Is Hypertrophy Limited in Elderly Muscle Fibers? A Comparison of Elderly and Young Strength-Trained Men

To investigate the capacity to hypertrophy and the satellite cell populations to change with age, the nucleo-cytoplasmic relationships and satellite cells were compared in skeletal muscles of young and elderly men before and after strength training. Vastus lateralis muscle biopsies were taken before and after 8 (young) or 16 weeks (elderly men) of strength training and compared to muscle from untrained men. The young men had more myonuclei in type II fibers than in type I, and the cytoplasm per nucleus was smaller. As muscle fibers hypertrophied with strength training, the larger cross-sectional area was matched by increasing nuclear numbers, maintaining a constant nucleus-to-cytoplasm ratio. The numbers of myonuclei were similar in elderly and young muscles, and strength training increased both, but not significantly. Myonuclear number was related to cross-sectional area in young untrained and trained muscles, but no relationship was found in untrained elderly men. With training, this relationship was restored in the elderly muscles. The relative increase in cross-sectional area in elderly muscles was similar to the young, but the elderly muscles began with such small fibers, that the hypertrophied fibers in the trained elderly muscles reached the size of untrained young muscle fibers. The results suggest that the increase in myonuclear number accompanies and is proportional to fiber hypertrophy in the young, but it is still unclear to what extent this occurs in the elderly. The percentage of satellite cells did not differ between young and elderly muscles. These satellite cells, responsible for contributing to myonuclear increase, did not decrease in numbers with aging.