Energetics of muscular exercise at work onset: the steady-state approach.

The energetics of muscular exercise at steady state is tightly coupled to, and dependent on, the events that occurred during the transient phase, of which the steady-state is therefore the "memory". The aim of the present study is to show that it is possible to utilize data obtained at exercise steady state to gain information on variables traditionally assessed during exercise transients. A theoretical model based on the steady-state relationships between mechanical power, O(2) uptake (VO(2)) and phosphocreatine (PC) split allows us to highlight three interdependent parameters: the time constant of the VO(2) on response at the muscular level (tau), the mechanical equivalent of PC splitting and the P/O(2) ratio. The model was applied to experimental data obtained during moderate calf exercise in humans inside an MR unit. For a P/O(2) from 5.6 to 6.2, the obtained tau values range from 10.6 to 24.9 s (for PC concentrations from 17.8 to 37.7 mmol/kg fresh muscle), and the corresponding mechanical equivalents of PC from 9.6 to 10.6 J/mmol. The analysis also shows that the time constant of the O(2) uptake kinetics is strictly dependent on the PC concentration at rest, whereas the mechanical equivalent of PC is unaffected by its concentration.