hence, the center of gravity may be defined as the point at which the three planes of the body intersect one another, and the line of gravity as the vertical line at which the two vertical planes intersect each other. When describing a movement in terms of a plane, such as “a movement of the forearm in the sagittal plane,” we mean that the movement occurs in a plane parallel to the sagittal plane. It does not necessarily imply that the movement occurs in a plane passing through the center of gravity. If the latter is intended, the term cardinal plane is used. Therefore, it may be concluded that there are only three cardinal planes but an infinite number of vertical and horizontal planes parallel to the cardinal planes. Thus, nodding the head is a movement occurring in the cardinal sagittal plane, but movement of either leg or arm forward-upward is movement occurring in the sagittal plane.

Axes

• 1. The bilateral axis passes horizontally from side to side.
• 2. The anteroposterior or AP axis passes horizontally from front to back.
• 3. The vertical (longitudinal) axis is perpendicular to the ground.

A rotary movement of a segment of the body occurs in a plane and around an axis. The axis around which the movement takes place is always at right angles to the plane in which it occurs. One may consider the axis to be an imaginary rod placed in such a position (perpendicular to each plane) so that the segment is permitted to move about this axis. Forward lifting of the leg (flexion) occurs in the sagittal plane about a bilateral axis, sideward raising of the arm (abduction) occurs in the frontal plane about an anteroposterior axis, and turning the head to the side (lateral rotation) is movement in the transverse plane about a vertical axis.

An understanding of planes and axes may guide the observer into the best position to observe or record a motion. Movement in a transverse plane is observed most easily from above or below. Movement within a sagittal plane is best observed from the side. These positions are parallel to the axis about which the movements are occurring.

Standard Starting Positions

Fundamental Standing Position

In this position the individual stands erect with the feet slightly separated and parallel, the arms hanging easily at the sides with palms facing the body (Figure 2.10a). This is the position usually accepted as the point of reference for analyzing all the movements of the body’s segments, except those of the forearm.

Anatomical Standing Position

This is the position usually depicted in anatomy textbooks. The individual is erect, with the elbows fully extended and the palms facing forward. The legs and feet are the same as for the fundamental standing position (Figure 2.10b). It is usually accepted as the point of reference for the movements of the forearm, hand, and fingers.

Being a multijointed structure, the human body consists of many movable segments. When we watch skillful acrobats, dancers, or basketball players, it might seem like a hopeless task to try to organize their movements into a single, meaningful classification. The task is greatly simplified, however, when we consider one segment at a time and visualize each movement as though it were performed from the anatomical standing position. This may take a bit of imagination, but knowing ahead of time the movements of which each joint in the body is capable is nine-tenths of the battle. These movements are described in Chapters 4, 5, 6, 7, 8, and 9 in the systematic discussions of the regions of the body. The following information is basic to an understanding of the movements of specific joints and segments.

Movements in the Sagittal Plane About a Bilateral Axis (Figure 2.11a)

Flexion

The angle at the joint diminishes. Examples include the following:

• 1. Tipping the head forward.
• 2. Lifting the foot and leg backward from the knee.
• 3. Raising the entire lower extremity forward-upward as though kicking.
• 4. With the upper arm remaining at the side, raising the forearm straight forward (Figure 2.11a).
• 5. With the elbow straight, raising the entire upper extremity forward-upward. The “diminishing angle” is hard to see in this movement until one views the raising of the arm from the shoulder in the same way that the raising of the thigh from the hip joint is viewed. In the latter, one automatically notices the angle that appears between the top of the thigh and the trunk. Similarly, when raising the arm, the angle to look for is the angle between the top of the arm and the neck–head segment, not the angle between the underside of the raised arm and the trunk. One must view the sagittal plane movements of the arm at the shoulder as being similar to those of the thigh at the hip joint.

For general purposes the upper arm may be considered fully flexed when it has reached the overhead vertical position. Later, when the role of the shoulder girdle in arm movements has been studied, it will be seen that the elevation of the arm does not occur solely at the shoulder joint. The movements of the scapula and clavicle are an important part of the total arm movement. Strictly speaking, the shoulder joint is in a fully flexed position when the humerus is raised until it is parallel with the long axis of the scapula (i.e., when it is in the same plane as the scapula). For the present, however, the upper arm will be considered fully flexed when it has been raised forward-upward until it has reached the vertical position and hyperflexed when it passes beyond this.

Extension

The return movement from flexion.

