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Humans have the capacity to produce a nearly infinite variety of postures and movements that require the structures of the human body to both generate and respond to forces that produce and control movement of the body’s joints. Although it is impossible to capture all the kinesiologic elements that contribute to human musculoskeletal function at a given point in time, knowledge of at least some of the physical principles that govern the body’s response to active and passive stresses is a prerequisite to an understanding of both human function and dysfunction.

We will examine some of the complexities related to human musculoskeletal function by examining the roles of the bony segments, joint-related connective tissue structures, and muscles, as well as the external forces applied to those structures. We will develop a conceptual framework that provides a basis for understanding the stresses on the body’s major joint complexes and the responses to those stresses. Case examples and clinical scenarios will be used to ground the reader’s understanding in relevant applications of the presented principles. The objective is to cover the key biomechanical principles necessary to understand individual joints and their interdependent functions in posture and locomotion. Although we acknowledge the role of the neurological system in motor control, we leave it to others to develop an understanding of the theories that govern the roles of the controller and feedback mechanisms.

This chapter will explore the biomechanical principles that must be considered in the examination of the internal and external forces that produce or control movement. The focus will be largely on rigid body analysis; the next two chapters explore how forces affect deformable connective tissues (Chapter 2) and how muscles create and are affected by forces (Chapter 3). Subsequent chapters then examine the interactive nature of force, stress, tissue behaviors, and function through a regional exploration of the joint complexes of the body. The final two chapters integrate the function of the joint complexes into the comprehensive tasks of posture (Chapter 13) and gait (Chapter 14).

To maintain our focus on clinically relevant applications of the biomechanical principles presented in this chapter, the following patient application will provide a framework within which to explore the relevant principles of biomechanics.

image Patient Application 1–1

John Alexander is 20 years old, 5 feet 9 inches (1.75 m) in height, and weighs 165 pounds (~75 kg or 734 N). John is a member of the university’s lacrosse team. He sustained an injury when another player fell onto the posterior-lateral aspect of his right knee. Physical examination and magnetic resonance imaging (MRI) resulted in a diagnosis of a tear of the medial collateral ligament, a partial tear of the anterior cruciate ligament (ACL), and a partial tear of the medial meniscus. John agreed with the orthopedist’s recommendation that a program of knee muscle strengthening was in order ...

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