Chapter 2

At the completion of this chapter, the reader will be able to:

1. Describe the various types of stress that can be applied to the body.

2. Describe the various physiological processes by which the body adapts to stress.

3. Define the various common mechanisms of injury.

4. Describe the etiology and pathophysiology of musculoskeletal injuries associated with various types of body tissue.

5. Outline the pathophysiology of the healing process and the various stages of healing.

6. Describe the methods used to enhance healing and the factors that can impede the healing process.

7. Outline the more common surgical procedures available for musculoskeletal injuries.

8. Outline the principles behind postsurgical rehabilitation.

9. Describe the detrimental effects of immobilization.

Tissues in the body are designed to function while undergoing stresses of everyday living. This ability to respond to stress is due to the differing viscoelastic properties of musculoskeletal tissue, with each tissue responding to stress in an individual manner based on design. One of the contributing factors to maintaining musculoskeletal health is the ability of the biological tissues to withstand various stresses and strains that occur during activity—body weight, friction, and air or water resistance are all types of stresses that commonly act on the body. Maintaining this health is a delicate balance, because insufficient, excessive, or repetitive stresses can prove deleterious. The healing process is an intricate phenomenon that occurs following an injury or disease.

Kinetics is the term applied to the forces acting on the body. Posture and movement are both governed by the control of forces. The same forces that move and stabilize the body also have the potential to deform and injure the body.1 A wide range of external and internal forces are either generated or resisted by the human body during the course of daily activities. Examples of these external forces include ground reaction force, gravity, and applied force through contact. Examples of internal forces include muscle contraction, joint contact, and joint shear forces (Fig. 2-1). Under the right circumstances, the body is able to respond to these stresses. The terms stress and strain have specific mechanical meanings. Stress or load is given in units of force per area, and is used to describe the type of force applied. Stress is independent of the amount of a material, but is directly related to the magnitude of force and inversely related to the unit area.2 Strain is defined as the change in length of a material due to an imposed load divided by the original length.2 The two basic types of strain are linear strain, which causes a change in the length of a structure, and shear strain, which causes a change in the angular relationships within a structure. It is the concentration of proteoglycans in solution (see Chap. 1) that is responsible for influencing the mechanical properties of the tissue, including compressive stiffness, sheer stiffness, osmotic pressure, and regulation of hydration.3

###### Figure 2-1
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