Intense unimanual contractions increase muscle excitation and local metabolic activity in the resting contralateral...

Intense unilateral muscle contractions evoke measurable activity within the contralateral neuroaxis, which can be detected with surface electromyographic activity in the resting homologous muscle. Physiological mirror activity (PMA), the unintentional increase in contralateral muscle excitation, has been implicated in cross-limb interactions and adaptations. Despite longstanding observations of PMA, it remains unknown whether this low-level muscle excitation influences local muscle metabolism. We addressed this question using a vascular occlusion test in 10 healthy adults. Surface electromyography and near-infrared spectroscopy-derived measures of tissue oxygen saturation and muscle oxygen consumption ( $m{\stackrel{.}{V}}_{O_2}$ ) were obtained from the resting left forearm during vascular occlusion at rest and during fatiguing unimanual contractions. PMA in the contralateral resting arm was greater during fatiguing unimanual contractions than during rest (mean difference: 8.9%, 95% CI: 4.1 to 13.8). This increase was accompanied by a steeper rate of tissue oxygen desaturation (mean difference: -0.132% s^{- 1}, 95% CI: -0.227 to -0.037) and greater $m{\stackrel{.}{V}}_{O_2}$ (mean difference: 0.188 mL O₂ min^{- 1} 100 g^{- 1}, 95% CI: 0.057 to 0.320). Greater PMA was associated with both a faster rate of oxygen desaturation (r = -0.85, 95% CI: -0.96 to -0.46) and greater $m{\stackrel{.}{V}}_{O_2}$ (r = 0.78, 95% CI: 0.28 to 0.94). These findings provide the first evidence that PMA is accompanied by increased local metabolic demand, consistent with a coupling between unintentional muscle excitation and oxygen extraction in the resting limb.