A Comparison of Muscle Sympathetic Nerve Activity to Non-contracting Muscle During Isometric Exercise in the Upper and Lower Limbs.

Daniel Boulton,Chloe E Taylor,Vaughan G Macefield,Simon Green

doi:10.3389/fnins.2019.00341

Abstract

Previous research indicates that greater sympathetic vasoconstrictor drive to skeletal muscle occurs during isometric upper limb exercise compared to lower limb exercise. However, potential disparity between blood flow and metaboreflex activation in contracting upper and lower limbs could contribute to the augmented sympathetic response during upper limb exercise. Therefore, the aim of this study was to examine MSNA responses during ankle dorsiflexion and handgrip exercise under ischaemic conditions, in order to standardize the conditions in terms of perfusion and metaboreflex activation. Eight healthy male subjects performed 4-min contractions of ischaemic isometric handgrip and ankle dorsiflexion at ∼10% maximal voluntary contraction, followed by 6 min of post-exercise ischaemia. MSNA was recorded continuously by microneurography of the common peroneal nerve of the non-contracting leg and quantified from negative-going sympathetic spikes in the neurogram, synchronized with the cardiac cycle. The time-course of MSNA exhibited parallel increases during exercise of the upper and lower limbs, rising throughout the contraction to peak at 4 min. This represented an increase of 100% relative to resting levels for handgrip exercise (66 ± 24 vs. 33 ± 7 spikes/min at rest) and 103% for dorsiflexion (63 ± 25 vs. 31 ± 8 spikes/min at rest; P < 0.01). In both conditions MSNA remained elevated during post-exercise ischaemia and returned to pre-contraction levels during recovery. These findings demonstrate that that the MSNA response to metaboreflex activation is similar for upper and lower limb exercise when perfusion is controlled for.

Highlights

Muscle metaboreflex activation during sustained isometric exercise causes a progressive decrease in muscle vascular conductance and an increase in arterial pressure (Mitchell et al, 1983; Andersen and Saltin, 1985)
In this study muscle sympathetic nerve activity (MSNA) was measured to non-contracting muscles during sustained dorsiflexion of the ankle and sustained handgrip exercise, both performed in the presence of occlusion of blood flow to the contracting muscle and followed by a period of post-exercise ischaemia
The present findings demonstrate that sympathetic responses during isometric contractions of the upper and lower limbs are similar when controlling for potential differences in blood flow and muscle metaboreflex activation

Summary

Introduction

Muscle metaboreflex activation during sustained isometric exercise causes a progressive decrease in muscle vascular conductance (via sympathetically mediated vasoconstriction) and an increase in arterial pressure (Mitchell et al, 1983; Andersen and Saltin, 1985). Saito (1995) compared MSNA responses to upper and lower limb exercise during isometric handgrip, dorsiflexion and plantarflexion, providing greater consistency with regards to muscle mass They reported that the increases in MSNA were greatest during handgrip, followed by dorsiflexion and plantarflexion, which they attributed to differences in fiber type composition and the metabolic capacity associated with these fibers. We propose that contractions are performed whilst occluding blood flow to, firstly, standardize the conditions for the two limbs as much as possible (i.e., in terms of perfusion) and, secondly, to maximize metaboreflex activation These conditions provide the opportunity to use lower contraction intensities, during which a more stable MSNA recording can be maintained and contractions can be sustained for longer (4 min)

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