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.
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
- 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
- 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
- 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.)
Frontal section of a diarthrodial joint. From W. H. Hollinshead
and D. B. Jenkins, Functional Anatomy of
the Limbs and Back, 5th ed. Copyright © 1981 W. B.
Saunders, Orlando, FL. Reprinted by permission.
Frontal section of a diarthrodial joint having fibrocartilage.
- 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
- 2. The third type (ligamentous) is not a true joint but is
a ligamentous connection between two bones, which may or may not
- 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.
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.
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
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 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.
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
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.
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
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).
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).
The range of motion at a joint can be determined using
images of the joint action. The angles at the knee and ankle joints
were measured at the point of the greatest flexion (a) and at the
peak of the jump. (b) The difference between the two measures (b
minus a) is the range of motion (ROM) for the given joint. The ROM
at the knee joint was 88 degrees and at the ankle joint was 64 degrees.
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.
Table 2.3 Average Ranges
of Joint Motion ||Download (.pdf)
Table 2.3 Average Ranges
of Joint Motion
|Rotation (arm in abduction)|
|Rotation (in extension)|
|Spine (thoracic and lumbar)|