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"Give me a lever long enough and a fulcrum on which to place it and I shall move the world"

Archimedes, 287 BC-212 BC Greek mathematician, physicist, engineer, inventor, and astronomer



This chapter identifies the mechanical and physics principles of force applications that are relevant to human movement. By the end of the chapter, you should be able to:

  • Identify the four forces that act upon the body;

  • Explain the three classes of levers and provide an example of each in the human body;

  • List Newton's laws of motion and distinguish how they apply to the human body;

  • Identify the elements of a force vector diagram and explain the tangential force and its significance in joint motion;

  • Describe a free body diagram and its function in kinesiology;

  • Explain why muscle and joint reaction forces are often larger than the external forces applied to the body;

  • Ascertain the ratio formulas for a right triangle;

  • Describe the differences between force and torque;

  • Provide a clinical example of the applications of Newton's laws of motion;

  • Offer a clinical example of how to alter a quadriceps exercise to increase resistance provided to a patient without increasing the weight.



William wished he had paid more attention to his physics professor during his sophomore year. He is now taking a kinesiology class that he really enjoys, but he does not recall much of the physics concepts his current professor is now reviewing. He is lost when Professor Violet starts discussing how bones are levers and muscles provide torque to move them. He is not sure he even remembers the difference between torque and force. As Professor Violet continues her presentation, William is intrigued when she states that we can change the amount of work a muscle must do just by changing where we apply the force against that muscle. William wonders, "How can that be?" and "What does that have to do with physics?"


So that we understand how muscles and their neural innervations create motion, we need to investigate how they respond when the body segments experience outside forces applied to them. Kinetics deals with forces that produce, stop, or modify motion of either the body as a whole or the individual body segments. Kinematics, as you learned in Chapter 1, deals with motion of the body and/or its segments without regard to the forces acting to produce those motions. This chapter advances from the ideas of the movements you learned in the first chapter to how those movements occur through the application of various forces. Forces within the body and forces outside the body both affect how a body moves. Muscles produce forces to move our body. Many factors influence how the muscles function to provide us with the mobility, ability, and variability we have to interact ...

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