Non-thermal modulation of sudomotor function during static exercise and the impact of intensity and muscle-mass recruitment

Aim: Static muscle activation elicits intensity-dependent, non-thermal sweating that is presumably controlled by feedforward (central command) mechanisms. However, it is currently unknown how the size of the recruited muscle mass interacts with that mechanism. To investigate the possible muscle-size dependency of that non-thermal sweating, the recruitment of two muscle groups of significantly different size was investigated in individuals within whom steady-state thermal sweating had been established and clamped. Methods: Fourteen passively heated subjects (climate chamber and water-perfusion garment) performed 60-s, static handgrip and knee-extension activations at 30% and 50% of maximal voluntary force, plus a handgrip at 40% intensity (143.4 N) and a third knee extension at the same absolute force. Local sweating from four body segments (averaged to represent whole-body sudomotor activity), three deep-body and eight skin temperatures, heart rates and perceptions of physical effort were measured continuously, and analyzed over the final 30 s of exercise. Results: In the presence of thermal clamping and low-level, steady-state sweating, static muscle activation resulted in exercise-intensity dependent changes in the whole-body sudomotor response during these handgrip and knee-extension actions (P < 0.05). However, there was no evidence of a dependency on the size of the recruited muscle mass (P > 0.05), yet both dependencies were apparent for heart rate, and partially evident for the sensations of physical effort. Conclusion: These observations represent the first evidence that exercise-related sudomotor feedforward is not influenced by the size of the activated muscle mass, but is instead primarily dictated by the intensity of the exercise itself.