ENHANCED FATIGUE RESISTANCE WITH AGE IS EVIDENT DURING ISOMETRIC AND DYNAMIC TASKS

Abstract

There may be a task-dependence for the fatigue resistance that has been observed in older vs. younger adults. For example, age-related neuromuscular changes (slower contractile properties, lower motor unit firing rates) may disadvantage older subjects during dynamic exercise because of the velocity-dependence and complexity of the dynamic task. PURPOSE We hypothesized that older vs. younger subjects would fatigue 1) less during isometric exercise and 2) more during dynamic exercise. As both central and peripheral activation failure have been suggested as mechanisms of fatigue in older adults, these were assessed before and after each protocol. METHODS 9 young (26.3 ±4.4 years, mean ±SD) and 9 older (72 ±4.4) healthy, relatively sedentary men completed two protocols: 1) isometric (3 min, 50% duty cycle) and 2) dynamic (90 @ 90°/s) MVCs of the ankle dorsiflexors. Fatigue=MVCpost/MVCpre. The completeness of central activation was measured using a superimposed stimulus during an isometric MVC. Peripheral excitability was assessed by changes in the amplitude and area of the M-wave. RESULTS Older subjects fatigued less than young during the isometric task (O=0.77, Y=0.66, p < 0.01) and dynamic exercise (O=0.45, Y=0.27, P=0.04). There was no evidence of central or peripheral activation failure in either group. CONCLUSIONS Older subjects fatigued less than young subjects during isometric exercise, and this fatigue resistance was observed during the dynamic task as well. Neither central nor peripheral activation failure appeared to play a role in fatigue in this study. These data suggest that the mechanisms responsible for the age differences in muscle fatigue lie distal to the muscle membrane, and that age-related neuromuscular changes may enhance fatigue resistance during a variety of tasks.