Human Skeletal Articulations

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C H A P T E R

5The Biomechanics of Human Skeletal Articulations After completing this chapter, you will be able to:

Categorize joints based on structure and movement capabilities.

Explain the functions of articular cartilage and fi brocartilage.

Describe the material properties of articular connective tissues.

Explain advantages and disadvantages of different approaches to increasing or maintaining joint fl exibility.

Describe the biomechanical contributions to common joint injuries and pathologies.

117

O N L I N E L E A R N I N G C E N T E R R E S O U R C E S

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118 BASIC BIOMECHANICS

T he joints of the human body largely govern the directional motion capabilities of body segments. The anatomical structure of a given joint, such as the uninjured knee, varies little from person to person; as do the directions in which the attached body segments, such as the thigh and lower leg, are permitted to move at the joint. However, dif- ferences in the relative tightness or laxity of the surrounding soft tis- sues result in differences in joint ranges of movement. This chapter discusses the biomechanical aspects of joint function, including the con- cepts of joint stability and joint fl exibility, and related implications for injury potential.

JOINT ARCHITECTURE

Anatomists have categorized joints in several ways, based on joint com- plexity, the number of axes present, joint geometry, or movement capa bili- ties (61). Since this book focuses on human movement, a joint classifi cation system based on motion capabilities is presented.

Immovable Joints

1. Synarthroses (immovable) (syn 5 together; arthron 5 joint): These fi - brous joints can attenuate force (absorb shock) but permit little or no movement of the articulating bones.

a. Sutures: In these joints, the irregularly grooved articulating bone sheets mate closely and are tightly connected by fi bers that are continuous with the periosteum. The fi bers begin to ossify in early adulthood and are eventually replaced completely by bone. The only example in the human body is the sutures of the skull.

b. Syndesmoses (syndesmosis 5 held by bands): In these joints, dense fi brous tissue binds the bones together, permitting ex- tremely limited movement. Examples include the coracoacromial, mid-radioulnar, mid-tibiofi bular, and inferior tibiofi bular joints.

Slightly Movable Joints

2. Amphiarthroses (slightly movable) (amphi 5 on both sides): These car- tilaginous joints attenuate applied forces and permit more motion of the adjacent bones than synarthrodial joints.

a. Synchondroses (synchondrosis 5 held by cartilage): In these joints, the articulating bones are held together by a thin layer of hyaline cartilage. Examples include the sternocostal joints and the epiphy- seal plates (before ossifi cation).

b. Symphyses: In these joints, thin plates of hyaline cartilage separate a disc of fi brocartilage from the bones. Examples include the verte- bral joints and the pubic symphysis.

Parietal bone

Suture

Occipital bone

Sutures between the occipital and parietal bones of the skull represent synarthroses (immovable joints). From Shier, Butler, and Lewis. Hole’s Human Anatomy and Physiology, © 1996. Reprinted by permission of The McGraw- Hill Companies, Inc.

Note the hyaline cartilage disc separating the bones of the pubic symphysis, typical of a symphysis joint. Courtesy of McGraw-Hill Companies, Inc.

The sternocostal joints are examples of synchondroses, wherein the articulating bones are joined by a thin layer of hyaline cartilage. Courtesy of McGraw-Hill Companies, Inc.

The mid-radioulnar joint is an example of a syndesmosis, where fi brous tissue binds the bones together. Courtesy of McGraw-Hill Companies, Inc.

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 119

Freely Movable Joints

3. Diarthroses or synovial (freely movable) (diarthrosis 5 “through joint,” indicating only slight limitations to movement capability): At these joints, the articulating bone surfaces are covered with articular cartilage, an articular capsule surrounds the joint, and a synovial membrane lining the interior of the joint capsule secretes a lubricant known as synovial fl uid (Figure 5-1). There are many types of syno- vial joints.

a. Gliding (plane; arthrodial): In these joints, the articulating bone surfaces are nearly fl at, and the only movement permitted is nonax- ial gliding. Examples include the intermetatarsal, intercarpal, and intertarsal joints, and the facet joints of the vertebrae.

b. Hinge (ginglymus): One articulating bone surface is convex and the other is concave in these joints. Strong collateral ligaments restrict movement to a planar, hingelike motion. Examples include the ul- nohumeral and interphalangeal joints.

c. Pivot (screw; trochoid): In these joints, rotation is permitted around one axis. Examples include the atlantoaxial joint and the proximal and distal radioulnar joints.

d. Condyloid (ovoid; ellipsoidal): One articulating bone surface is an ovular convex shape, and the other is a reciprocally shaped concave surface in these joints. Flexion, extension, abduction, adduction, and circumduction are permitted. Examples include the second through fi fth metacarpophalangeal joints and the radiocarpal joints.

e. Saddle (sellar): The articulating bone surfaces are both shaped like the seat of a riding saddle in these joints. Movement capability is the same as that of the condyloid joint, but greater range of move- ment is allowed. An example is the carpometacarpal joint of the thumb.

f. Ball and socket (spheroidal): In these joints, the surfaces of the ar- ticulating bones are reciprocally convex and concave. Rotation in all three planes of movement is permitted. Examples include the hip and shoulder joints.

