A critical appraisal of methods to determine the Maximal Metabolic Steady State in cycling

This thesis aimed to examine the validity of different performance markers, namely the maximal lactate steady state (MLSS), functional threshold power (FTP) and critical power (CP), in representing the maximal metabolic steady state or the upper boundary of the heavy intensity domain from a physiological perspective. Study one (Chapter 3) reported that six out of thirteen participants’ blood lactate change between the 15th and 30th minute was below 1 mM (BLCΔ1530 < 1mM) when exercising 15 W above the MLSS (MLSS+15W) determined using the conventional protocol (BLCΔ1030 < 1 mM). It demonstrated that the conventional MLSS determination protocol failed to account for the delayed steady state in blood lactate as the intensity increased. Additionally, the overall VO2 response (VO2 kinetics and the amplitude of VO2 slow component; VO2sc) corresponding to MLSS and MLSS+15W matched the characteristics of the heavy intensity domain. Study two (Chapter 4) examined the VO2 and blood lactate response when exercising at and 15 W above the FTP (FTP+15W). The overall VO2 response (VO2 kinetics and the amplitude of VO2sc) corresponding to FTP and FTP+15W matched the characteristics of the heavy intensity domain. Conversely, the blood lactate appeared to stabilise when exercising at FTP but not during FTP+15W. Thus, there is an apparent dissociation between the blood lactate and VO2 kinetics, and the intensity corresponding to FTP does not represent the upper boundary of the heavy intensity domain. In study 3 (Chapter 5), it was demonstrated that VO2 only stabilised when exercised at the intensity corresponding to the CP but not CP+15W. In addition to failing to stabilise, the highest VO2 achieved during all CP+15W trials were not significantly different from the VO2peak. Moreover, the amplitude of VO2sc corresponding to all CP+15W trials was significantly higher than exercising at CP. The validity of the 2-parameter CP model in calculating the time to task failure (TTF) at CP+15W was also examined. The actual TTF at three CP+15W trials was significantly shorter than the predicted TTF. However, the actual TTF was not significantly different from the predicted TTF corresponding to CP+15W after one week of HIT training. Therefore, the CP is a valid representation of the upper boundary of the heavy intensity domain, but its validity in calculating the constant severe intensity exercise performance remains unclear. In conclusion, the present thesis provides scientific evidence that blood lactate cannot accurately reflect the VO2 kinetics. Although there are some limitations of each maker, CP is the only marker examined in the present thesis that can be considered a valid representation of the MMSS or the upper boundary of the heavy intensity domain.