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 VO₂ kinetics for different sections of the motor unit pool. It was hypothesized that [PCr] and VO₂ kinetics would be unaffected by priming during R-->M exercise, but that the time constants (tau) describing the fundamental [PCr] response and the phase II VO₂ 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 5min rest. Two trials were performed with measurement of pulmonary VO₂ 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 (31)P-MRS assessment of muscle metabolic responses. Priming did not significantly affect the [PCr] or VO₂ tau during R-->M ([PCr] tau Unprimed: 24+/-16 vs. Primed: 22+/-14s; VO₂ tau Unprimed: 26+/-8 vs. Primed: 25+/-9s) or M-->H transitions ([PCr] tau Unprimed: 30+/-5 vs. Primed: 32+/-7s; VO₂ tau Unprimed: 37+/-5 vs. Primed: 38+/-9s). However, it did reduce the amplitudes of the [PCr] and VO₂ 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 tau for the initial exponential change of muscle [PCr] and pulmonary VO₂ following the transition from moderate-to-high-intensity prone knee-extension exercise is not altered by priming exercise.