Human muscle energetics during voluntary and electrically induced isometric contractions as measured by P-31 NMR spectroscopy

Electrical stimulation (ES) and voluntary contraction (VC) were compared in the quadriceps muscle of ten male volunteers. In both modes, a workload co rresponding to 20% of maximal voluntary contraction was applied during 64 i sometric contraction (5.5 s)-relaxation (5.5 s) cycles. The protocols were performed in a 1.5T whole-body magnet. The Pi/PCr ratio and the intracellul ar pH (pHi) were monitored by P-31 NMR spectroscopy during baseline, exerci se and recovery periods, in a superficial region of the vastus medialis. Du ring baseline, the Pi/PCr ratio (0.12 vs. 0.10) and the pHi (7.01 vs. 7.00) were comparable in both conditions. During exercise, the Pi/PCr ratio was higher (0.36 vs. 0.14) and the pHi was lower (6.85 vs. 7.07) during ES than during VC. For the same external work production, these results reflect a different metabolic solicitation in the ES quadriceps than in the VC ones.