Articular capsule

Synovial cavity

Femur Synovial membrane

Suprapatellar bursa

Patella

Prepatellar bursa

Subpatellar fat

Articular cartilages

Menisci

Infrapatellar bursa

Subchondrial plate

Tibia

FIGURE 5-1

The knee is an example of a synovial joint, with a ligamentous capsule, an articular cavity, and articular cartilage. From Shier, Butler, and Lewis, Hole’s Human Anatomy and Physiology, © 1996. Reprinted by permission of The McGraw-Hill Companies, Inc.

articular cartilage protective layer of dense white connective tissue covering the articulating bone surfaces at diarthrodial joints

articular capsule double-layered membrane that surrounds every synovial joint

synovial fl uid clear, slightly yellow liquid that provides lubrication inside the articular capsule at synovial joints

120 BASIC BIOMECHANICS

Synovial joints vary widely in structure and movement capabilities, as shown in Figure 5-2. They are commonly categorized according to the number of axes of rotation present. Joints that allow motion around one, two, and three axes of rotation are referred to respectively as uniaxial, biaxial, and triaxial joints. A few joints where only limited motion is per- mitted in any direction are termed nonaxial joints. Joint motion capabili- ties are also sometimes described in terms of degrees of freedom (df ), or the number of planes in which the joint allows motion. A uniaxial joint has one df, a biaxial joint has two df, and a triaxial joint has three df.

Two synovial structures often associated with diarthrodial joints are bursae and tendon sheaths. Bursae are small capsules, lined with syno- vial membranes and fi lled with synovial fl uid, that cushion the structures they separate. Most bursae separate tendons from bone, reducing the fric- tion on the tendons during joint motion. Some bursae, such as the olec- ranon bursa of the elbow, separate bone from skin. Tendon sheaths are double-layered synovial structures that surround tendons positioned in close association with bones. Many of the long muscle tendons crossing the wrist and fi nger joints are protected by tendon sheaths.

Hipbone

Metacarpal

Phalanx

A Ball-and-socket joint

C Gliding joint

E Pivot joint

D Hinge joint

B Condyloid joint

Head of femur in acetabulum

Femur

Carpals

F Saddle joint

First metacarpal

Trapezium

Humerus

Dens Transverse ligament

Ulna

Radius

Atlas Axis

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 121

Articular Cartilage

The joints of a mechanical device must be properly lubricated if the mov- able parts of the machine are to move freely and not wear against each other. In the human body, a special type of dense, white connective tis- sue known as articular cartilage provides a protective lubrication. A 1- to 5-mm-thick protective layer of this material coats the ends of bones ar- ticulating at diarthrodial joints. Articular cartilage serves two important purposes: (a) It spreads loads at the joint over a wide area so that the amount of stress at any contact point between the bones is reduced, and (b) it allows movement of the articulating bones at the joint with minimal friction and wear (4).

Articular cartilage is a soft, porous, and permeable tissue that is hy- drated. It consists of specialized cells called chondrocytes embedded in a matrix of collagen fi bers, proteoglycans, and noncollagenous proteins. The matrix protects the chondrocytes and also signals changes in local pressure to the chondrocytes (6). The chondrocytes maintain and restore cartilage from wear, although this ability diminishes with aging, disease, and injury (68). Chondrocyte densities and matrix structures have been found to vary across joints, as well as within a given joint, depending on the mechanical loading sustained (51).

Under loading at the joint, articular cartilage deforms, exuding syno- vial fl uid. At healthy synovial joints, where the articulating bone ends are covered with articular cartilage, motion of one bone end over the other is typically accompanied by a fl ow of synovial fl uid that is pressed out ahead of the moving contact area and also sucked in behind the contact area (44). At the same time, the permeability of the cartilage is reduced in the area of direct contact, providing a surface on which fl uid fi lm can form under the load (44). When joint loading occurs at a low rate, the solid components of the cartilage matrix resist the load. When loading is faster, however, it is the fl uid within the matrix that primarily sustains the pressure (35, 49).

Cartilage can reduce the maximum contact stress acting at a joint by 50% or more (71). The lubrication supplied by the articular cartilage is so effective that the friction present at a joint is only approximately 17–33% of the friction of a skate on ice under the same load, and only one-half that of a lubricated bearing (5, 47).

During normal growth, articular cartilage at a joint such as the knee increases in volume as the child’s height increases (28). Interestingly, there is no relationship between cartilage accrual at the knee and weight change. Children participating in vigorous sport activities accumulate knee cartilage faster than those who do not, and males tend to gain knee cartilage faster than do females (28).

Unfortunately, once damaged, articular cartilage has little to no ability to heal or regenerate on its own (55). Instead, injuries to this tissue tend to progress, with more and more of the protective coating of the articulating bone ends worn away, resulting in degenerative arthritis. A promising ap- proach for repairing damage to articular cartilage is autologous cartilage regeneration, a procedure through which healthy chondrocytes (cartilage cells) are arthroscopically removed from the patient’s joint and then cul- tured in a laboratory using principles of tissue engineering (8). After a few weeks, the cells have grown into articular cartilage plugs that can be arthroscopically inserted into the damaged area of cartilage. A review of 20 studies revealed that among athletes treated for joint damage with this procedure, 73% suffi ciently recovered joint function to return to sports participation (42). Factors infl uencing an athlete’s ability to participate in

122 BASIC BIOMECHANICS

competitive sports include the athlete’s age, duration of injury, level of play, extent of cartilage damage, and repair tissue morphology (42). Research is under way to investigate the potential for a variety of new approaches for treating degenerated cartilage, including the use of mesenchymal stem cells, tissue engineering, and gene transfer technology (53, 66, 69).

Articular Fibrocartilage

At some joints, articular fi brocartilage, in the form of either a fi brocarti- laginous disc or partial discs known as menisci, is also present between the articulating bones. The intervertebral discs (Figure 5-3) and the me- nisci of the knee (Figure 5-4) are examples. Although the function of discs and menisci is not clear, possible roles include the following:

1. Distribution of loads over the joint surfaces 2. Improvement of the fi t of the articulating surfaces 3. Limitation of translation or slip of one bone with respect to another 4. Protection of the periphery of the articulation 5. Lubrication 6. Shock absorption

Fibrocartilaginous disk of symphysis pubis

Pubic bone

Gelatinous core

Band of fibrocartilage

Spinous process

Body of vertebra

Intervertebral disks

Lateral meniscus

Tubercle of tibia

Posterior cruciate ligament

Popliteus tendon Medial meniscus

Anterior cruciate ligament

FIGURE 5-3 Fibrocartilage is present in (A) the symphysis pubis that separates the pubic bones and (B) the intervertebral discs between adjacent vertebrae. From Shier, Butler, and Lewis, Hole’s Human Anatomy and Physiology, © 1996. Reprinted by permission of The McGraw-Hill Companies, Inc.

