Programme Contents

Sudomotor responses during isometric exercise appear to be intensity- and muscle mass-dependent

Cardiac frequency, mean arterial pressure and skin sympathetic nerve activity during isometric exercise, increase in proportion to exercise intensity (Vissing et al., 1991), while pressor responses also appear to be modulated by the size of the active muscle mass (Ray and Wilson, 2004). Sweating also responds to exercise intensity (Kondo et al., 2000), however, there is no information relating to the affect of muscle mass recruitment on sudomotor function. The hypothesis was tested that non-thermal sudomotor drive in the heat would be influenced by both exercise-intensity and the size of the recruited muscle mass.

Seven, resting (upright) males were heated (60 min) using a water-perfusion suit (37.2°C) and a climate-controlled chamber (36.7°C, 58% relative humidity) to induce steady-state sweating. Body temperature was clamped thereafter. Isometric handgrip and knee extension activations (60 s with 10 min rest) were performed at 30% and 50% maximal voluntary contraction (MVC) in a balanced order. Sweat rate (_sw) was measured (1 Hz: 3.16 cm² capsules) at four sites (forehead, chest, and inactive forearm and thigh), and averaged. Cardiac frequency was monitored continuously (0.2 Hz), and mean arterial pressure was measured beat-by-beat.

Oesophageal and mean skin temperatures did not change during either rest or isometric exercise, verifying the veracity of the thermal clamp. Cardiac frequency displayed both an intensity- and a mass-dependence, resulting in the following pre- to post-activation changes (1 min): handgrip (5.9±1.4, 22.4±2.0 b.min^-1, 30 and 50% MVC; P<0.05); knee extension (14.5±1.4, 26.6±2.5 b.min^-1, 30 and 50% MVC; P<0.05). Similar to cardiac frequency, mean arterial pressure increased significantly during handgrip (10.1±1.9, 23.7±4.1 mmHg, 30 and 50% MVC; P<0.05), and knee extension (20.1±1.6, 32.1±2.8 mmHg, 30 and 50% MVC; P<0.05). Whilst pre-activation _sw baselines were similar, normalised increases in _sw from baseline, were intensity-dependent, but not mass-dependent: handgrip (0.093±0.027 and 0.212±0.035 mg.cm^-2.min^-1, 30 and 50% MVC; P<0.05); knee extension (0.140±0.017 and 0.198±0.026 mg.cm^-2.min^-1; P>0.05). However, the integrated sudomotor responses during isometric exercise appeared to reveal an intensity- and muscle mass-dependency: handgrip (3.15±0.70 mg.cm^-2 and 4.61±0.87 mg.cm^-2, 30 and 50% MVC); knee extension (4.17±0.48 and 5.53±0.89 mg.cm^-2). Whilst differences between handgrip and knee extension were non-significant (30% MVC P=0.09; 50% MVC P=0.08), post hoc analyses reveal our design to be under-powered; further testing is underway. In addition, following knee extension, _sw remained elevated compared to handgrip exercise. The possibility exists that the delayed _sw recovery, was mediated by intramuscular changes, which may be mass-dependent.

This study provides evidence that sudomotor responses to isometric exercise, during heat stress, may be exercise-intensity and muscle mass-dependent. If real, this latter observation is both novel and significant. Non-thermal factors have been suggested to modulate sweating during isometric exercise (Kondo et al., 2000). We now propose that motor unit recruitment may also influence sweating. In addition, the continued elevation of _sw, but not body temperature, after isometric exercise, in particular knee extension exercise, may indicate that metaboreceptor stimulation, or an unidentified thermal factor, has augmented post-exercise sweating. This appears to also be mass-dedendent.

Kondo, N., Tominaga, H., Shibasaki, M., Aoki, K., Okada, S. & Nishiyasu, T. (2000) Journal of Applied Physiology 88: 1590­1596.

Ray, C.A. & Wilson, T.E. (2004) Journal of Applied Physiology 97: 160-164.

Vissing, S.F., Scherrer, U. & Victor, R.G. (1991) Circulation Research, 69: 228-238.