Localized metabolic and t2 changes induced by voluntary and evoked contractions.

PURPOSE: This study compared the metabolic and activation changes induced by electrically evoked (neuromuscular electrical stimulation (NMES)) and voluntary (VOL) contractions performed at the same submaximal intensity using P chemical shift imaging (CSI) and T2 mapping investigations.
METHODS: Fifteen healthy subjects were asked to perform both NMES and VOL protocols with the knee extensors (i.e., 232 isometric contractions at 30% of maximal force) inside a 3-T scanner for two experimental sessions. During the first session, metabolic variations, i.e., phosphocreatine (PCr), inorganic phosphate (Pi), and pH, were recorded using localized P CSI. During a second session, T2 maps of the knee extensors were obtained at rest and immediately after each exercise. Voxels of interest were selected from the directly stimulated vastus lateralis and from the nondirectly stimulated rectus femoris/vastus intermedius muscles.
RESULTS: PCr depletion recorded throughout the NMES session was significantly larger in the vastus lateralis as compared with the rectus femoris/vastus intermedius muscles for both conditions (VOL and NMES). A higher occurrence of Pi splitting and a greater acidosis was found during NMES as compared with VOL exercise, illustrating the heterogeneous activation of both slow and fast muscle fibers. T2 changes were greater after NMES as compared with VOL for both muscles but were not necessarily related to the localized metabolic demand.
CONCLUSION: We provided direct evidence that the metabolic demand was strongly related to both the exercise modality and the site of stimulation. On the basis of the occurrence of Pi splitting, we suggested that NMES can activate fast muscle fibers even at low force levels.