ACUTE EFFECTS OF RESISTANCE EXERCISE ON MUSCLE PROTEIN-SYNTHESIS RATEIN YOUNG AND ELDERLY MEN AND WOMEN

Muscle mass and function are improved in the elderly during resistance exercise training. These improvements must, result from alterations i n the rates of muscle protein synthesis and breakdown. We determined t he rate of quadriceps muscle protein synthesis using the in vivo rate of incorporation of intravenously infused [C-13]leucine into mixed-mus cle protein in both young (24 yr) and elderly (63-66 yr) men and women before and at the end of 2 wk of resistance exercise training. Before training, the fractional rate of muscle protein synthesis was lower i n the elderly than in the young (0.030 +/- 0.003 vs. 0.049 +/- 0.004 % /h; P = 0.004) but increased (P < 0.03) to a comparable rate of muscle protein synthesis in both young (0.075 +/- 0.009 %/h) and elderly sub jects (0.076 +/- 0.011 %/h) after 2 wk of exercise. In the elderly, mu scle mass, 24-h urinary 3-methylhistidine and creatinine excretion, an d whole body protein breakdown rate determined during the [C-13]leucin e infusion were not changed after 2 wk of exercise. These finding demo nstrate that, during the initial phase of a resistance exercise traini ng program, a marked increase in quadriceps muscle protein synthesis r ate occurs in elderly and young adults without an increase in the rate of whole body protein breakdown. In the elderly, this was not accompa nied by an increase in urinary 3-methylhistidine excretion, an index o f myofibrillar protein breakdown.