Bonus: La regla procesal establecida por el Tribunal puede generar problemas prácticos

cross the joint. The ways in which joints can move are determined by their structure and sur-rounding soft tissues (Radomski & Trombly Latham, 2008). For instance, hinge joints such as those of the proximal and distal interpha-langeal joints of the fingers (i.e., the two joints nearest the tip of each finger) allow movement in one plane (i.e., extension and flexion). Ball and socket joints, such as the shoulder and hip joints, allow movement in several planes. These joints are not only capable of flexion and exten-sion but also adduction (movement toward the midline of the body), abduction (movement away from the body midline), and rotation (movement about a longitudinal axis). Thus, the structure of each joint determines the move-ments that are possible at the joint.

The connective tissue (e.g., ligaments), muscle, and skin that surround joints have elas-ticity, which is the ability to stretch and to return to original shape and size after movement. The amount of elasticity of these tissues also affects the extent of possible movement.

Range of Motion

Active range of motion refers to the range of movement that a person can produce using vol-untary muscle contraction. Passive range of motion refers to the range of movement that is possible when an outside force moves a joint.

The potential for motion in the body’s joints allows the body to assume positions and engage in actions that are necessary for functional performance. Range of motion makes possible such actions as bending, reaching, pulling, lift-ing, and grasping.

Strength

Stability and motion are produced when skeletal muscles act on the joints of the body. Muscles cross one or more joints and exert force to con-trol or produce movements allowed by the struc-ture of the joints. Thus, tension produced in the muscles is necessary to stabilize or move joints.

Muscles move joints when muscles contract to produce forces that act on one side or aspect of the joint. For example, the muscles that connect to the bones of the fingers on the palm side of the hand flex the fingers, while those muscles

that connect to the finger bones on the back of the hand cause the fingers to extend. For exam-ple, as shown in Figure 7.1, the extensor digito-rum muscle attaches to the bones of the second, third, fourth, and fifth fingers on the back of the hand and when it contracts it extends the fingers at all three finger joints. Muscles stabilize joints when they produce forces that act with equal tension from all directions in which the joint is capable of moving.

The strength or ability of a muscle to pro-duce tension is heavily influenced by the number and size of fibers in the muscle. The diameter of muscle fibers increases when the muscle is used to produce tension. Thus, the use to which a muscle is put during the course of everyday activities affects its strength.

In daily life, normal movement is not limited to the action of a single muscle across a single

FIGURE 7.1 Extensor digitorum muscle. From Lippert, L. (2006). Clinical Kinesiology and Anatomy.

Philadelphia: F.A. Davis.

joint. Rather, performance depends on the simul-taneous action of muscles across many joints.

This produces the combination of stability and movement required for a task. Moreover, groups of muscles work together to produce each move-ment (Pendleton & Schultz-Krohn, 2006).

The extent of muscle strength determines what kind of functional movement one’s body can perform. For example, a certain amount of strength is necessary to lift an arm against grav-ity. More strength is needed when lifting a heavy object overhead. Along with range of mo-tion, strength determines the extent to which a person is able to execute necessary tasks. To perform optimally, one must have adequate strength to do the tasks that make up one’s occupations.

Endurance

The ability to sustain muscle activity (i.e., endurance) is a function of muscle physiology and the underlying cardiopulmonary functions that supply oxygen and energy materials. Thus, endurance is a somewhat more complex phenom-enon than strength and motion since it depends on the musculoskeletal system but also entails the functions of other body systems. Two types of endurance are recognized. Muscle endurance refers to the ability of a muscle to contract repeat-edly to do work. Cardiorespiratory endurance refers more broadly to the ability to sustain activ-ity over time as when walking or running.

Like range of motion and strength, endurance determines the extent to which persons can do the tasks that make up occupational life.

Endurance is most important when an occupa-tion requires repeated mooccupa-tion or sustained effort over time. Walking to school or work, vacuum-ing or moppvacuum-ing the house, and workvacuum-ing on an assembly line are obvious examples of occupa-tions that require one to sustain motion over time. Nonetheless, all activities require a certain amount of endurance since they call for one to stabilize and move oneself for some duration.

