Muscle sympathetic nerve activity (MSNA) during high-intensity, isometric leg exercise in humans

Muscle sympathetic nerve activity (MSNA) is activated during mild-intensity isometric handgrip exercise, but not during isometric leg exercise at a similarly relative intensity. It is unclear whether MSNA is increased by leg exercise at heavy intensity in humans. We aimed to examine how MSNA responds to high-intensity, isometric leg exercise in humans. Eight young healthy male volunteers performed isometric plantar flexions of high intensity (70% of MVC) for 2 min, followed by 2 min of post-exercise muscle ischemia (PEMI; isolated muscle metaboreflex stimulation), with monitoring of muscle sympathetic nerve activity (MSNA), heart rate and arterial blood pressure. MSNA was increased at the onset of exercise (p<0.05), and further increased to 512 +/- 111% of baseline at the second min of exercise (p<0.05). Furthermore, MSNA remained elevated above baseline during PEMI (259 +/- 41 % of baseline, p<0.05). Heart rate also rose at the onset of the test (P<0.05) and gradually increased all through the test (p<0.05), but decreased to baseline level during PEMI. Mean blood pressure was progressively increased from the onset to the end of test (p<0.05), and remained elevated above baseline during PEMI (p<0.05). In summary, MSNA was increased progressively throughout high-intensity, isometric plantar flexion in humans, primarily due to muscle metaboreflex.