Hydration status and repetitive head impacts: neurophysiological responses and functional implications

Combat sport athletes frequently experience two distinct yet potentially interacting physiological stressors: repetitive head impacts (RHIs) and rapid weight loss through hypo-hydration. While each has been studied independently, limited research has examined their combined neurophysiological consequences. This thesis aimed to investigate how RHIs and hypo-hydration influence neuromuscular and cortical function, using transcranial magnetic stimulation (TMS) and motor nerve stimulation (MNS) as assessment tools. An initial survey of over 100 combat sport athletes highlighted widespread engagement in rapid weight loss practices, with many reporting overlapping symptoms of hypo-hydration and concussion. In addition, some athletes reported exacerbation of concussion symptoms after undergoing a weight-cut. A reliability study confirmed that TMS and MNS measures could be used with sufficient precision in ensuring robustness for subsequent investigations. In the first experimental study, exercise and heat-induced hypo-hydration impaired maximal strength and voluntary activation but did not significantly alter corticospinal excitability or inhibition, suggesting preserved central drive under fluid-restricted conditions. In contrast, the following study examined responses to a single sparring session and found significant increases in cortical inhibition and motor thresholds, along with reduced maximal strength and voluntary activation, all of which resolved within 48 hours. A follow-up longitudinal study using instrumented mouthguards revealed that impact magnitude e.g., cumulative angular acceleration predicted the degree of increased cortical inhibition. Finally, a 10-week case series tracked three fighters through their fight camps. Voluntary activation and strength showed the most frequent meaningful changes, and over 200 RHIs were recorded in approximately 35% of their monitored sparring sessions. In summary, repeated head impacts elevated cortical inhibitory activity and central fatigue, while heat-induced hypo-hydration produced a contrasting neurophysiological response. These opposing effects may suggest a complex physiological interplay if they occur concurrently, whereby head-impact-related neural inhibition could disrupt or override compensatory mechanisms associated with fluid loss, potentially increasing vulnerability to injury and impairing athletic performance.