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

Abstract

The mechanisms for the loss in limb muscle power in old (60-79 years) and very old (>80 years) adults and whether the mechanisms differ between men and women are not well-understood. We compared maximal 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 loss of power. 31 young (22.9+/-3.0 years, 15 women), 83 old (70.4+/-4.9 years, 39 women), and 16 very old adults (85.8+/-4.2 years, 9 women) performed maximal isokinetic contractions at 14 different velocities (30-450deg/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 power was ~6.5 W/year for men (R2=0.62, p<0.001), which was a greater rate of decline (p=0.002) than the ~4.2 W/year for women (R2=0.77, p<0.001). Contractile properties were the most closely associated variables with power output 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), whereas neuromuscular activation (EMG amplitude) during the maximal power contraction was not associated with power in men (p=0.347) or women (p=0.106). These data suggest that the age-related loss in 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.