Central command increases muscle sympathetic nerve activity more to contracting than noncontracting muscle during rhythmic isotonic leg exercise

We have previously shown that the increase in muscle sympathetic nerve activity (MSNA) to contracting muscle during sustained isometric exercise is due primarily to central command, and that contracting muscle does not express a metaboreceptor-driven increase in MSNA. Here, we tested the hypothesis that MSNA increases to the contracting muscle also during rhythmic isotonic exercise, in which muscle metabolites will not accumulate because the contraction is performed without external load. MSNA was recorded from the common peroneal nerve in 10 participants and negative-going sympathetic spikes extracted during 50 cycles of sinusoidal (0.15 Hz) isotonic dorsiflexions of the ipsilateral or contralateral ankle. Electromyographic activity (EMG) was recorded from the tibialis anterior muscle on both sides. Cross-correlation analysis between the MSNA and EMG revealed a marked cyclic modulation of MSNA to the contracting (ipsilateral) muscle. This modulation, in which MSNA increased during the contraction phase, was 3 times greater than that to the non-contracting muscle (modulation index = 27.4 ± 3.2 vs 9.2 ± 1.5 %; P<0.002). There were no differences in either the intensity or magnitude of modulation of EMG during ipsilateral and contralateral contractions. We conclude that central command increases MSNA to the contracting muscle during rhythmic isotonic exercise.