Training principles to improve muscular strength

The two guiding principles for planning exercise programs to improve muscular strength and endurance are overload and specificity. The overload principle states that to make improvements, a muscle must lift a weight heavier than that to which it is accustomed. As your muscles adapt to the new load, you must overload the muscles again to achieve additional strength gains.

The principle of specificity states that the type of adaptation (i.e., muscular strength or muscular endurance) is related to the type of exercise. If a person lifts heavy weights, the primary change is an increase in the size of the muscle cell, an adaptation called hypertrophy. This increase in muscle size is associated with the increase in muscle strength. On the other hand, if a person does many repetitions with lighter weights (e.g., 50 percent of 1-RM), the primary changes in muscle are found in the energy-producing parts, called mitochondria, and in the number of capillaries that bring oxygen to the muscle cell. Muscle size does not increase very much with endurance exercise. Though in general the effects of training are specific to the type of exercise performed, there is some overlap in the areas of strength and endurance because a person cannot completely isolate one from the other.

Strength increases over the course of a training program, but hypertrophy doesn't occur until later (more than 10 weeks) in the program. The strength gain early in the program comes primarily from neural changes related to coordination of the major muscle groups and the ability to recruit more muscle fibers. Men typically experience more hypertrophy than women as a result of a long-term training program. This is primarily because men have more testosterone, a hormone that has more anabolic (tissue-building) properties than estrogen.