As one ages the mass of skeletal muscle decreases throughout the body. The muscles atrophy resulting in a decreased number of muscle fibers and decrease in the size of the remaining fibers. As skeletal muscles are postmitotic they can not reproduce to produce new fibers. As a result the fibers are replaced with fat tissue. The muscle fiber loss has been reported to be as high as 30% between the ages of 30 and 80. Proper nutrition and exercise can slow the loss muscle cells, but heredity also seems to be a factor.
Along with the loss of skeletal muscle mass comes the loss of skeletal muscle strength. For most individuals there is only a ten to twenty percent reduction in strength up to the age of seventy. After the age of seventy the reduction in strength may increase to fifty percent.