The Dynamics of Movement Capacity

Several factors are considered in understanding movement. The potential for movement (joint

range of motion) is a function of the anatomy of joints and soft tissues around joints.

Bones constitute a system of rigid levers that are moved by forces produced by the mus-cles attached to those bones (Hall, 2003). Bones are arranged as levers in which force is applied somewhere along the length of the bone to move it on its pivot point or fulcrum. The production of movement is a function of how muscles act on the levers created by bones. Functional movement requires a complex interaction of forces produced by muscles acting on many levers simultaneously to stabilize and move them according to the task being performed.

Endurance while doing a functional activity is a function of muscle physiology and the ability of the body systems to transport needed material into and waste material out of muscle tissue.

Early understanding of how muscles pro-duced movement was based primarily on the anatomical study of their position with respect to the skeletal system (i.e., where the muscles attached to bones and how they crossed joints).

Such observations of anatomical organization led to the belief that specific movements and muscles would be used to perform a given task (Radomski & Trombly Latham, 2008).

As more sophisticated methods have become available to study the process of movement, the understanding of how movement is used to accomplish occupations has changed. The actual movements produced during occupational per-formance can be described in terms of kinemat-ics. For example, movement can be characterized by the actual movement path (e.g., the forward and backward movements such as those used for walking) or displacement of a body part (e.g., the degrees of motion involved in flexing the elbow) and the velocity (i.e., speed) and acceleration (i.e., rate of change in speed).

Kinematics is an important part of the understanding of how the body actually accom-plishes functional movements. For example, it is now understood that different persons use dif-ferent combinations of movements to do the same task and that a person uses different com-binations of movements to perform the same task at different times (Trombly, 1995). Despite this complex variability in how functional movements are executed, all movements require

the foundation of the potential for range of mo-tion in the structure of the skeletal system and its joints, the necessary strength for accomplish-ing functional movements provided by muscles, and the endurance to sustain motion over the course of task performance provided by muscle physiology and the supporting cardiopulmonary system.

Maintaining Biomechanical Capacity One of the most important observations of the biomechanical model is that the capacity for movement (i.e., strength, range of motion, and endurance) not only affects but is affected by occupational performance. That is, muscle strength increases and decreases according to how much muscles are stressed (i.e., used to produce motion) in the course of everyday occu-pations. Similarly, the structure of bones is pos-itively affected by how much weight-bearing they do, and joint mobility is affected by the nature of ongoing joint movement. Finally, the capacity for endurance waxes and wanes over time with changes in activity level.

While the biomechanical model is used with clients whose musculoskeletal capacities have been compromised due to disease or trauma, the fundamental principle, that biome-chanical capacity increases or decreases accord-ing to use, is important not only to increasaccord-ing capacities that have been reduced, but also to ensure that capacities are not decreased through disuse.

Problems and Challenges

The biomechanical model addresses problems and challenges related to producing the stability and movement for the performance of one’s occupations. Occupational performance gener-ally requires that we stabilize some part of the body while moving others. For instance, while typing at a keyboard one must keep the back, shoulders, elbow, and wrist relatively stable while the fingers move.

Problems with stability and motion emanate from biomechanical impairments (i.e., restric-tions of joint motion, strength, and/or endurance) (Radomski & Trombly Latham, 2008). Thus, the central concern of the model is with problems

that occur when persons cannot generate and/or sustain the stability or movement needed to per-form their occupations. A wide range of diseases or traumas may lead to such problems. Addi-tionally, disuse or overuse of the musculoskeletal system can create problems.

Joint range of motion may be limited because of joint damage, edema of tissues around the joint, pain, skin tightness, muscle spasticity (i.e., excess muscle tone producing tightness), or muscle and tendon shortening as a conse-quence of prolonged immobilization. Examples of conditions that affect joint mobility are arthri-tis, trauma to the joint or to the surrounding connective tissue, and burns that limit the elas-ticity of skin over the joint.

