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 CORE ) 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 CORE attained at fatigue in Wistar rats (n = 218) subjected to incremental-speed treadmill running: ambient temperature (T AMB ), distance traveled, initial T CORE , 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 AMB , distance traveled, initial T CORE , and measurement site were the variables with predictive power. Next, regression analyses were conducted with data for each of the following T CORE indices: abdominal (T ABD ), brain cortex (T BRAIN ), or colonic (T COL ) temperature. Our findings indicated that T AMB , distance traveled (i.e., an exercise performance-related variable), initial T CORE , and HLI predicted the three T CORE indices at fatigue. Most intriguingly, HLI was inversely related to T ABD and T BRAIN but positively associated with T COL . Lastly, we compared the temperature values at fatigue among these T CORE indices, and the following descendent order was noticed - T COL , T ABD , and T BRAIN - irrespective of T AMB where experiments were conducted. In conclusion, T CORE in rats exercised to fatigue depends primarily on environmental conditions, performance, pre-exercise T CORE , and measurement site. Moreover, the influence of cutaneous heat loss on T COL is qualitatively different from the influence on T ABD and T BRAIN , and the temperature values at fatigue are not homogenous within the body core.