The human body contains more than 600 skeletal muscles, which constitute 40% to 50% of the total body weight (61). Skeletal muscle performs three important functions: (1) force generation for locomo— tion and breathing, (2) force generation for postural support, and (3) heat production during cold stress. The most obvious function of skeletal muscle is to enable an individual to move freely and breathe. New evidence also suggests that skeletal muscles are endo— crine organs and play an important role in the regula— tion of a variety of organ systems in the body. This concept was introduced in Chap. 5 and will be mentioned again in this chapter.