Neural and muscular contributions to the age-related differences in peak power of the knee extensors in men and women.

Abstract

The mechanisms for the loss in limb muscle power output in old (60-79years) and very old (≥80years) adults and whether the mechanisms differ between men and women are not well-understood. We compared maximal peak power of the knee extensor muscles between young, old, and very old men and women and identified the neural and muscular factors contributing to the age-related differences in power. 31 young (22.9±3.0years, 15women), 82 old (70.3±4.9years, 38women), and 16 very old adults (85.8±4.2years, 9women) performed maximal isokinetic contractions at 14 different velocities (30-450°/s) to identify peak power. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation to the motor cortex and electrical stimulation of the femoral nerve. The age-related loss in peak power was ~6.5 W·year^-1 for men (R²=0.62, p<0.001), which was a greater rate of decline (p=0.002) than the ~4.2 W·year^-1 for women (R²=0.77, p<0.001). Contractile properties were the most closely associated variables with peak power for both sexes, such as the rate of torque development of the potentiated twitch (men: R²=0.69, p<0.001; women: R²=0.57, p<0.001). VA was weakly associated with power in women (R²=0.13, p=0.012) but not men (p=0.191). Similarly, neuromuscular activation (rates of EMG rise) during the maximal power contraction was associated with power in women (R²=0.07, p=0.042) but not men (p=0.456). These data suggest that the age-related differences in maximal peak power of the knee extensor muscles is due primarily to factors within the muscle for both sexes, although neural factors may play a minor role in older women.