FIGURE 5-4

The menisci at the knee joint help to distribute loads, lessening the stress transmitted across the joint.

articular fi brocartilage soft-tissue discs or menisci that intervene between articulating bones

•Intervertebral discs act as cushions between the vertebrae, reducing stress levels by spreading loads.

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 123

Articular Connective Tissue

Tendons, which connect muscles to bones, and ligaments, which connect bones to other bones, are passive tissues composed primarily of collagen and elastic fi bers. Tendons and ligaments do not have the ability to con- tract like muscle tissue, but they are slightly extensible. These tissues are elastic and will return to their original length after being stretched, unless they are stretched beyond their elastic limits (see Chapter 3). A tendon or ligament stretched beyond its elastic limit during an injury remains stretched and can be restored to its original length only through surgery. The results of modeling studies suggest that tendons routinely undergo healing to repair internal microfailures over the course of the life span in order to remain intact (37).

Tendons and ligaments, like bone, respond to altered habitual mechan- ical stress by hypertrophying or atrophying. Research has shown that regular exercise over time results in increased size and strength of both tendons (57) and ligaments (36), as well as increased strength of the junc- tions between tendons or ligaments and bone (65).

Evidence also suggests that the size of a ligament such as the anterior cruciate ligament (ACL) is proportionate to the strength of its antago- nists (in this case, the quadriceps muscles) (1). Tendons and ligaments can not only heal following rupturing but in some cases regenerate in their entirety, as evidenced by examples of complete regeneration of the semitendinosus tendon following its surgical removal for repair of ante- rior cruciate ligament ruptures (14, 16, 48).

JOINT STABILITY

The stability of an articulation is its ability to resist dislocation. Specifi - cally, it is the ability to resist the displacement of one bone end with respect to another while preventing injury to the ligaments, muscles, and muscle tendons surrounding the joint. Different factors infl uence joint stability.

Shape of the Articulating Bone Surfaces

In many mechanical joints, the articulating parts are exact opposites in shape so that they fi t tightly together (Figure 5-5). In the human body, the articulating ends of bones are usually shaped as mating convex and concave surfaces.

Although most joints have reciprocally shaped articulating surfaces, these surfaces are not symmetrical, and there is typically one position of best fi t in which the area of contact is maximum. This is known as the close-packed position, and it is in this position that joint stability is usually greatest. Any movement of the bones at the joint away from the

•A material stretched beyond its elastic limit remains lengthened beyond its original length after tension is released.

Ball and socket Saddle joint Hinge

FIGURE 5-5

Mechanical joints are often composed of reciprocally shaped parts.

joint stability ability of a joint to resist abnormal displacement of the articulating bones

•The articulating bone surfaces at all joints are of approximately matching (reciprocal) shapes.

close-packed position joint orientation for which the contact between the articulating bone surfaces is maximum

124 BASIC BIOMECHANICS

close-packed position results in a loose-packed position, with reduction of the area of contact.

Some articulating surfaces are shaped so that in both close- and loose- packed positions, there is either a large or a small amount of contact area and consequently more or less stability. For example, the acetabulum provides a relatively deep socket for the head of the femur, and there is always a relatively large amount of contact area between the two bones, which is one reason the hip is a stable joint. At the shoulder, however, the small glenoid fossa has a vertical diameter that is approximately 75% of the vertical diameter of the humeral head and a horizontal diameter that is 60% of the size of the humeral head (46). Therefore, the area of contact between these two bones is relatively small, contributing to the relative instability of the shoulder complex. Slight anatomical variations in shapes and sizes of the articulating bone surfaces at any given joint among individuals are found; therefore, some people have joints that are more or less stable than average.

Arrangement of Ligaments and Muscles

Ligaments, muscles, and muscle tendons affect the relative stability of joints. At joints such as the knee and the shoulder, in which the bone con- fi guration is not particularly stable, the tension in ligaments and muscles contributes signifi cantly to joint stability by helping to hold the articulat- ing bone ends together. If these tissues are weak from disuse or lax from being overstretched, the stability of the joint is reduced. Strong ligaments and muscles often increase joint stability. For example, strengthening of the quadriceps and hamstring groups enhances the stability of the knee (52). The complex array of ligaments and tendons crossing the knee is il- lustrated in Figure 5-6.

loose-packed position any joint orientation other than the close-packed position

Femur

Posterior cruciate ligament

Medial condyle

Anterior cruciate ligament

Medial meniscus

Medial condyle

Tibial collateral ligament

Patellar ligament (cut)

Lateral condyle

Fibular collateral ligament

Fibula

Tibia

Lateral meniscus Oblique

popliteal ligament

Fibular collateral ligament

Arcuate popliteal ligament

Fibula

Femur

Joint capsule

Tibial collateral ligament

Tibia

FIGURE 5-6 At the knee joint, stability is derived primarily from the tension in the ligaments and muscles that cross the joint. From Shier, Butler, and Lewis, Hole’s Human Anatomy and Physiology, © 1996. Reprinted by permission of The McGraw-Hill Companies, Inc.

•Stretching or rupturing of the ligaments at a joint can result in abnormal motion of the articulating bone ends, with subsequent damage to the articular cartilage.

