Purpose: The purpose of this study was to determine if %Type I fibers and/or aerobic fitness (as peak .VO(2)) would predict Delta efficiency (DeltaEff) and Delta.VO(2)/Deltawork rate (WR) for moderate (below lactate threshold <LT) constant work rate exercise in subjects with diverse level of fitness and fiber type.
Methods: Twenty-two subjects (15 male, 23 +/- 6 yr) heterogenous for fitness (peak .VO(2): 43.9 +/- 7.1 mL.kg(-1).min(-1)) were tested. On 3 different days, each subject performed constant load exercise for 6 min at work rates corresponding to 30, 50, 70, and 90% LT, separated by 6 min of 10% LT pedaling. After a short rest, each subject then completed 8 min of exercise at a WR corresponding to 50% of the difference between LT and peak .VO(2) (Delta50%). The mean .VO(2) determined for the last 3 min of each <LT WR was regressed against the absolute WR for each subject and trial.
Results: The group mean Delta.VO(2)/DeltaWR was 10.1 mL.min(-1).W(-1) (range 8.6-11.2 mL.min(-1).W(-1)), whereas the mean DeltaEff was 28.0% (range 24.5-32.0%); both were significantly inversely correlated with each other (r = 0.972, P < 0.0001). DeltaEff was significantly negatively, and Delta.VO(2)/DeltaWR positively, correlated with peak .VO(2)(r = -0.51 and 0.57, respectively; both P < 0.01), but not to % Type I fibers (r = 0.01 and 0.11, P > 0.05).
Conclusion: These results suggest that aerobic fitness affects the energetic response to changes in power output during moderate exercise, such that the more aerobically fit a subject, the greater the increase in oxygen cost (.VO(2)) (reduced efficiency) as work rate increases. Further, Delta.VO(2)/DeltaWR reflects the inverse of DeltaEff for moderate-intensity exercise in healthy fed subjects.