Muscle weakness (i.e., reduced tension-producing capacity) can occur as a result of disuse or because of disease affecting muscle physiology. Loss of muscle strength may be due to disease (e.g., polio or amyotrophic lateral sclerosis) or trauma (spinal cord or peripheral nerve injury) that affects the nervous system stimulation of muscle contraction. Muscle dis-eases, such as muscular dystrophy, directly affect muscle tissue and its ability to contract.

Finally, extended disuse or immobilization can result in the shrinking of muscle fibers that produces weakness that impairs everyday per-formance (Pendleton & Schultz-Krohn, 2006;

Radomski & Trombly Latham, 2008).

Like strength, endurance can be reduced with any extended confinement or limitation of activity. Other factors, including pathology of the cardiovascular or respiratory systems and muscular diseases, can also reduce endurance.

Sensory Problems

Although problems of sensation are not—

strictly speaking—biomechanical, they are often intertwined with movement problems. Tactile sensations or touch are often affected by the same diseases or traumas that affect muscles (i.e., peripheral nerve injuries or spinal cord injuries). Thus, it is very common that sensory loss and loss of motion co-occur. Since tactile sensations are used to direct many movements and to protect the body from harm during per-formance, they are closely tied to being able to

move effectively without injuring oneself (Radomski & Trombly Latham, 2008).

Another important aspect of sensation that is closely tied to movement is pain. While pain ordinarily occurs as a warning against injurious actions, it can be chronically or periodically present in association with disease or trauma that affects the musculoskeletal system. Arthritic pain is a common example. Because pain can affect a person’s tolerance for exerting and sus-taining movement and because movement can worsen pain, the two must often be considered carefully together.

Rationale for Therapeutic Intervention

Interventions based on the biomechanical model focus on the intersection of motion and occupa-tional performance. These interventions can be divided into three different rationales:

•Preventing deformity and maintaining existing capac-ity for motion

•Restoring the capacity for motion

•Compensating for limited range of motion, strength, and/or endurance

These three rationales are often used in combi-nation. The three rationales share the aims of minimizing any gap between persons’ existing limited capacity for movement and the move-ment requiremove-ments of their ordinary occupa-tional tasks. The first, preventative, approach seeks to avoid the development of a gap or to prevent the gap from becoming larger. The sec-ond, restorative, approach seeks to narrow the gap by increasing the capacity for motion. The third, compensatory, approach seeks to bridge the gap through means external to the musculoskeletal system. Each of these approaches is examined here.

Maintenance and Prevention

As already noted, reasonable use is necessary to maintain function of the musculoskeletal sys-tem. The biomechanical model extends this observation to the principle that muscles that are

still able to produce contractions and joints that allow motion should be used to maintain their capacity for functional motion. When the person is not able to move joints through muscle con-traction, joint range of motion is maintained passively (i.e., by externally manipulating joints through their ranges of motion). Joint position-ing, including the use of splints that maintain joints in proper alignment, is also used to pre-vent joint deformity.

Research has shown, and more people are becoming aware, that many biomechanical problems are caused by how persons perform tasks in their daily occupations. Examples of this are back injury due to use of poor body mechanics while lifting and damage to soft tissues due to repetitive motions performed in work (Radomski & Trombly Latham, 2008).

This awareness has fueled efforts to prevent occurrence or recurrence of such problems, especially in the workplace and schools. Occu-pational therapists can teach proper body mechanics or recommend task, work-site, or classroom modifica-tions as preventative measures to avoid such biomechanical problems.

Restoration

Restoration aims at increasing available motion, strength, and endurance. Principles of restora-tion are based on the understanding of normal biomechanical functioning. Because movement maintains normal range of motion, strength, and endurance, movement in therapeutically designed activities or tasks can be used to restore or improve range of motion, strength, or endurance.

Strategies for restoring strength, range of motion, and endurance are described later in the chapter (see Intervention).

Goals for increasing motion, strength, and endurance are determined according to the residual potential (i.e., how much improvement a person is likely to be able to achieve based on the nature of the disease or trauma underlying the impairment) and the movement demands of the occupations the person needs and/or wants to perform. For this approach to work, there

Interventions based on