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.