Key result
Isometric knee extension and handgrip exercise at the same relative intensity elicited similar increases in skin sympathetic nerve activity (50% vs 52%), suggesting relative effort drives the response.
Why the study?
Does isometric leg exercise elicit greater skin sympathetic nerve activity responses than isometric arm exercise in humans?
Does isometric leg exercise elicit greater skin sympathetic nerve activity responses than isometric arm exercise in humans?
Absolute Event Rate: 50% vs 52%
Relative effort, rather than muscle mass, appears to be the main determinant of exercise-induced skin sympathetic nerve activity responses in humans.
Challenges assumption of greater sympathetic drive from leg versus arm exercise; leaves open confirmation in larger autonomic studies.
Measurement of skin sympathetic nerve activity (SSNA) during isometric exercise has been previously limited to handgrip. We hypothesized that isometric leg exercise due to the greater muscle mass of the leg would elicit greater SSNA responses than arm exercise because of presumably greater central command and muscle mechanoreceptor activation. To compare the effect of isometric arm and leg exercise on SSNA and cutaneous end-organ responses, 10 subjects performed 2 min of isometric knee extension (IKE) and handgrip (IHG) at 30% of maximal voluntary contraction followed by 2 min of postexercise muscle ischemia (PEMI) in a normothermic environment. SSNA was recorded from the peroneal nerve. Cutaneous vascular conductance (laser-Doppler flux/mean arterial pressure) and electrodermal activity were measured within the field of cutaneous afferent discharge. Heart rate and mean arterial pressure significantly increased by 16 +/- 3 and 23 +/- 3 beats/min and by 22 +/- 2 and 27 +/- 3 mmHg from baseline during IHG and IKE, respectively. Heart rate and mean arterial pressure responses were significantly greater during IKE compared with IHG. SSNA increased significantly and comparably during IHG and IKE (52 +/- 20 and 50 +/- 13%, respectively). During PEMI, SSNA and heart rate returned to baseline, whereas mean arterial pressure remained significantly elevated (Delta12 +/- 2 and Delta13 +/- 2 mmHg from baseline for IHG and IKE, respectively). Neither cutaneous vascular conductance nor electrodermal activity was significantly altered by either exercise or PEMI. These results indicate that, despite cardiovascular differences in response to IHG and IKE, SSNA responses are similar at the same exercise intensity. Therefore, the findings suggest that relative effort and not muscle mass is the main determinant of exercise-induced SSNA responses in humans.
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Ray et al. (2004) studied this question. Isometric knee extension (IKE) vs. Isometric handgrip (IHG) was evaluated on Skin sympathetic nerve activity (SSNA). Isometric knee extension and handgrip exercise at the same relative intensity elicited similar increases in skin sympathetic nerve activity (50% vs 52%), suggesting relative effort drives the response.
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