Neural and Muscular Function in the Heat

The development of hyperthermia impacts on several aspects of neural and muscular function. Hyperthermia has been shown to impair cognitive performances during complex planning or working memory tasks. This is likely due to the cognitive load imposed by heat stress limiting the available resources available to perform such tasks. However, cognitive function in the heat may be protected or preserved when chronically exposed to heat stress. During acute exercise in the heat, cerebral blood flow may become compromised when intensity is elevated, leading to a suppression of arousal and disengagement to a task. The development of hyperthermia also induces a supraspinal failure in sustaining neural drive during prolonged muscle contractions. As with some aspects of cognitive function, part of this supraspinal failure may be recovered following heat acclimation. Hyperthermia also induces peripheral alterations in neural drive transmission. These peripheral alterations are likely due to a faster nerve conduction velocity when temperature increases and are not affected by heat acclimation. There are also changes occurring at the level of the skeletal muscle when temperature increases, with an increase in contraction and relaxation velocity. Although a rise in muscle temperature can increase muscle power production, whole-body hyperthermia reduces performance and may affect motor control. Finally, an increase in muscle temperature has also been shown to trigger signalling cascades regulating myofibrillar and mitochondrial mass.