[Aging changes in muscle mass of Japanese]

The purpose of this study is To examine the influence of age on muscle mass in a Japanese population for health promotion.

Subjects were 4,003 community-dwelling Japanese men and women. We employed four-frequency bioelectrical impedance analysis to estimate upper and lower limbs, trunk and whole body muscle mass.

Men showed significantly more muscle mass in all parts of the body compared to women. There was a curvilinear relationship between age and muscle mass in both men and women. For all parts of the body, the slope of the regression line between age and muscle mass was greater in men than women. The changes in muscle mass with advancing age were different in each part of the body. In the upper limbs, there was little change with advancing age in both men and women. In the lower limbs, the decrease in muscle mass began after two decades, with the reduction in this muscle mass the greatest of all parts of the body with advancing age. In the trunk, the slope of the regression line increased from the second to the fifth decade, after which the slope decreased.

These findings indicated that lower limb muscle mass was the first to begin to decrease and also showed the greatest decrease. For health promotion, it was seen as important to maintain muscle mass from at least middle age, with particular emphasis on the lower limbs.