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.