•The close-packed position occurs for the knee, wrist, and interphalangeal joints at full extension and for the ankle at full dorsifl exion (30).

•One factor enhancing the stability of the glenohumeral joint is a posteriorly tilted glenoid fossa and humeral head. Individuals with anteriorly tilted glenoids and humeral heads are predisposed to shoulder dislocation.

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 125

The angle of attachment of most tendons to bones is arranged so that when the muscle exerts tension, the articulating ends of the bones at the joint crossed are pulled closer together, enhancing joint stability. This situation is usually found when the muscles on opposite sides of a joint produce tension simultaneously. When muscles are fatigued, however, they are less able to contribute to joint stability, and injuries are more likely to occur (13). Rupture of the cruciate ligaments is most likely when the tension in fatigued muscles surrounding the knee is inadequate to protect the cruciate ligaments from being stretched beyond their elastic limits (52).

Other Connective Tissues

White fi brous connective tissue known as fascia surrounds muscles and the bundles of muscle fi bers within muscles, providing protection and support. A particularly strong, prominent tract of fascia known as the iliotibial band crosses the lateral aspect of the knee, contributing to its stability (Figure 5-7). The fascia and the skin on the exterior of the body are other tissues that contribute to joint integrity.

JOINT FLEXIBILITY

Joint fl exibility is a term used to describe the range of motion (ROM) al- lowed in each of the planes of motion at a joint. Static fl exibility refers to the ROM present when a body segment is passively moved (by an exercise

•Engaging in athletic participation with fatigued muscles increases the likelihood of injury.

Patella

Anterior view

Iliotibial tract of fascia lata

FIGURE 5-7

The iliotibial band is a strong, thickened region of the fascia lata that crosses the knee, contributing to the knee’s stability.

joint fl exibility a term representing the relative ranges of motion allowed at a joint

range of motion angle through which a joint moves from anatomical position to the extreme limit of segment motion in a particular direction

126 BASIC BIOMECHANICS

partner or clinician), whereas dynamic fl exibility refers to the ROM that can be achieved by actively moving a body segment by virtue of muscle contraction. Static fl exibility is considered to be the better indicator of the relative tightness or laxity of a joint in terms of implications for injury potential. Dynamic fl exibility, however, must be suffi cient not to restrict the ROM needed for daily living, work, or sport activities. Research in- dicates that these two components of fl exibility are independent of each other (26).

Although people’s general fl exibility is often compared, fl exibility is ac- tually joint-specifi c. That is, an extreme amount of fl exibility at one joint does not guarantee the same degree of fl exibility at all joints.

Measuring Joint Range of Motion

Joint ROM is measured directionally in units of degrees. In anatomi- cal position, all joints are considered to be at zero degrees. The ROM for fl exion at the hip is therefore considered to be the size of the angle through which the extended leg moves from zero degrees to the point of maximum fl exion (Figure 5-8). The ROM for extension (return to ana- tomical position) is the same as that for fl exion, with movement past anatomical position in the other direction quantifi ed as the ROM for hyperextension. A goniometer used for measuring joint ROM is shown in Figure 5-9.

Factors Influencing Joint Flexibility

Different factors infl uence joint fl exibility. The shapes of the articulat- ing bone surfaces and intervening muscle or fatty tissue may termi- nate movement at the extreme of a ROM. When the elbow is in extreme hyperextension, for example, contact of the olecranon of the ulna with

FIGURE 5-8

The range of motion for fl exion at the hip is typically measured with the individual supine.

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 127

the olecranon fossa of the humerus restricts further motion in that di- rection. Muscle and/or fat on the anterior aspect of the arm may ter- minate elbow fl exion. Regular participants in bilaterally asymmetrical sports such as tennis are likely to have less range of motion for the dominant arm than for the nondominant arm at the glenohumeral joint of the shoulder (15).

For most individuals, joint fl exibility is primarily a function of the rel- ative laxity and/or extensibility of the collagenous tissues and muscles crossing the joint. Tight ligaments and muscles with limited extensibil- ity are the most common inhibitors of a joint’s ROM (41). In one study researchers showed that a four-week stretching protocol resulted in in- creased joint fl exibility but with no change in muscle compliance, or ex- tensibility, suggesting that it was the ligaments and tendons that became easier to stretch (75).

Laboratory studies have shown that the extensibility of collagenous tissues increases slightly with temperature elevation (54). Although this fi nding suggests that “warm-up” exercises should increase joint ROM, this has not been well documented in humans. In a study comparing the effects of static stretching on ankle range of motion, as compared to static stretching preceded by exercise warm-up, superfi cial heat application, or ultrasound, all protocols produced similar effects (33). More research is needed to identify the specifi c mechanism responsible for the effects of warm-up on joint ROM.

Flexibility and Injury

Research has shown that the risk of injury is heightened when joint fl exibility is extremely low, extremely high, or signifi cantly imbalanced between dominant and nondominant sides of the body (32). Severely limited joint fl exibility is undesirable because, if the collagenous tissues and muscles crossing the joint are tight, the likelihood of their tearing or rupturing if the joint is forced beyond its normal ROM increases. Tight ligaments and muscles were found to be related to lower-extremity injury incidence among male, but not female, college athletes, possibly because the female athletes studied were more fl exible and less tight at the lower- extremity joints (34). In a study of competitive female gymnasts, those in a highly injury-prone category had less fl exibility of the shoulder, elbow, wrist, hip, and knee joints than those in a low injury incidence category (62). Alternatively, an extremely loose, lax joint is lacking in stability and, therefore, prone to displacement-related injuries. Among U.S. Army in- fantry recruits assessed for hip/low back fl exibility with the sit-and-reach test, both the least fl exible and the most fl exible were over two times as likely to get injured as soldiers in the middle of the fl exibility range.

