Neural and muscular contributions to the age-related loss in power of the knee extensors in men and women
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- 450{degrees}/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{middle dot}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{middle dot}year-1 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. NEW & NOTEWORTHYThe accelerated age-related loss in power relative to the loss in muscle mass of the knee extensors was primarily due to factors altering the contractile properties of the muscle for both old and very old ([≥]80 yr) adults. The mechanisms for the decrements in power with aging appear largely similar for men and women, although neural factors may play more of a role in older women.