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Twenty-three-year-old Julius was admitted to the hospital with a bacterial infection of unknown origin. He is now medically stable but weak. He is able to use his arms to help move himself in bed, but he is unable to lift his legs off the bed without assistance. The task today in therapy is to help Julius stand in the parallel bars from a wheelchair. At 6’8”, Julius doesn’t believe there is any way his 5’6” therapist, Heather, can get him into a standing position and keep him from falling.

How can Heather create a mechanical advantage that will allow her to assist Julius safely and effectively in rising to a standing position?

To be well equipped to assist patients in mobility tasks, you must have a basic understanding of the rules that govern movement. This chapter introduces fundamental principles of physics, examines the relationship between mobility and stability, and applies these principles to body mechanics and examples of patient mobility tasks.

In patient-care techniques, your body becomes the most essential instrument you have for performing tasks and achieving functional goals. To use your body effectively to achieve patient mobility while protecting yourself against injury, and to educate patients in the safe and effective use of their bodies, the clinician must have a fundamental understanding of certain laws of physics.

For the most part, we already “know” these laws as evidenced by the fact that we work with them every day. We demonstrate our understanding of lever arms when we hold a heavy bag of groceries close instead of with an outstretched arm. We intuitively adjust our center of mass when we use our arms to steady ourselves when standing on an unstable surface. But to apply this knowledge to other people and in performing unfamiliar and complex tasks, the clinician must have a working knowledge that includes the ability to analyze forces, reason through hypothetical situations, and make informed choices about stability vs. mobility.

Biomechanical Aspects of Human Movement

image An overview of some basic principles of biomechanics, the application of engineering and physical science to human or animal movement, provides an important foundation for patient mobility activities. For those who have not yet studied biomechanics, this chapter introduces terms and principles. For those who are already familiar with these topics, the chapter serves as a review and provides patient-related applications.

Accuracy in the use of terminology is essential to effective clinical problem-solving. Just as a surgeon must know the name and function of each surgical tool, a therapist needs to be able to communicate accurately about the use of the mechanical body as a mobility instrument.


Some terms have multiple meanings depending on the context and on what group is using them. When a term has multiple meanings, some of those meanings are presented in ...

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