In rigor mortis , the death of the individual means that ATP cannot be supplied and the crossbridges remain locked in place. During electrical shocks , calcium removal is impeded, and muscles contract continuously and involuntarily. Cramps result from ionic imbalances in the blood and body fluids, causing an involuntary release of calcium. Any one muscle is made up of a combination of both types of cells. Although muscles do change in response to training, fiber type appears to be largely determined by genetics.
White fibers are big, powerful, fast, with few mitochondria, capillaries, and no myoglobin.
Reliance on anaerobic glycolysis causes buildup of lactic acid and rapid fatigue. Performance correlates of fiber type - variation among individuals predisposes athletes to success in specific events. Fiber type does not change with either of these. In endurance training, muscles become more oxidative, the heart is larger,and the blood volume increases. This means that well-trained endurance athletes have low resting heart rates and blood pressure.