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Forty-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, Ashley, can get him into a standing position and keep him from falling.

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

In patient care techniques, your body becomes the most essential instrument you have for performing mobility tasks. A fundamental understanding of the rules that govern movement, that is, the laws of physics, allows you to achieve safe and effective patient mobility while protecting yourself against injury. It also allows you to educate patients and care partners in the safe and effective use of their bodies. This chapter introduces fundamental principles of physics, examines the relationship between mobility and stability, and applies these principles to body mechanics and patient mobility tasks.

For the most part, you already “know” these laws. You demonstrate your understanding of lever arms when you hold a heavy bag of groceries close instead of with an outstretched arm. You intuitively adjust your center of mass (CoM) when you use your arms to steady yourself when standing on an unstable surface. But to apply this knowledge to other people who are performing unfamiliar and complex tasks, you, as the clinician, must have a working knowledge that includes the ability to analyze forces, reason through hypothetical situations, and make informed choices about stability versus mobility.


Biomechanics is the application of engineering and physical science to human and animal movement and, as such, 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.

Understanding common biomechanical terminology allows for a clearer discussion of biomechanical principles. Professional language is held to a higher standard of accuracy and precision than lay language because 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.

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Some terms have multiple meanings depending on the context and the person who is using them. Terms in this book that have different meanings outside biomechanics or healthcare are defined for this ...

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