At the cellular level, the mixture of slow and fast twitch muscle fibers influence what type of exercise you will excel at. Slow-twitch fibers (type I) have a greater aerobic capacity and do best at endurance-strength activities. Fast-twitch fibers (type IIb) have a higher anaerobic. capacity and are better at power-strength exercises. One kind of fiber cannot be converted into another. All muscle fibers can make energy aerobically and anaerobically in varying degrees. Because of this, the fast-twitch fibers tire quickly, while the slow-twitch fibers take much longer to fatigue. All skeletal muscle possesses both types of fibers in varying amounts. It is the total proportion of this mixture that influences the type of activity a person will do best.

Recently, scientists have found cross-over fibers in the muscle. These fast-slow-twitch fibers (type IIa) can emphasis either characteristic according to the style of exercise done. There is a theory that these fibers can permanently change into one type or the other after prolong aerobic or anaerobic use.

The heart, lungs, and bones of males are larger than those of women. Men have a slower heart rate which, coupled with their larger hearts, enables them to pump more blood with fewer beats. The stroke volume of a well conditioned woman can still exceed that of a deconditioned man. The blood of a typical male is thicker then a woman's. It carries 6 percent more red oxygenated blood cells per cubic millimeter. Men also have more and bigger muscle fibers then women. In addition, the male hormonal system produces testosterone that increases the growth of muscle size. Because of this, the muscles of women who lift weights do not hypertrophy to the extent that a man's does.

On the other hand a woman's muscles grow stronger from regular exercise just like a man. A woman's heart rate, cardiovascular capacity, strength and endurance will improve from regular exercise just like a man's will. She stands to gain as much as any man from exercise. With the personal prescriptive workout planning available from The BioFitness Institute both men and women are guaranteed to enjoy fitness success.