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Articles in PresS, published online ahead of print April 19, 2002
J Appl Physiol, 10.1152/jap.01010.2001
Submitted on October 3, 2001
Accepted on March 13, 2002
1 Department of Exercise Science, University of Massachusetts, Amherst, MA, USA
2 Department of Physical Therapy, University of Florida, Gainesville, FL, USA
3 Department of Physical Therapy, University of Delaware, Newark, DE, USA
* To whom correspondence should be addressed. E-mail: sbinder{at}udel.edu.
During an electrically-elicited, isometric contraction, the metabolic cost of attaining is greater than of maintaining force. Thus, the fatigue produced during such stimulation may not simply be a function of the force-time integral (FTI), as previously suggested. The goal of the present study was to evaluate the fatigue produced in the human medial gastrocnemius by intermittent, isometric electrical stimulation with trains of different frequencies (20, 40, or 80 Hz) and durations (300, 600, or 1200 ms), that produced different peak forces and force-time integrals. Each subject (n=10) participated in a total of 6 sessions. During each session, the subjects received a pre- and post-fatigue testing protocol and a different, 150-train fatiguing protocol. Each fatiguing protocol used only a single frequency and duration. The fatigue produced by the different protocols was correlated to the initial peak force of the fatiguing protocols (r2= 0.74-0.85), but not to the initial or total force time integral. All of the protocols tested produced a proportionately greater impairment of force in response to low- vs. high-frequency stimulation (i.e., low-frequency fatigue). There was no effect of protocol on low-frequency fatigue, suggesting that all the protocols produced comparable levels of impairment in excitation-contraction coupling. These results suggest that for brief stimulated contractions, peak force is a better predictor of fatigue than FTI, possibly due to the different metabolic demands of attaining and maintaining force.
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