Clinical Biomechanics: Mechanical Concepts and Terms
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This is Chapter 2 from RC’s best-selling book:
Second Edition ~ Wiliams & Wilkins
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Chapter 2: Mechanical Concepts and TermsAll motor activities such as walking, running, jumping, squatting, pushing, pulling, lifting, and throwing are examples of dynamic musculoskeletal mechanics. To better appreciate the sometimes simple and often complex factors involved, this chapter reviews the basic concepts and terms involved in maintaining static equilibrium. Static equilibrium is the starting point for all dynamic activities.
Energy and Mass
Biomechanics is constantly concerned with a quantity of matter (whatever occupies space, a mass) to which a force has been applied. Such a mass is often the body as a whole, a part of the body such as a limb or segment, or an object such as a load to be lifted or an exercise weight. By the same token, the word “body” refers to any mass; ie, the human body, a body part, or any object.
Energy is the power to work or to act. Body energy is that force which enables it to overcome resistance to motion, to produce a physical effect, and to accomplish work. The body’s kinetic energy, the energy level of the body due to its motion, is reflected solely in its velocity, and its potential energy is reflected solely in its position. Mathematically, kinetic energy is half the mass times the square of the velocity: m/2 X V524. In a closed system where there are no external forces being applied, the law of conservation of mechanical energy states that the sum of kinetic energy and potential energy is equal to a constant for that system.
Potential energy (PE), measured in newton meters or joules, is also stored in the body as a result of tissue displacement or deformation, like a wound spring or a stretched bowstring or tendon. It is expressed mathematically in the equation PE = mass X gravitational acceleration X height of the mass relative to a chosen reference level (eg, the earth’s surface). Thus, a 100-lb upper body balanced on L5 of a 6-ft person has a potential energy of about 300 ft-lb relative the ground.
The Center of Mass
The exact center of an object’s mass is sometimes referred to as the object’s center of gravity. When an object’s mass is evenly distributed throughout, the center of mass is located at the object’s geometric center. In the human body, however, this is infrequently true, and the center of mass is located towards the heavier, often larger, aspect. When considering the body as a whole, the center of mass in the anatomic position, for instance, is constantly shifted during activity when weight is shifted from one area to another during locomotion or when weight is added to or subtracted from the body.
The term weight is not synonymous with the word mass. Body weight refers to the pull of gravity on body mass. Mass is the quotient obtained by dividing the weight of a body by the acceleration due to gravity (32 ft/sec524). Each of these terms has a different unit of measurement. Weight is measured in pounds or kilograms, while mass is measured by a body’s weight divided by the gravitational constant. The potential energy of gravity can be simply visualized as an invisible spring attached between the body’s center of mass and the center of the earth. The pull is always straight downward so that more work is required to move the body upward than horizontally (Fig. 2.1).
Newton’s Laws of Mechanics