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Received July 30, 2007
Revised September 7, 2007
Accepted after revision October 23, 2007
1 University of Wisconsin-Madison
* To whom correspondence should be addressed. E-mail: amann{at}wisc.edu.
We asked whether the central effects of fatiguing locomotor muscle fatigue exert an inhibitory influence on central motor drive to regulate the total degree of peripheral fatigue development. Eight cyclists performed constant-workload pre-fatigue trials a) to exhaustion [83% of peak power output (Wpeak), 10 ± 1 min; PFT83%], and b) for the identical duration but at 67% Wpeak (PFT67%). Exercise-induced peripheral quadriceps fatigue was assessed via changes in potentiated quadriceps twitch-force (
Qtw,pot) from pre- to post-exercise in response to supra-maximal femoral nerve stimulation (Qtw,pot). On different days, each subject randomly performed three 5-km time-trials. First, subjects repeated PFT83% and the time-trial was started 4 min later with a known level of pre-existing locomotor muscle fatigue (Qtw,pot -36%) (PFT83%-TT). Second, subjects repeated PFT67% and the time-trial was started 4 min later with a known level of pre-existing locomotor muscle fatigue (Qtw,pot -20%) (PFT67%-TT). Finally, a control time-trial was performed without any pre-existing level of fatigue. Central neural drive during the three TTs was estimated via quadriceps electromyogram. Increases in pre-existing locomotor muscle fatigue from control time-trial to PFT83%-TT resulted in significant dose-dependent changes in central motor drive (-23%), power output (-14%), and performance time (+6%) during the time-trials. However, the magnitude of locomotor muscle fatigue following various time-trials was not different (Qtw,pot of -35 to -37%, P=0.35). We suggest that feedback from fatiguing muscle plays an important role in the determination of central motor drive and force output, so that the development of peripheral fatigue is confined to a certain level.
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