FIGURE 5-9

A goniometer is basically a protractor with two arms. The point where the arms intersect is aligned over the joint center while the arms are aligned with the longitudinal axes of the body segments, to measure the angle present at a joint.

•A joint with an unusually large range of motion is termed hypermobile.

128 BASIC BIOMECHANICS

Soldiers who participated in a stretching program for the hamstrings, however, sustained 12.4% fewer lower extremity overuse injuries than those who did not participate (21). Female college athletes with a hip ex- tension fl exibility imbalance of 15% or more were 2.6 times more likely to suffer lower-extremity injuries (31).

The desirable amount of joint fl exibility is largely dependent on the activities in which an individual wishes to engage. Gymnasts and danc- ers obviously require greater joint fl exibility than do nonathletes. How- ever, these athletes also require strong muscles, tendons, and ligaments to perform well and avoid injury. Although large, bulky muscles may in- hibit joint ROM, an extremely strong, stable joint can also enable large ROMs.

Athletes and recreational runners commonly stretch before engag- ing in activity for purposes of reducing the likelihood of injury. There is some evidence that preparticipation stretching reduces the incidence of muscle strains, and recent research shows that increased joint fl exibil- ity translates to a lower incidence of eccentric exercise-induced muscle damage (7, 40). Stretching has no effect, however, on overuse-type injuries (40).

Although people usually become less fl exible as they age, this phenom- enon appears to be primarily related to decreased levels of physical ac- tivity rather than to changes inherent in the aging process. No changes in fl exibility have been found to be associated with growth during ad- olescence (17). Regardless of the age of the individual, however, if the collagenous tissues crossing a joint are not stretched, they will shorten. Conversely, when these tissues are regularly stretched, they lengthen and fl exibility is increased. Among women, signifi cant, positive relationships have been found between weekly hours of participation in a sport and knee ROM, with active knee extension ROM increasing among swim- mers and competitive gymnasts, and active knee fl exion ROM increasing among basketball players (20). The results of several studies indicate that fl exibility can be signifi cantly increased among elderly individuals who participate in a program of regular stretching and exercise (19, 45).

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 129

TECHNIQUES FOR INCREASING JOINT FLEXIBILITY

Increasing joint fl exibility is often an important component of thera- peutic and rehabilitative programs and programs designed to train athletes for a particular sport. Increasing or maintaining fl exibility in- volves stretching the tissues that limit the ROM at a joint. Several ap- proaches for stretching these tissues can be used, with some being more effective than others because of differential neuromuscular responses elicited.

Neuromuscular Response to Stretch

Sensory receptors known as Golgi tendon organs (GTOs) are located in the muscle–tendon junctions and in the tendons at both ends of muscles (Figure 5-10). Approximately 10–15 muscle fi bers are connected in di- rect line, or in series, with each GTO. These receptors are stimulated by tension in the muscle–tendon unit. Although both tension produced by muscle contraction and tension produced by passive muscle stretch can stimulate GTOs, the threshold for stimulation by passive stretch is much higher. Whereas the muscle force arising from passive stretch must reach approximately 2 N, the activation of a single muscle fi ber with a force production of 30–90 �N is suffi cient to stimulate a GTO (3). The GTOs respond through their neural connections by inhibiting tension develop- ment in the activated muscle (promoting muscle relaxation) and by initi- ating tension development in the antagonist muscles.

Other sensory receptors are interspersed throughout the fi bers of muscles. These receptors, which are oriented parallel to the fi bers, are known as muscle spindles because of their shape (Figure 5-11). Each muscle spindle is composed of approximately 3–10 small muscle fi bers, termed intrafusal fi bers, that are encased in a sheath of connective tissue.

Muscle spindles respond to both the amount of muscle lengthening (static response) and the rate of muscle lengthening (dynamic response). Intrafusal fi bers known as nuclear chain fi bers are primarily responsible for the static component, and intrafusal fi bers known as nuclear bag fi - bers are responsible for the dynamic component. These two types of in- trafusal fi ber have been shown to function independently, but because the dynamic response is much stronger than the static response, a slow

Golgi tendon organ sensory receptor that inhibits tension development in a muscle and initiates tension development in antagonist muscles

FIGURE 5-10

Golgi tendon organ

Tendon

Sensory nerve fiber

Skeletal muscle fiber

muscle spindle sensory receptor that provokes refl ex contraction in a stretched muscle and inhibits tension development in antagonist muscles

130 BASIC BIOMECHANICS

rate of stretching does not activate the muscle spindle response until the muscle is signifi cantly stretched (9). Some muscles receive greater muscle spindle response than others. For example, the soleus receives more mus- cle spindle feedback than the gastrocnemius during both rest and muscle activation (67).

The spindle response includes activation of the stretch refl ex and inhi- bition of tension development in the antagonist muscle group, a process known as reciprocal inhibition. The stretch refl ex, also known as the myo- tatic refl ex, is provoked by the activation of the spindles in a stretched muscle. This rapid response involves neural transmission across a single synapse, with afferent nerves carrying stimuli from the spindles to the spinal cord and efferent nerves, returning an excitatory signal directly from the spinal cord to the muscle, resulting in tension development in the muscle. The knee-jerk test, a common neurological test of motor func- tion, is an example of muscle spindle function producing a quick, brief contraction in a stretched muscle. A tap on the patellar tendon initiates the stretch refl ex, resulting in the jerk caused by the immediate develop- ment of tension in the quadriceps group (Figure 5-12).

