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 males were tested (cycle-ergometer) during incremental exercises (IE) up to voluntary exhaustion, and moderate- and heavy- constant-load exercises (CLE). Measurements were carried out before and after a 8-week training program by isometric bilateral NMES (quadriceps muscles), which induced an ~25% increase in maximal isometric force. Breath-by-breath pulmonary O(2) uptake (V'O(2)) and vastus lateralis oxygenation indices (by near-infrared spectroscopy [NIRS]) were determined. Skeletal muscle fractional O(2) extraction was estimated by NIRS on the basis of changes in concentration of deoxygenated haemoglobin+myoglobin (Delta[deoxy(Hb+Mb)]). Values obtained at exhaustion were considered "peak" values. The following functional evaluation variables were unaffected by NMES: V'O(2)peak; gas exchange threshold; the V'O(2) vs. work rate relationship (O(2) cost of cycling); the Delta[deoxy(Hb+Mb)] vs. work rate relationship (related to the matching between O(2) delivery and O(2) uptake); peak fractional O(2) extraction; V'O(2) 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 O(2) diffusion, possibly counterbalancing, in-vivo, the fast-to-slow phenotypic changes which were observed in vitro, in a previous work, in the same subjects of the present study.