Human Body as a Machine

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The human body can be seen as a machine that consumes food and converts it to mechanical work and heat (See Thermal Energy). Power is transmitted through the action of forces on the muscles, bones, and joints that make body’s lever system. As muscles contract, they pull a series of tendons and bones (levers) across joints (fulcrums, pivots, or hinges) to perform various tasks. Muscles always act as pairs, so when one contracts, its antagonist extends. Examples of the first, second and third class levers are the joint between skull and vertebrae (neck joints), the Achilles tendons, and the elbow joints, respectively. Most of the movements of the body are produced by third class levers, where the force is between fulcrum and weight. This design lends itself to speed of movement rather than force. For example, when we lift a weight, the biceps and triceps work as a pair; when the biceps flex the arm (effort) or lift a weight (load or resistance), the triceps relax to extend the arm (Figure 1 from Mechanical Efficiency and Mechanical Advantage).

 A human elbow acts as a third class lever during the lifting of a weight
Figure 1: A human elbow acts as a third class lever during the lifting of a weight

Question: A man is lifting a heavy weight by contracting the bicep muscle, and at the same time relaxing his triceps muscle. The elbow acts as the fulcrum. What is the force needed to lift a 10 pound weight?

Answer: Force needed is higher by ratio of the arms (40 to 4); we need ten times the weight of the object to support the weight. In the discussions above we assumed that there are no losses involved. When losses are present, mechanical advantage will be reduced by a factor equal to the mechanical efficiency.

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