Lack of functional effects of neuromuscular electrical stimulation on skeletal muscle oxidative metabolism in healthy humans.

A recent study has demonstrated that neuromuscular electrical stimulation (NMES) determines, in vitro, a fast-to-slow shift in the metabolic profile of muscle fibers. The aim of the present study was to evaluate if, in the same subjects, these changes would translate, in vivo, into an enhanced skeletal muscle oxidative metabolism. Seven young men were tested (cycle ergometer) during incremental exercises up to voluntary exhaustion and moderate and heavy constant-load exercises (CLE). Measurements were carried out before and after an 8-wk training program by isometric bilateral NMES (quadriceps muscles), which induced an 25% increase in maximal isometric force. Breath by breath pulmonary O2 uptake (V˙ O2) and vastus lateralis oxygenation indexes (by near-infrared spectroscopy) were determined. Skeletal muscle fractional O2 extraction was estimated by near-infrared spectroscopy on the basis of changes in concentration of deoxygenated hemoglobin + myoglobin. Values obtained at exhaustion were considered “peak” values. The following functional evaluation variables were unaffected by NMES: peak V ˙ O2; gas exchange threshold; the V ˙ O2 vs. work rate relationship (O2 cost of cycling); changes in concentration of deoxygenated hemoglobin + myoglobin vs. work rate relationship (related to the matching between O2 delivery and V ˙O2); peak fractional O2 extraction; V ˙ O2 kinetics (during moderate and heavy CLE) and the amplitude of its slow component (during heavy CLE). Thus NMES did not affect several variables of functional evaluation of skeletal muscle oxidative metabolism. Muscle hypertrophy induced by NMES could impair peripheral O2 diffusion, possibly counterbalancing, in vivo, the fast-to-slow phenotypic changes that were observed in vitro, in a previous work, in the same subjects of the present study.