Abstract
PurposeTo examine quadriceps muscle fatigue and central motor output during fatiguing single joint exercise at 40% and 80% maximal torque output in resistance trained men.MethodTen resistance trained men performed fatiguing isometric knee extensor exercise at 40% and 80% of maximal torque output. Maximal torque, rate of torque development, and measures of central motor output and peripheral muscle fatigue were recorded at two matched volumes of exercise, and after a final contraction performed to exhaustion. Central motor output was quantified from changes in voluntary activation, normalized surface electromyograms (EMG), and V-waves. Quadriceps muscle fatigue was assessed from changes in the size and shape of the resting potentiated twitch (Q.pot.tw). Central motor output during the exercise protocols was estimated from EMG and interpolated twitches applied during the task (VAsub).ResultsGreater reductions in maximal torque and rate of torque development were observed during the 40% protocol (p<0.05). Maximal central motor output did not change for either protocol. For the 40% protocol reductions from pre-exercise in rate and amplitude variables calculated from the Q.pot.tw between 66.2 to 70.8% (p<0.001) exceeded those observed during the 80% protocol (p<0.01). V-waves only declined during the 80% protocol between 56.8 ± 35.8% to 53.6 ± 37.4% (p<0.05). At the end of the final 80% contraction VAsub had increased from 91.2 ± 6.2% to 94.9 ± 4.7% (p = 0.005), but a greater increase was observed during the 40% contraction where VAsub had increased from 67.1 ± 6.1% to 88.9 ± 9.6% (p<0.001).ConclusionMaximal central motor output in resistance trained men is well preserved despite varying levels of peripheral muscle fatigue. Upregulated central motor output during the 40% contraction protocol appeared to elicit greater peripheral fatigue. V-waves declines during the 80% protocol suggest intensity dependent modulation of the Ia afferent pathway.
Highlights
IntroductionMechanisms of muscular fatigue are generally considered from peripheral factors associated with maintaining muscle contraction (e.g. blood flow, oxygen delivery, contraction efficiency), and factors associated with maintaining central motor output to the muscle from the nervous system (e.g. cortical and motoneuron output) [1,2]
Mechanisms of muscular fatigue are generally considered from peripheral factors associated with maintaining muscle contraction, and factors associated with maintaining central motor output to the muscle from the nervous system [1,2]
Maximal central motor output in resistance trained men is well preserved despite varying levels of peripheral muscle fatigue
Summary
Mechanisms of muscular fatigue are generally considered from peripheral factors associated with maintaining muscle contraction (e.g. blood flow, oxygen delivery, contraction efficiency), and factors associated with maintaining central motor output to the muscle from the nervous system (e.g. cortical and motoneuron output) [1,2]. Research investigating whether or not the relative contraction intensity (e.g. high or low) of an exercise protocol elicits a differential effect on this relationship is contradictory [3,4,5,6] All of these studies tested relatively untrained healthy or university student populations, limiting the generalizability to resistance trained populations where fatiguing exercise at high and low contraction intensities are routinely performed. This demographic is of particular interest for examination of, and comparison between, different contraction intensities during fatiguing exercise considering the chronic adaptation unique to the nervous system of resistance trained individuals. Investigating the interaction between peripheral muscle fatigue and central motor output in a resistance trained population during such exercise may help clarify the discrepancy between studies previously investigating the influence of contraction intensity [3,4,5,6]
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