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In general, as people grow older, their muscle fibers shrink in number and in size (atrophy) and become less sensitive to messages from the central nervous system. This contributes to a decrease in strength, balance, and coordination. Although there is no question that people experience at least some of these declines at about age 40, the extent to which they occur depends on a number of factors, including genetics, diet, smoking and alcohol use, and -- most important -- physical activity level. Indeed, recent research has indicated that inactivity is responsible for the majority of age associated muscle loss. Fortunately, resistance exercise can reverse much of this decline by increasing the size of shrunken muscle fibers.