Introduction: Aerobic fitness and oxygen uptake kinetics (tVO2) at the onset of exercise appear to be inversely correlated, however, the mechanisms underlying changes in tVO2 across different levels of aerobic fitness have not been elucidated. The purpose of this study was to investigate the relationship between maximal VO2 (VO2max) and tVO2 and determine whether the capacity to deliver or to utilize O2 limits tVO2 in an aerobic fitness dependent manner. Methods: Twenty-three healthy, young males (25 ± 4 years) with a VO2max classified as superior (S; VO2max > 60 mL·kg-1·min-1, n = 7), good (G; VO2max = 45-55 mL·kg-1·min-1, n = 8) or poor (P; VO2max < 40 mL·kg-1·min-1, n = 8) performed two moderate-intensity knee-extension (KE) exercise transitions (80% of gas exchange threshold) on a custom-built KE ergometer. VO2 was measured breath-by-breath. Leg blood flow (BF) was measured by doppler ultrasound at the femoral artery, and leg vascular conductance (LVC) was calculated as BF·mean arterial pressure (MAP)-1. Near-infrared spectroscopy derived-[HHb] was measured on the vastus lateralis muscle. tVO2, tLVC, and t[HHb] data were averaged and fit with a mono-exponential function. Results: tVO2 was faster in the S (P < 0.01) and G (P < 0.05) fitness groups compared with the P fitness group. t[HHb] was faster in the S (P < 0.05) compared with the P fitness group. VO2max was inversely correlated to tVO2 (r = -0.71; P < 0.001) and t[HHb] (r = -0.55; P < 0.01), but not with tLVC (r = -0.12; P > 0.05). tVO2 was positively correlated with t[HHb] (r = -0.57; P < 0.01), but not with tLVC (r = -0.25; P > 0.05). Conclusion: VO2max and tVO2 were inversely correlated across fitness levels. These findings indicate that O2 delivery is not rate-limiting for tVO2 across fitness levels and suggest that the intracellular capacity to utilize O2 may be the primary limiting factor for tVO2 in healthy young adults, regardless of aerobic fitness.
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