It has been hypothesized that because the muscle is working harder in weight training, more blood is required to supply that muscle with oxygen and other nutrients. Thus it is thought that the number of capillaries is increased. This hypothesis is only partially correct; few new capillaries are formed during strength training, but a number of dormant capillaries may become filled with blood to meet this increased demand for blood supply.
A third theory to explain this increase in muscle size seems the most credible. Muscle fibers are composed primarily of small protein filaments, called myofilaments, which are the contractile elements in muscle. These myofilaments increase in both size and number as a result of strength training, causing the individual muscle fibers themselves to increase in cross - sectional diameter. This is particularly true in men.
In addition to muscle hypertrophy there are a number of other physiological adaptations to resistance training. The strength of noncontractile structures, including tendons and ligaments, is increased. The mineral content of bone is increased, making the bone stronger and more resistant to fracture. Maximal oxygen uptake is improved when resistance training is of sufficient intensity to elicit heart rates at or above training levels. There is also an increase in several enzymes important in aerobic and anaerobic metabolism.