Sensory nerve fiber

Nerve endings

Skeletal muscle fiber

Muscle spindle

Connective tissue sheath

FIGURE 5-11

stretch refl ex monosynaptic refl ex initiated by stretching of muscle spindles and resulting in immediate development of muscle tension

reciprocal inhibition inhibition of tension development in the antagonist muscles resulting from activation of muscle spindles

Cell body of motor neuron

Effector—quadriceps femoris muscle group

Axon of sensory neuron

Cell body of sensory neuron

Dendrite of sensory neuron

Receptor—muscle spindle

Patella

Patellar ligament

Direction of impulse

Axion of motor neuron

Dendrite of motor neuron

Spinal cord

FIGURE 5-12

The myotatic (stretch) refl ex is initiated by stretching of the muscle spindles. From Shier, Butler, and Lewis, Hole’s Human Anatomy and Physiology, © 1996. Reprinted by permission of The McGraw-Hill Companies, Inc.

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 131

Because muscle spindle activation produces tension development in stretching muscle, whereas GTO activation promotes relaxation of mus- cle developing tension, the general goals of any procedure for stretching muscle are minimizing the spindle effect and maximizing the GTO ef- fect. A summary comparison of GTOs and muscle spindles is presented in Table 5-1.

Active and Passive Stretching

Stretching can be done either actively or passively. Active stretching is pro- duced by contraction of the antagonist muscles (those on the side of the joint opposite the muscles, tendons, and ligaments to be stretched). Thus, to actively stretch the hamstrings (the primary knee fl exors), the quadriceps (primary knee extensors) should be contracted. Passive stretching involves the use of gravitational force, force applied by another body segment, or force applied by another person, to move a body segment to the end of the ROM. Active stretching provides the advantage of exercising the muscle groups used to develop force. With passive stretching, movement can be carried farther beyond the existing ROM than with active stretching, but with the concomitant disadvantage of increased injury potential.

Ballistic and Static Stretching

Ballistic stretching, or performance of bouncing stretches, makes use of the momentum of body segments to repeatedly extend joint position to or beyond the extremes of the ROM. Because a ballistic stretch activates the stretch refl ex and results in the immediate development of tension in the muscle being stretched, microtearing of the stretched muscle tissue may occur. Because the extent of the stretch is not controlled, the potential for injury to all of the stretched tissues is heightened.

With static stretching, the movement is slow, and when the desired joint position is reached, it is maintained statically, usually for about 30–60 seconds (s). There seems to be general agreement that for optimal effect, the static stretch of each muscle group should be sequentially re- peated three to fi ve times (46). Other research demonstrates that it is the total stretch time during each day, rather than the stretching protocol, that determines the effect on tissue extensibility (10).

Although static stretching has been shown to be effective for in- creasing joint fl exibility, there is also overwhelming evidence that a single, 30-s bout of static stretching has a noticeably detrimental ef- fect on muscle strength, with additional stretching further decreasing

active stretching stretching of muscles, tendons, and ligaments produced by active development of tension in the antagonist muscles

passive stretching stretching of muscles, tendons, and ligaments produced by a stretching force other than tension in the antagonist muscles

CHARACTERISTIC GOLGI TENDON ORGANS MUSCLE SPINDLES

Location Within tendons near the muscle– tendon junction in series with

muscle fi bers

Interspersed among muscle fi bers in parallel with the

fi bers

Stimulus Increase in muscle tension Increase in muscle length

Response 1. Inhibits tension development in stretched muscle,

2. Initiates tension development in antagonist muscles

1. Initiates rapid contraction of stretched muscle,

2. Inhibits tension development in antagonist muscles

Overall effect Promotes relaxation in muscle developing tension

Inhibits stretch in muscle being stretched

TABLE 5-1

Golgi Tendon Organs (GTOs) and Muscle Spindles: How Do They Compare?

ballistic stretching a series of quick, bouncing-type stretches

•Ballistic, bouncing types of stretches can be dangerous because they tend to promote contraction of the muscles being stretched, and the momentum generated may carry the body segments far enough beyond the normal ROM to tear or rupture collagenous tissues.

static stretching maintaining a slow, controlled, sustained stretch over time, usually about 30 seconds

132 BASIC BIOMECHANICS

strength (40, 74). Following static stretching this decrease in muscle strength has also been shown to translate to a signifi cant decrement in performance in both 60- and 100-m sprints, as well as in endur- ance running events (30, 73). Although some coaches seem to believe that performing concentric contraction exercises after stretching will ameliorate the negative effects of stretching on muscular strength, re- search shows this to be false, even when the exercises involve maximal contractions (29, 70). Studies comparing static and ballistic stretching have shown that static stretching is more effective in increasing joint range of motion, both after a single bout of stretching and after a four- week stretching protocol (2, 11). However, whereas static stretching produces a transient decrease in muscle strength, there is no such ef- fect with ballistic stretching (2).

Dynamic stretching involves motion of the body as in ballistic stretch- ing, but unlike ballistic stretching, the motion is controlled and not a bouncing-type movement. Recent research demonstrates that following a bout of dynamic stretching there is a benefi cial effect for activities requir- ing muscular power (12, 18, 38, 56). The current literature suggests that prior to athletic competition a warm-up including dynamic stretching may be desirable, with static stretching being most benefi cial following a perfor- mance to maintain or increase joint range of motion. Both forms of stretch- ing can induce soreness in muscles that are not habitually stretched (60).

Proprioceptive Neuromuscular Facilitation

The most effective stretching procedures are known collectively as proprio- ceptive neuromuscular facilitation (PNF). PNF techniques were originally used by physical therapists for treating patients with neuromuscular pa- ralysis. All PNF procedures involve some pattern of alternating contrac- tion and relaxation of agonist and antagonist muscles designed to take advantage of the GTO response. All PNF techniques require a partner or clinician. Stretching the hamstrings from a supine position provides a good illustration for several of the popular PNF approaches (see Figure 5-12).

