Why does excessive exercise cause muscles to become stiff and sore?
During vigorous exercise, the circulatory system cannot supply oxygen to muscle fibers quickly enough. In the absence of oxygen, the muscle cells begin to produce a substance called lactic acid, which accumulates in the muscle. It is this build-up of lactic acid that causes soreness and stiffness. Sources: Tortora, Gerard J. Introduction to the Human Body, p. 150; Tortora, Gerard J. Principles of Human Physiology, 2nd ed., pp. 202-3.
