Skeletal muscle function and hypertrophy are diminished in old age.

Muscle loss occurs during aging. To investigate whether the hypertrophic response is attenuated at old age, we used male Fischer 344 (26 months old; n = 5) and Fischer 344 x Brown Norway rats (6, 9, and 33 months old; n = 8, 10, and 6, respectively). Hypertrophy of the left plantaris muscle was induced by surgical denervation of its agonists. The right leg served as control. The mass and maximal tetanic force (P(0)) of control muscles declined by approximately 30% between 9 and 26 months (P < 0.05). Fatigue resistance during intermittent isometric contractions was reduced by approximately 60% at 33 months. At 33 months, the attenuated hypertrophy was accompanied by a decrease rather than an increase in P(0). Yet, hypertrophy was accompanied by a 25% rise in fatigue resistance at all ages (P = 0.001). Thus, aging is associated with a decline in muscle mass and function. In addition, at advanced age, the hypertrophic response is reduced and a hypertrophic stimulus even aggravates the age-associated muscle weakening.