The contract-relax-antagonist-contract technique (also referred to as slow-reversal-hold-relax), involves passive static stretch of the ham- strings by a partner, followed by active contraction of the hamstrings against the partner’s resistance. Next, the hamstrings are relaxed and

Active, static stretching involves holding a position at the extreme of the range of motion. Photo © Lars A. Niki.

proprioceptive neuro- muscular facilitation a group of stretching procedures involving alternating contraction and relaxation of the muscles being stretched

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 133

the quadriceps are contracted as the partner pushes the leg into increas- ing fl exion at the hip. There is then a phase of complete relaxation, with the leg held in the new position of increased hip fl exion. Each phase of this process is typically maintained for a duration of 5–10 s, and the entire sequence is carried out at least four times.

The contract-relax and hold-relax procedures begin as in the slow-reversal- hold method, with a partner applying passive stretch to the hamstrings, followed by active contraction of the hamstrings against the partner’s resis- tance. With the contract-relax approach, the contraction of the hamstrings is isotonic, resulting in slow movement of the leg in the direction of hip ex- tension. In the hold-relax method, the contraction of the hamstrings is iso- metric against the partner’s unmoving resistance. Following contraction, both methods involve relaxation of the hamstrings and quadriceps while the hamstrings are passively stretched. Again, the duration of each phase is usually 5–10 s, and the entire sequence is repeated several times.

The agonist-contract-relax method is another PNF variation, with 5–20 s sequential phases. This procedure begins with active, maximal contrac- tion of the quadriceps to extend the knee, followed by relaxation as the partner manually supports the leg in the position actively attained.

Studies show that PNF techniques can signifi cantly increase joint ROM transiently after a single stretching session and with a more long-lasting effect when three bouts of PNF stretching are performed three times per week (24). Researchers have found the optimal contraction intensity for individuals using PNF techniques to be approximately 65% of maximum voluntary isometric contraction (58).

COMMON JOINT INJURIES AND PATHOLOGIES

The joints of the human body support weight, are loaded by muscle forces, and at the same time provide range of movement for the body segments. They are consequently subject to both acute and overuse injuries, as well as to infection and degenerative conditions.

Passive stretching can be accomplished with the assistance of a partner. Photo courtesy Royalty-Free/CORBIS.

134 BASIC BIOMECHANICS

Sprains

Sprains are injuries caused by abnormal displacement or twisting of the ar- ticulating bones that results in stretching or tearing of ligaments, tendons, and connective tissues crossing a joint. Sprains can occur at any joint, but are most common at the ankle. Lateral ankle sprains are particularly com- mon, because the ankle is a major weight-bearing joint and because there is less ligamentous support on the lateral than on the medial side of the ankle. Sprains can be classifi ed as fi rst, second, and third degree, depend- ing on the severity of the injury. First-degree sprains are the mildest, with symptoms of tenderness and slight swelling, but little loss of joint ROM. With second-degree sprains, more damage to the tissues is present, and there is usually swelling, bruising, localized tenderness, moderate pain, and some restriction of joint ROM. Third-degree sprains involve partial to complete tearing of the ligaments, accompanied by swelling, pain, and typically joint instability. The traditional treatment for sprains is rest, ice, compression, and elevation.

Dislocations

Displacement of the articulating bones at a joint is termed dislocation. These injuries usually result from falls or other mishaps involving a large magnitude of force. Common sites for dislocations include the shoulders, fi ngers, knees, elbows, and jaw. Symptoms include visible joint deformity, intense pain, swelling, numbness or tingling, and some loss of joint move- ment capability. A dislocated joint may result in damage to the surround- ing ligaments, nerves, and blood vessels. It is important to reduce (or properly relocate) a dislocated joint as soon as possible both to alleviate the pain and to ensure that the blood supply to the joint is not impeded. Reduction of a dislocated joint should be performed by a trained medical professional.

Bursitis

The bursae are sacs fi lled with fl uid that function to cushion points where muscles or tendons slide over bone. Under normal conditions, the bursae create a smooth, nearly frictionless gliding surface. With bursitis, or in- fl ammation of a bursa, movement around the affected area becomes pain- ful, with more movement increasing the infl ammation and aggravating the problem. Bursitis can be caused by overuse-type, repetitive, minor impacts on the area, or from acute injuries, with subsequent infl amma- tion of the surrounding bursae. The condition is treated with rest, ice, and anti-infl ammatory medications. For example, runners who increase training mileage too abruptly may experience infl ammation of the bursa between the Achilles tendon and the calcaneous. Pain and possibly some swelling are symptoms of bursitis.

Arthritis

Arthritis is a pathology involving joint infl ammation accompanied by pain and swelling. It is extremely common with aging, with over 100 dif- ferent types of arthritis identifi ed.

Rheumatoid Arthritis

The most debilitating and painful form of arthritis is rheumatoid arthri- tis, an autoimmune disorder that involves the body’s immune system at- tacking healthy tissues. It is more common in adults, but there is also a

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 135

juvenile rheumatoid arthritis. Characteristics include infl ammation and thickening of the synovial membranes and breakdown of the articular cartilage, resulting in limitation of motion and eventually ossifi cation or fusing of the articulating bones. Other symptoms include anemia, fatigue, muscular atrophy, osteoporosis, and other systemic changes.

Osteoarthritis

Osteoarthritis, or degenerative joint disease, is the most common form of arthritis. It is increasingly believed to be an entire family of related disor- ders that result in progressive degradation of the biomechanical proper- ties of articular cartilage (50). In the early stages of the disorder, the joint cartilage loses its smooth, glistening appearance and becomes rough and irregular. Eventually, the cartilage completely wears away, leaving the articulating bone surfaces bare. Thickening of the subchondral bone and the formation of osteophytes, or bone spurs, are accompanying features (59). Pain, swelling, ROM restriction, and stiffness are all symptoms, with the pain typically relieved by rest, and joint stiffness improved by activity.

