Physiologic responses to training.

Endurance training improves exercise tolerance principally by inducing structural and biochemical changes in the exercising muscles. These changes yield improved oxygen delivery to the site of muscle metabolism, which improves oxygen use and forestalls the onset of anaerobic metabolism. The physiologic stress of heavy exercise is thereby reduced. The molecular mechanisms that induce these changes are unclear, but the characteristics of an effective training program have been well described. Cessation of training leads to prompt regression of the accumulated benefits.