Figure 33.15: Competitive Athletes Tend to Have Muscle Fiber Types Corresponding to Their Sports (A) People vary in the cellular composition of their skeletal muscles. In these microscopic images, cells were stained with a reagent that shows slow oxidative cells as dark and fast glycolytic cells as light. (B) At the level of international or national competition, athletes successful in sustained aerobic events tend to have a high percent of slow oxidative cells, whereas successful sprinters tend to have a high percent of fast glycolytic cells. These differences are caused by heredity, early development, and training. Differences among individuals in muscle cell composition do not matter in sports done for fun.