The cause of osteoarthritis is usually unknown. Although articular car- tilage appears to adapt to changes in habitual loading patterns, efforts to associate the incidence of osteoarthritis with lifestyle factors have pro- duced confl icting results (22, 23, 25, 63). Whereas occupations requiring heavy lifting, farming, and participation in elite sports have been associ- ated with higher incidences of hip osteoarthritis, no relationship has been found between levels of regular physical activity throughout life and the incidence of knee osteoarthritis (25, 63). It has been shown, however, that malalignment of the hip-knee-ankle increases the progression of osteo- arthritis at the knee, with varus and valgus alignments respectively in- creasing loading and osteoarthritis progression on the medial and lateral aspects of the knee (see Chapter 8) (57).

Because articular cartilage is avascular in adults, it relies on cyclic mechanical loading for fl uid exchange to deliver nutrients and remove waste products. Consequently, too little cyclic mechanical stress at syno- vial joints results in deterioration of the cartilage. Research suggests that some degenerative joint disease may actually stem from remodeling and related vascular insuffi ciency in the underlying subchondral bone, a pat- tern also associated with disuse (27, 39, 43). Current thinking is that both too little mechanical stress and excessive mechanical stress can promote the development of osteoarthritis, with an intermediate zone of regular cyclic loading that optimizes the health of articular cartilage (72).

SUMMARY

The anatomical confi gurations of the joints of the human body govern the directional movement capabilities of the articulating body segments. From the perspective of movements permitted, there are three major categories of joints: synarthroses (immovable joints), amphiarthroses (slightly movable joints), and diarthroses (freely movable joints). Each major category is further subdivided into classes of joints with common anatomical characteristics.

The ends of bones articulating at diarthrodial joints are covered with articular cartilage, which reduces contact stress and regulates joint lu- brication. Fibrocartilaginous discs or menisci present at some joints also may contribute to these functions.

Tendons and ligaments are strong collagenous tissues that are slightly extensible and elastic. These tissues are similar to muscle and bone in

that they adapt to levels of increased or decreased mechanical stress by hypertrophying or atrophying.

Joint stability is the ability of the joint to resist displacement of the ar- ticulating bones. The major factors infl uencing joint stability are the size and shape of the articulating bone surfaces, and the arrangement and strength of the surrounding muscles, tendons, and ligaments.

Joint fl exibility is primarily a function of the relative tightness of the muscles and ligaments that span the joint. If these tissues are not stretched, they tend to shorten. Approaches for increasing fl exibility include active versus passive stretching, and static versus dynamic stretching. PNF is a particularly effective procedure for stretching muscles and ligaments.

INTRODUCTORY PROBLEMS

(Reference may be made to Chapters 7–9 for additional information on specifi c joints.)

1. Construct a table that identifi es joint type and the plane or planes of allowed movement for the shoulder (glenohumeral joint), elbow, wrist, hip, knee, and ankle.

2. Describe the directions and approximate ranges of movement that occur at the joints of the human body during each of the following movements:

a. Walking b. Running c. Performing a jumping jack d. Rising from a seated position 3. What factors contribute to joint stability? 4. Explain why athletes’ joints are often taped before the athletes par-

ticipate in an activity. What are some possible advantages and disad- vantages of taping?

5. What factors contribute to fl exibility? 6. What degree of joint fl exibility is desirable? 7. How is fl exibility related to the likelihood of injury? 8. Discuss the relationship between joint stability and joint fl exibility. 9. Explain why grip strength diminishes as the wrist is hyperextended. 10. Why is ballistic stretching contraindicated?

ADDITIONAL PROBLEMS

1. Construct a table that identifi es joint type and the plane or planes of movement for the atlanto-occipital joint, the L5-S1 vertebral joint, the metacarpophalangeal joints, the interphalangeal joints, the carpo- metacarpal joint of the thumb, the radioulnar joint, and the talocrural joint.

2. Identify the position (for example, full extension, 90° of fl exion) for which each of the following joints is close packed:

a. Shoulder b. Elbow c. Knee d. Ankle 3. How is articular cartilage similar to and different from ordinary

sponge? (You may wish to consult the Annotated Readings.) 4. Comparatively discuss the properties of muscle, tendon, and ligament.

(You may wish to consult the Annotated Readings.)

136 BASIC BIOMECHANICS

5. Discuss the relative importance of joint stability and joint mobility for athletes participating in each of the following sports:

a. Gymnastics b. Football c. Swimming 6. What specifi c exercises would you recommend for increasing the sta-

bility of each of the following joints? a. Shoulder b. Knee c. Ankle Explain the rationale for your recommendations. 7. What specifi c exercises would you recommend for increasing the fl ex-

ibility of each of the following joints? a. Hip b. Shoulder c. Ankle Explain the rationale for your recommendations. 8. In which sports are athletes more likely to incur injuries that are

related to insuffi cient joint stability? Explain your answer. 9. In which sports are athletes likely to incur injuries related to insuf-

fi cient joint fl exibility? Explain your answer. 10. What exercises would you recommend for senior citizens interested

in maintaining an appropriate level of joint fl exibility?

CHAPTER 5: THE BIOMECHANICS OF HUMAN SKELETAL ARTICULATIONS 137

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139

NAME _________________________________________________________

DATE _________________________________________________________

LABORATORY EXPERIENCES

1. Using a skeleton, an anatomical model, or the Dynamic Human CD, locate and provide a brief de- scription for an example of each type of joint.

a. Synarthroses (immovable joints)

Suture: _______________________________________________________________________________________