Influence of initial metabolic rate on pulmonary O2 uptake on-kinetics during severe intensity exercise.

We hypothesised that the fundamental (Phase II) component of pulmonary oxygen uptake (VO(2)) kinetics would be significantly slower when step transitions to severe intensity cycle exercise were initiated from elevated baseline metabolic rates, and that this would be associated with evidence for a greater activation of higher-order (i.e. type II) muscle fibres. Seven male subjects (age 22-34 years) completed repeat step transitions to a severe (S) work rate, estimated to require 100% VO(2) peak, from a baseline of: (1) 3 min of unloaded cycling (L-->S); (2) 6 min of moderate exercise (M-->S); (3) 6 min of heavy exercise (H-->S). Pulmonary gas exchange and the electromyogram (EMG) of the m. vastus lateralis were measured throughout all exercise tests. The Phase II VO(2) kinetics became progressively slower at higher baseline metabolic rates (tau was 37 +/- 6, 59 +/- 23, and 93 +/- 50 s for L-->S, M-->S, and H-->S, respectively; P < 0.05 between L-->S and H-->S). Both the integrated EMG and the mean power frequency were significantly higher immediately before the step transition to severe exercise when it was initiated from higher metabolic rates. Although indirect, these data suggest that the slower Phase II VO(2) kinetics observed at higher baseline metabolic rates was related to alterations in muscle activation and fibre recruitment patterns.