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1 Department of Exercise Science, University of Massachusetts, Amherst, MA, USA
* To whom correspondence should be addressed. E-mail: janekb{at}excsci.umass.edu.
It has been suggested that the effects of old age on the ability to resist fatigue may be task-dependent. To test one aspect of this hypothesis, we compared the neuromuscular responses of 9 young (26 ± 4 years, mean ± SD) and 9 older (72 ± 4) healthy, relatively sedentary men to intermittent isometric (3 min, 5s contract/ 5s rest) and dynamic (90 @ 90°/s) maximum voluntary contractions (MVC) of the ankle dorsiflexor muscles. To assess the mechanisms of fatigue (defined as MVCpost/MVCpre), we also measured isometric central activation ratios (CAR), tetanic torque, contractile properties and compound muscle action potentials before and immediately after exercise. Because dynamic contractions are more neurally complex and metabolically demanding than isometric contractions, we expected an age-related fatigue resistance observed during isometric exercise to be absent during dynamic exercise. In contrast, older men (O) fatigued less than young (Y) during both isometric (O = 0.77 ± 0.07, Y = 0.66 ± 0.02, mean ±
SE, P 
0.01) and dynamic (O = 0.45 ± 0.07, Y = 0.27 ± 0.02, P = 0.04) contractions, with no evidence of peripheral activation failure in either group. We observed no obvious limitations in central activation in either group, as assessed using isometric CAR methods, following both isometric and dynamic contractions. Pre-exercise half-time of tetanic torque relaxation, which was longer in O compared with Y, was linearly associated with fatigue resistance during both protocols (r = 0.62 and 0.66, p 0.004, n = 18). These results suggest that relative fatigue resistance is enhanced in older adults during both isometric and isokinetic contractions, and that age-related changes in fatigue may be due largely to differences within the muscle itself.
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