Determinants of body core temperatures at fatigue in rats subjected to incremental-speed exercise: The prominent roles of ambient temperature, distance traveled, initial core temperature, and measurement site.

Understanding the factors that underlie the physical exercise-induced increase in body core temperature (T) is essential to developing strategies to counteract hyperthermic fatigue and reduce the risk of exertional heatstroke. This study analyzed the contribution of six factors to T attained at fatigue in Wistar rats (n = 218) subjected to incremental-speed treadmill running: ambient temperature (T), distance traveled, initial T, body mass, measurement site, and heat loss index (HLI). First, we ran hierarchical multiple linear regression analyses with data from different studies conducted in our laboratory (n = 353 recordings). We observed that T, distance traveled, initial T, and measurement site were the variables with predictive power. Next, regression analyses were conducted with data for each of the following T indices: abdominal (T), brain cortex (T), or colonic (T) temperature. Our findings indicated that T, distance traveled (i.e., an exercise performance-related variable), initial T, and HLI predicted the three T indices at fatigue. Most intriguingly, HLI was inversely related to T and T but positively associated with T. Lastly, we compared the temperature values at fatigue among these T indices, and the following descendent order was noticed - T, T, and T - irrespective of T where experiments were conducted. In conclusion, T in rats exercised to fatigue depends primarily on environmental conditions, performance, pre-exercise T, and measurement site. Moreover, the influence of cutaneous heat loss on T is qualitatively different from the influence on T and T, and the temperature values at fatigue are not homogenous within the body core.