All persons have genetic predispositions to exercise. No aerobic , weight training, or drug regimen can change this. At the cellular level, the mixture of slow and fast twitch muscle fibers influence what type of exercise you will excel at. One kind of fiber cannot be converted into another.
All muscle fibers can make energy aerobically and anaerobically in varying degrees.
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. 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.