V(O2) max is unaffected by altering the temporal pattern of stimulation frequency in rat hindlimb in situ.
It might be anticipated that fatiguing contractions would impair the aerobic metabolic response in skeletal muscle if significant fatigue developed before full activation of aerobic metabolism. On the basis of this premise, we examined two groups of rats to test the hypothesis that a gradual increase in stimulation frequency would yield a higher maximal O2 uptake (Vo2 max) than beginning immediately with an intense stimulation frequency because of 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 for 4 min (F60; n = 6 rats); in the other group, the muscles were incrementally stimulated for 1 min at each of 7.5, 15, 30, and 60 tetani/min and for 2 min at 90 tetani/min (FInc; n = 5 rats). Despite large differences in rate of fatigue [time to 60% of initial force was 47 +/- 3 (SE) vs. 188 +/- 1 s in F60 and FInc, respectively] and the time at which Vo2 max occurred (120 +/- 15 vs. 264 +/- 6 s), Vo2 max was not different (419 +/- 24 vs. 381 +/- 44 micromol x min-1. 100 g-1). Furthermore, time x tension integral at Vo2 max (3.82 +/- 0.41 vs. 4.07 +/- 0.31 N. s) and peak lactate efflux (910 +/- 45 vs. 800 +/- 98 micromol x 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, O2 uptake and lactate efflux declined after Vo2 max 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 Vo2 max was reached.
Hepple
Russell T
RT
Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada T2N 1N4. hepple@ucalgary.ca
Krause
Daniel J
DJ
Hagen
Jason L
JL
Jackson
Cory C
CC
D000818:Animals; D004558:Electric Stimulation / Q000379:methods; D006614:Hindlimb; D066298:In Vitro Techniques; D019344:Lactic Acid / Q000378:metabolism; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*; D010477:Perfusion; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D019344:Lactic Acid