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) over dot(O2) kinetics for different sections of the motor unit pool. It was hypothesized that [PCr] and (V) over dot(O2) 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 (V) over dot(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) over dotO(2) 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 P-31-MRS assessment of muscle metabolic responses. Priming did not significantly affect the [PCr] or (V) over dot(O2) tau during R -> M ([PCr] tau Unprimed: 24 +/- 16 vs. Primed: 22 +/- 14 s; (V) over dot(O2) tau Unprimed: 26 +/- 8 vs. Primed: 25 +/- 9 s) or M -> H transitions ([PCr] tau Unprimed: 30 +/- 5 vs. Primed: 32 +/- 7 s; (V) over dot(O2) tau Unprimed: 37 +/- 5 vs. Primed: 38 +/- 9 s). However, it did reduce the amplitudes of the [PCr] and (V) over dot(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 tau for the initial exponential change of muscle [PCr] and pulmonary (V) over dot(O2) following the transition from moderate-to-high-intensity prone knee-extension exercise is not altered by priming exercise. (C) 2010 Elsevier B.V. All rights reserved.