Influence of priming exercise on muscle [PCr] and pulmonary O2 uptake dynamics during ‘work-to-work’ knee-extension exercise

Metabolic transitions from rest to high-intensity exercise were divided into two discrete steps (i.e., rest-to-moderate-intensity (R → M) and moderate-to-high-intensity (M → H)) to explore the effect of prior high-intensity ‘priming’ exercise on intramuscular [PCr] and pulmonary V˙O2 kinetics for different sections of the motor unit pool. It was hypothesized that [PCr] and V˙O2 kinetics would be unaffected by priming during R → M exercise, but that the time constants (τ  ) describing the fundamental [PCr] response and the phase II V˙O2 response would be significantly reduced by priming for M → H exercise. On three separate occasions, six male subjects completed two identical R → M/M → H ‘work-to-work’ prone knee-extension exercise bouts separated by 5 min rest. Two trials were performed with measurement of pulmonary V˙O2 and the integrated electromyogram (iEMG) of the right m. vastus lateralis. The third trial was performed within the bore of a 1.5-T superconducting magnet for 31P-MRS assessment of muscle metabolic responses. Priming did not significantly affect the [PCr] or V˙O2τ during R → M ([PCr] τ Unprimed: 24 ± 16 vs. Primed: 22 ± 14 s; V˙O2τ Unprimed: 26 ± 8 vs. Primed: 25 ± 9 s) or M → H transitions ([PCr] τ Unprimed: 30 ± 5 vs. Primed: 32 ± 7 s; V˙O2τ Unprimed: 37 ± 5 vs. Primed: 38 ± 9 s). However, it did reduce the amplitudes of the [PCr] and V˙O2 slow components by 50% and 46%, respectively, during M → H (P < 0.05 for both comparisons). These effects were accompanied by iEMG changes suggesting reduced muscle fiber activation during M → H exercise after priming. It is concluded that the τ   for the initial exponential change of muscle [PCr] and pulmonary V˙O2 following the transition from moderate-to-high-intensity prone knee-extension exercise is not altered by priming exercise.