Increased Fatigability Of Older Women Performing High-velocity Contractions Is Explained By Mechanisms Within The Muscle

Abstract

Older adults are often more fatigable than young adults during high-velocity concentric contractions. This age-related difference in fatigability, however, has been studied primarily in men, and the contribution of supraspinal mechanisms are unknown. PURPOSE: The purpose of this study was to quantify the changes in voluntary activation and contractile properties of the knee extensors in response to high-velocity dynamic contractions in 12 young (22.5 ± 0.6 [SE] yrs) and 18 old women (71.7 ± 1.5 yrs). METHODS: Subjects performed 80 maximal velocity concentric contractions (1 every 3 s) with a load set at 20% of the maximal voluntary isometric contraction (MVC). Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were used to measure voluntary activation and the contractile properties, respectively, before and immediately (<10 s) following the fatiguing task. RESULTS: Old women generated 55% less power (P<0.01) and 43% lower isometric MVC torque (P<0.01) than the young women. At the end of the fatiguing task, power was reduced by 35 ± 4.7% in the old compared to 20 ± 3.7% in the young (P=0.02). The reduction in MVC torque, however, was similar for the old and young (25 ± 2.5% vs 22 ± 1.6%, respectively, P=0.28). Voluntary activation was similar between the old and young (97 ± 0.7% vs 97 ± 0.6%, respectively, P=0.90) before fatigue, but was reduced only in the old after the fatiguing task by 3 ± 1.1% (P=0.04). Potentiated twitch amplitude was reduced by 30 ± 4.7% in the old compared with 13 ± 5.4% in the young (P=0.02). Similarly, twitch half-relaxation times increased more in the old compared with the young women (72 ± 12.8% vs 32 ± 9.4%, respectively, P=0.02). CONCLUSIONS: Both a loss in voluntary activation from supraspinal centers and fatigue within the muscle are responsible for the greater loss in power during high-velocity dynamic contractions in old compared with young women. However, the large age-related reductions in the contractile properties indicate that muscular mechanisms are primarily responsible for the greater loss in power during knee extension in the old compared with the young women. Supported by NIA (R21 AG045766) to SK Hunter.