Heavy resistance training is associated with increased body weight, lean body mass, and muscle cross-sectional area. The increased muscle cross-sectional area is mainly brought about by hypertrophy of individual muscle fibers. There is a greater increase in the area of fast twitch fibers compared to slow twitch fibers. In addition, long-term heavy resistance training may produce fiber proliferation. Mitochondrial volume density decreases in proportion to muscle hypertrophy in response to training. Typically, no capillary neoformation occurs during strength training. Therefore, capillary density decreases consequent to heavy resistance training. It appears, though, that bodybuilders, relying on a high repetition training system, in contrast to Olympic weight- and power lifters, display a small increase in number of capillaries per fiber. Enzyme activities, reflecting oxidative potential; decrease during long-term heavy resistance training, resulting in muscle hypertrophy. Although glycogen storage capacity is enhanced in strength trained athletes, enzyme activities reflecting anaerobic metabolism do not increase in response to heavy resistance exercise.