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.