Sigmoidal VO2 on-kinetics: A new pattern in VO2 responses at the lower district of extreme exercise domain

The aim of the study was to analyse the VO2 on-kinetics belonging to the work rates within the lower district of extreme exercise domain. Maximal O-2 utilisation and peak power outputs of eight well-trained cyclists were revealed by multisession trails. Critical threshold (CT) as the lower boundary of severe domain and aerobic limit power (ALP) as the upper boundary of severe domain were determined by multisession constant-load exercises. VO2 on-kinetics over time were best fitted by multicomponent exponential models described by an initial concave-up response known as cardio-dynamic phase (tau = 18.2 +/- 2.88 s; alpha = 1.56 + 0.39 L.min(-1)) before a primary concave-up phase (tau = 35.4 +/- 12.4 s; alpha = 1.53 +/- 0.36 L.min(-1)), and then a slow component in two of the participants (tau = 80.8 +/- 37 s; alpha = 0.47 +/- 0.05 L.min(-1)) or without a slow component in six of the participants during exercises performed at 50 W above the CT (R-2 >= 0.96; SEE <= 0.24; p < 0.001). However, VO2 on-kinetics over time belonging to exercises performed at 50 W above the ALP were best fitted by sigmoidal model (R-2 >0.98; SEE < 0.14; p < 0.001) in comparison with linear (R-2 = 0.37-0.66; SEE = 0.46-0.64; p < 0.01), or exponential functions (p> 0.05). Indeed, during those exercises, a short period of convex-up response = 16.8 +/- 3.1 s; alpha = 1.72 + 0.39 L.min(-1)) was determined just before a concave-up primary phase in VO2 over time (r = 24.6 +/- 5.86 s; alpha = 1.31 +/- 0.20 L.min(-1)). It was shown that multicomponent exponential trend in VO2 transformed into a sigmoidal shape, once the work rate exceeded the upper boundary of severe exercise domain.