Does intramuscular thermal feedback modulate eccrine sweating in exercising humans?

AIM: Few investigators have considered the possibility that skeletal muscles might contain thermosensitive elements capable of modifying thermoeffector responses. In this experiment, the temporal relationships between dynamic changes in deep-body and intramuscular temperatures and eccrine sweat secretion were explored during rhythmical and reproducible variations in heat production. METHODS: Eight subjects performed semi-recumbent cycling (25 °C) at a constant load to first establish whole-body thermal and sudomotor steady states (35 min), followed by a 24-min block of sinusoidal workload variations (three, 8-min periods) and then returning to steady-state cycling (20 min). Individual oesophageal, mean skin and intramuscular (vastus lateralis) temperatures were independently cross-correlated with simultaneously measured forehead sweat rates to evaluate the possible thermal modulation of sudomotor activity. RESULTS: Both intramuscular and oesophageal temperatures showed strong correlations with sinusoidal variations in sweating with respective maximal cross-correlation coefficients of 0.807 (±0.044) and 0.845 (±0.035), but these were not different (P = 0.40). However, the phase delay between intramuscular temperature changes and sweat secretion was significantly shorter than the delay between oesophageal temperature and sweating 25.6 s (±12.6) vs. 46.9 s (±11.3); P = 0.03. CONCLUSION: The temporal coupling of eccrine sweating to intramuscular temperature, combined with a shorter phase delay, was consistent with the presence of thermosensitive elements within skeletal muscles that appear to participate in the modulation of thermal sweating.