The effects of fitness status and exercise intensity, on VO2 Kinetics, within the Moderate-intensity Domain

This study investigated the effects of fitness status and exercise intensity, on oxygen-uptake (V O2) kinetics, within the moderate-intensity m . g ‘ g - ’ (mean ± standard deviation: age: 29 ± 5 years; height: 181.7 ± 5.6 cm; body-mass: 76.7 ± 9.2 kg; V O2peak: 58.2 ± 3.3 mL.kg-1.min.-1) and four ‘low- ’ p p s initially volunteered. However, as the low-fitness group were unable to perform all of the intensity transitions, a further eight ‘m te-fitness’ individuals (age: 28 ± 3 years; height: 177.4 ± 7.2 cm; body-mass: 82.5 10.0 g; V O2peak: 44.7 ± 3.4 mL.kg-1.min.-1) were recruited (statistical comparisons were only made between the high- and moderate-fitness groups). Using a cycle ergometer, the participants performed either three (high-fitness) or four (moderate-fitness) square-wave exercise transitions from unloaded pedalling to each of the desired intensities (70% gas exchange threshold [GET], 82.5% GET, and 95% GET). There was no suggestion that either group consistently displayed an overshoot in V O2 at exercise onset. Therefore, a mono-exponential function was used to model the data. A 2 x 3 mixed ANOVA was then used to assess the effects of fitness status and exercise intensity on the phase II time constant () and the functional gain (G). The results displayed no evidence of an intensity by fitness status interaction (F(2,28) = 2.019, p = 0.152; G: F(2,28) = 0.01, p = 0.990) or a main effect of fitness status (F(1,14) = 0.197, p = 0.664; G: F(1,14) = 1.000, p = 0.312). However, there was a significant main effect of exercise intensity for both variables (F(2,28) = 7.226, p = 0.003; G: F(2,28) = 5.931, p = 0.007). Post hoc analyses revealed a significant difference in and G between the 70% GET and 95% GET transitions (mean difference = 3.1 s; 95% likely range 1.6 to 4.6 s; G: mean difference = 1.1 mL.min-1.W-1; 95% likely range 0.3 to 1.8 mL.min-1.W-1). These findings provide evidence that within the moderate-intensity domain the V O2 response does not conform to linear first-order kinetics. The accompanying increase in G alongside also adds some support for the suggestion that a hierarchical muscle-fibres recruitment pattern may exist.