We verified the hypothesis that the muscular phosphocreatine concentration (pre-transition [PCr]) before the onset of a moderate-intensity exercise sets the time course of the adjustment of oxidative metabolism.

To this aim, in 7 young active males (24±2 years), we measured breath by breath alveolar-to-capillary oxygen transfer kinetics during a sequence of 6 moderate intensity step transitions, each lasting 6 min. Pre-transition intramuscular [PCr] was manipulated by progressively reducing the recovery time, in random order, at 30 s, 60 s, 90 s, 120 s and 300 s. VO₂ data from the 6 repetitions of the same transition were time-aligned, interpolated at 1 s, combined in 5 s bins and fitted with a monoexponential model (after the exclusion of the initial 20s of data) to calculate the time constant of phase 2 (t₂). Changes [PCr], from a baseline value of 25 mmol•Kg^-1 of muscle were estimated based on the calculated oxygen deficit and on the subjects' anthropometrics.

Absolute variations of PCr concentration (D[PCr]) and pre-transition [PCr] increased linearly with recovery time up to 120 s (r²=0.94 and 0.98 respectively). On the contrary, t₂ VO₂ decreased with increased recovery time up to 120 s (r²=0.72). As a consequence, D[PCr] and pre-transition [PCr] were negatively correlated with t₂ (r²=0.47 and 0.57 respectively).

These results are in contrast with the conclusions drawn from a dynamical model of oxidative phosphorylation in skeletal muscle that suggested a linear positive relationship between t₂ and pre-transition [PCr] and D[PCr] during step transitions.