Hyperflexion

This term refers only to the movement of the upper arm. When the arm is flexed beyond the vertical, it is considered to be hyperflexed. In other joints of the body, flexion is terminated by contact of the moving segment with another part of the body—for example, the forearm against the upper arm, the lower leg against the thigh, or by structural limitations of the joints themselves (e.g., flexion of the thoracic and lumbar spine).

Hyperextension

The continuation of extension beyond the starting position or beyond the straight line. Examples include the following:

• 1. Hyperextension of the upper arm is said to occur when the arm is extended backward beyond the body.
• 2. The forearm is considered to be hyperextended when the angle at the elbow joint has exceeded 180 degrees.

Reduction of Hyperextension

Return movement from hyperextension. This movement could also be called flexion to the starting position—that is, the fundamental or the anatomical starting position, as the case may be.

Movements in the Frontal Plane About an Anteroposterior Axis (Figure 2.11b)

Viewed from the front or back.

Abduction

Sideward movement away from the midline or sagittal plane or, in the case of the fingers, away from the midline of the hand. This term is used most commonly for sideward movements of the upper arm away from the trunk—in other words, sideward elevation of the arm—and for sideward elevation of the lower extremity. The jumping-jack exercise involves both of these.

Adduction

The return movement from abduction.

Lateral Flexion

This term refers to the lateral bending of the head or trunk. It may also be used for sideward movements of the middle finger, but the more specific terms radial or ulnar flexion are usually used for these.

Hyperabduction

Like hyperflexion, this term usually refers to the upper arm when the latter is abducted beyond the vertical, as seen from the front or back.

Hyperadduction

The trunk blocks hyperadduction of the upper extremity, and the presence of the supporting lower extremity blocks hyperadduction of the other lower extremity. By combining slight flexion with hyperadduction, the upper extremities can move across the front of the body, and one lower extremity can move across in front of the supporting one.

Reduction of Hyperadduction

The return movement from hyperadduction.

Reduction of Lateral Flexion

The return movement from lateral flexion.

Movements in the Transverse Plane About a Vertical Axis (Figure 2.11c)

Viewed or visualized from overhead or from directly beneath, as through a glass platform. The point of reference for all rotations of the upper extremities is the midposition as in the fundamental (not anatomical) standing position.

Rotation Left and Right

Applies to rotation of the head, neck, or pelvis in such a way that the anterior aspect turns, respectively, to the left or right.

Outward (Lateral) and Inward (Medial) Rotation

Applies to rotation of the thigh, the upper arm, or the upper or lower extremity as a whole in such a way that the anterior aspect of the segment turns laterally or medially.

Supination and Pronation

Apply, respectively, to outward (lateral) and inward (medial) rotation of the forearm.

Reduction of Outward Rotation, Inward Rotation, Supination, or Pronation

Rotation of the segment back to the midposition.

Movements in a Combination of Planes

Circumduction

An orderly sequence of the movements that occur in the sagittal, frontal, and intermediate oblique planes so that the segment as a whole describes a cone is known as circumduction. It consists of an axial movement that may occur in any plane. The movement may occur at biaxial or triaxial joints. Arm circling and trunk circling are examples of circumduction.

Naming Joint Actions in Complex Movements

It is important to note that although all movements do not begin from the anatomical standing position, the joint actions are still named as if they were occurring in the anatomical standing position; however, the plane and axis are identified as those in which the movement actually occurs. An example is flexion and extension of the forearm at the elbow joint. Beginning from the anatomical standing position, this joint action occurs within the sagittal plane about a bilateral axis. If, however, the elbow movement is preceded by 90 degrees of flexion and inward rotation of the arm at the shoulder joint, elbow flexion and extension are described as occurring within the horizontal plane about a longitudinal axis.

Nonaxial Movements

Movements permitted at any diarthrodial joint except the arthrodial or plane joints are rotary movements and occur in a plane about an axis. Movements in plane joints are nonaxial. An example is the gliding movement that occurs between the articular facets of the spinal column. Nonaxial movements of body segments may occur when they are moved as the result of rotary action of adjacent segments. In a pushing action, the hand moves linearly forward in the sagittal plane because of the axial movements of the arm segments at the shoulder, elbow, and wrist joints. Similarly, in a deep knee bend, the trunk and head move linearly because of the rotary actions of the leg segments.

