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The Mystery of Muscle Fatigue

Posted by Matt Fitzgerald on Jan 2, 2008 1:54:52 PM


Previously here I have reported on research demonstrating that exercise fatigue is caused not by "catastrophic" events within the muscles themselves but instead by reduced motor output from the brain to the working muscles, which contribute to fatigue by providing feedback indicating local, peripheral fatigue to the brain. An elegant new study conducted by researchers at the University of Wisconsin and published in the Journal of Physiology provides some of the best proof yet of this increasingly undeniable fact.


Eight trained cyclists performed 5K time trials on three separate occasions. Before one time trial they pre-fatigued their legs by riding to exhaustion at 83% of peak power and then began the time trial after four minutes of rest. Before a second time trial they rode at 67% of peak power for a duration equal to that achieved in the ride to exhaustion at 83% of peak power and rested for four minutes (leaving them fatigued but less fatigued than in the other trial). The third time trial was performed in a fresh state. The researchers used EMG sensors to determine the level of motor output from the brain to the quadriceps muscles during each time trial and also measured power output and finishing time.


Compared to the fresh time trial, central motor drive was reduced by 23%, power output was reduced by 14%, and finishing time increased by 6% in the time trial that followed the exhaustive pre-fatiguing ride. The loss of motor output, power, and performance was smaller but still significant in the other, non-exhaustive pre-fatigued time trial. Interestingly, the quadriceps muscles exhibited precisely the same level of fatigue (as measured by a test of maximal twitch force) after all three time trials. The study's authors concluded, "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."



The ice is getting thin underneath the feet of those who continue to cling to the conservative vew that exercise fatigue is caused strictly by physiological events occuring from the neck down!



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