Mechanisms For The Age-related Loss In Power Of The Knee Extensors In Men And Women

Abstract

The mechanisms for the loss in limb muscle power output in old (60-79 years) and very old adults (≥80 years) and whether these mechanisms differ between older men and women are not well-understood. PURPOSE: Compare peak power output of the knee extensor muscles between young, old, and very old adults and determine the physiological mechanisms for the age-related loss of power in men compared with women. METHODS: 31 young (22.9 ± 3.0 years, 16 men), 83 old (70.4 ± 4.9 years, 44 men), and 16 very old adults (85.8 ± 4.2 years, 7 men) performed maximal isokinetic contractions at 15 different velocities (0 - 450°/s) to identify peak power output across the torque-velocity relationship. Voluntary activation (VA) and contractile properties were assessed with transcranial magnetic stimulation and electrical stimulation of the femoral nerve. RESULTS: Old and very old men and women generated less power than young across all velocities (p < 0.01). Compared with young men (673 ± 41 W), peak power output was ~42% lower in old (399 ± 18 W, p < 0.01) and ~ 67% lower in very old (225 ± 19 W, p < 0.01). For women, peak power output was ~49% lower in old (223 ± 9 W, p < 0.01) and ~ 60% lower in very old (170 ± 15 W) compared with young (428 ± 20 W, p < 0.01). This corresponded with an age-related loss in peak power output of ~6.5 W/year for men (R2 = 0.62, p < 0.01) and ~ 4.2 W/year for women (R2 = 0.77, p < 0.01). Factors within the muscle were the most closely associated variables with peak power for both sexes including; thigh lean tissue mass (men: R2 = 0.53, p < 0.01; women: R2 = 0.34, p < 0.01), potentiated twitch amplitude (men: R2 = 0.60, p < 0.01; women: R2 = 0.55, p < 0.01), and rate of torque development of the potentiated twitch (men: R2 = 0.60, p < 0.01; women: R2 = 0.48, p < 0.01). In contrast, VA was weakly associated with peak power output for women (R2 = 0.13, p = 0.01) but not for men (R2 = 0.03, p = 0.18), and muscle activation (relative EMG amplitude) of the vastus lateralis during the maximal power contraction was not associated with peak power output for either men (R2 = 0.01, p = 0.35) or women (R2 = 0.04, p = 0.11). CONCLUSION: 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 men and women, and that impaired neural activation may play a minor role in the loss in power output for women.