Strength may be defined as the ability to do work against resistance. The amount of muscular force which can be exerted against a resistance depends upon the size of the muscles involved, the proportion of the constituent fibers engaged in the action, the coordination of the muscle groups, the physical condition of the muscles and the mechanical advantage of the levers employed.
The strength of a muscle is proportional to its cross-section area. Although absolute values vary so much from muscle to muscle that it would be hazardous to apply the figure for one muscle to any other muscle, 1 the work capacity of the muscles has been calculated to be on the order of magnitude of 6 kilogram meters per minute per square centimeter of tissue. Fluctuations in this value among individuals arc unrelated to age from 13 to 48 years. Arm muscles are capable of lifting approximately 1,500 times their own weight. Such great strength as this is required in the human body because of the very poor mechanical advantage of most of its levers.
Two muscles having the same cross-section area may differ in strength due to difference in the amounts of fatty tissue which they contain. Fat not only lacks contractile power, but it also acts as a friction brake, limiting the rate and extent of shortening of the muscle fibers. Strength, speed, endurance and skill are all reduced as the percentage of fat in the body of a muscle is increased.
The cross-section area of a muscle is increased most rapidly by activities in which heavy loads are moved, such as weight lifting, wrestling and gymnastics. The optimum amount of muscular strength for an individual is slightly above that needed to meet the requirements of daily activity. If muscles are frequently called on to execute near-maximum contractions, fatigue and reduced physical efficiency result. An adequate reserve of muscular strength permits the necessary, work to be accomplished with only slight displacement of homeostatic equilibria and fatigue occurs only after long periods of continuous work.
Excessive amounts of muscular tissue, on the other hand, constitute an extra load to be supported and moved. A disproportionately great amount of time and effort in training is necessary to maintain such large muscles. When the optimum degree of strength has been attained in a body-building program, the emphasis may well be shifted to activities requiring larger measures of skill or endurance.