Journal of Applied Physiology
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J Appl Physiol (April 18, 2003). doi:10.1152/japplphysiol.00058.2003
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Submitted on January 21, 2003
Accepted on April 8, 2003

VO2max is Unaffected by Altering the Temporal Pattern of Stimulation Frequency in Rat Hindlimb In Situ

Russell T Hepple1*, Daniel J Krause2, Jason L Hagen3, and Cory C Jackson3

1 Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada; Faculty of Medicine, University of Calgary, Calgary, AB, Canada
2 Faculty of Medicine, University of Calgary, Calgary, AB, Canada
3 Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada

* To whom correspondence should be addressed. E-mail: hepple{at}ucalgary.ca.

It might be anticipated that fatiguing contractions would impair the aerobic metabolic response in skeletal muscle if significant fatigue developed prior to full activation of aerobic metabolism. Based upon this premise, we examined two groups of rats to test the hypothesis that a gradual increase in stimulation frequency would yield a higher maximal aerobic power (VO2max) than beginning immediately with an intense stimulation frequency, due to a slower progression of fatigue under the former conditions. In one group of animals the distal hindlimb muscles were electrically stimulated at a frequency of 60 tetani.min-1 for 4 min (F60; n = 6); in the other group the muscles were incrementally stimulated for 1 min at each of 7.5, 15, 30, and 60 tetani.min-1, and 2 min at 90 tetani.min-1 (FInc; n = 5). Despite large differences in rate of fatigue (time to 60% of initial force was 47 ± 3 vs 188 ± 1 s in F60 and FInc, respectively; mean ± SE) and the time at which VO2max occurred (120 ± 15 vs 264 ± 6 s), VO2max was not different (419 ± 24 vs 381 ± 44 µmol.min-1.100 g-1). Furthermore, time x tension integral at VO2max (3.82 ± 0.41 vs 4.07 ± 0.31 N.s) and peak lactate efflux (910 ± 45 vs 800 ± 98 µmol.min-1.100 g-1) were not different between groups. Thus, our results show that the more rapid progression of fatigue in F60 did not compromise the aerobic metabolic response in electrically stimulated rat hindlimb muscles. However, in both groups VO2 and lactate efflux declined after VO2max was attained in similar proportion to a further fall in force, suggesting that ongoing fatigue with intense contractions reduced ATP demand below that requiring maximal aerobic and glycolytic metabolic responses once VO2max was reached.




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