HUMAN AGING, MUSCLE MASS, AND FIBER-TYPE COMPOSITION

To assess the age-related loss of muscle mass and to determine the mec hanisms behind this aging atrophy, the muscle structure and fiber type composition have been estimated, using invasive and noninvasive techn iques. Limb muscles from older men and women are 25-35% smaller and ha ve significantly more fat and connective tissue than limb muscles from younger individuals. Comparisons of muscle biopsies from younger and older individuals reveal that type 2 (fast-twitch) fibers are smaller in the old, while the size of type I (slow-twitch) fibers is much less affected. Studies of whole muscle cross sections also show a signific antly smaller number of muscle fibers, a significantly lower relative type 2 fiber area, and a significant increase in fiber type grouping w ith increasing age. These results indicate a gradual decrease in size/ volume with advancing age, accompanied by a replacement by fat and con nective tissue. This aging atrophy seems to be due to a reduction in b oth number and size of muscle fibers, mainly of type 2, and is to some extent caused by a slowly progressive neurogenic process.