The first four columns of the anatomical analysis model presented in Table 1.2 deal primarily with the orientation of the body segments and the patterns of joint motion. For each phase of the technique and for each joint participating in the phase, the precise joint action and segment being moved should be identified and recorded. The plane of motion for each joint should be stipulated, along with the axis of joint motion. If it seems desirable to measure the range of motion in a given joint, this can be done from sequential motion pictures.

Once the information used in the anatomical model has been collected and examined, the analyst can begin to evaluate the performance in terms of joint motions. This evaluation should take into account some basic anatomical principles. Returning to the long jump example, several joint action principles apply.

Alignment

The proper alignment of segments may differ from movement to movement. The optimum segmental alignment for any skill should be determined based on efficiency, effectiveness, and safety. In the standing long jump example, the joints of the lower extremity need to be fully extended at takeoff to impart maximum jumping force. Because this is a jump for horizontal distance, the joints should be aligned in such a way as to ensure that the center of gravity is in front of the feet at the moment of takeoff.

Range of Motion

Each joint in the body has a limit to its range of motion. The range of motion limitations of the body and the range of motion demands of the activity must be compatible to avoid injury while still producing an effective performance. A limited range of motion in the standing long jump will shorten the amount of extension possible and thereby decrease the length of time over which the push-off force will be applied.

Flexibility

Flexibility reduces the internal resistance to motion. The maintenance of normal flexibility is an important consideration for all. In the jump example, flexibility in the joints of the lower extremity will allow for optimum flexion in the lower extremities while reducing the risk of injury.

• 1. By studying a skeleton and observing a living subject, classify the following joints without referring to the textbook: hip, elbow, knee, ankle, wrist, radioulnar, metacarpophalangeal joint of finger, shoulder joint. (Do not confuse the motion at the elbow or wrist joints with that of the radioulnar joints.)
• 2. Take turns with a partner performing simple movements of the head, trunk, upper extremity, and lower extremity, and identify the planes and axes concerned.
• 3. Construct a simple device to illustrate the planes and axes in relation to the human body.
• 4. Study the soccer throw-in (Exercise 5.4 in Appendix G). The action between (a) and (b) represents the “force phase,” and between (b) and (c) the “follow-through phase.” Following the example given for the right metatarsophalangeal joint, complete the following table for the force phase. Repeat for the follow-through.
• 5. Measure the joint ranges of motion of 10 to 15 subjects for the movements indicated, using either a goniometer or a flexometer. Compare the results obtained with the averages given in Table 2.3. Explain why you think the differences among individuals exist. (The figure numbers refer to Appendix B.)
  • a. Elbow flexion and hyperextension (Figure B.1)
  • b. Shoulder flexion and hyperextension (Figure B.4b)
  • c. Shoulder rotation (Figure B.5)
  • d. Lateral flexion of trunk (Figure B.11b)
  • e. Hip flexion (Figure B.7)
  • f. Medial and lateral hip rotation (Figure B.8b)
  • g. Plantar flexion and dorsiflexion (Figure B.10)
• 6. Select one or two joint actions (e.g., shoulder flexion and hip flexion). Perform a comparison study of the range of motion for the actions selected using two groups of people, such as the following.
  • a. Males versus females
  • b. Varsity athletes from different sports (swimming vs. basketball or wrestling vs. gymnastics)
  • c. Different age groups (20–30 vs. 50–60)
  • d. Joggers versus sedentary people
  • Subjects in each group should be matched on the basis of age and gender. Are any “group” differences apparent? Explain your results.
• 7. Measure the range of motion in one ankle. Tape the ankle and repeat the measurements immediately and then following 10 to 15 minutes of exercise during which the ankle is used (e.g., jogging, basketball, handball). Record your results and compare the differences. What effect, if any, did the tape have on range of motion for plantar and dorsiflexion? Inversion and eversion? Using the results of your measurements as the basis, discuss the value of taping as a means of preventing ankle injuries.
• 8. Measure the ranges of motion at the hip and knee joints during the back somersault depicted in Exercise 5.6 in Appendix G.
  • a. On each tracing, mark the location of the right hip, knee, and ankle joints with a dot. Also place a dot at the midpoint of the waist. Connect the dots with straight lines so as to make a stick figure of the leg.
  • b. Using a protractor, measure and record the angles between the trunk and thigh, and the thigh and lower leg, on each tracing.
  • c. Determine the range of motion for each joint action from start to end of takeoff [(a) to (c)] and from peak to touchdown [(b) to (e)].
  • d. What effect does range of motion in these joints have on skillful performance of this technique? As a coach or teacher, what specific training techniques would you include in the program of the performer to improve the range of motion where needed?