Sarcopenia is the most obvious beneficiary of exercise. Because muscles are costly (right now, you are spending about one out of five calories simply maintaining your muscles73), they are the classic example of the “Use It or Lose It” principle of energy allocation. When we demand more from our muscles, especially contractions involving resistance, we activate genes that increase the size of fibers as well as repair and maintain muscle cells. As soon as we stop using them, muscles dwindle. Thus although aging affects hormone levels and nerve properties that inexorably diminish strength,
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