Effects of head impact exposure on in-game heart rate dynamics in Canadian varsity football athletes

Objectives: To characterize the association between head impact exposure and heart rate dynamics during varsity football games. Design: Prospective observational study following a convenience sample of 72 athletes over two seasons. Methods: Athletes wore instrumented mouthguards and chest-strap sensors to record head impact exposure, heart rate, and velocity. After filtering for physiologically relevant recovery segments (n=7693), linear mixed-effects models assessed associations between head impact exposure and heart rate dynamics, controlling for physical exertion and inter-individual variability. Exploratory non-linear associations were modeled using generalized additive mixed models. Results: Greater acute impact magnitude was associated with elevated heart rate and delayed time to peak heart rate recovery (p<0.001), independent of running speed. Random slopes analysis further indicated substantial inter-individual variability in the immediate cardiac response to impacts. Higher accumulated session-wide impact exposure demonstrated a non-linear relationship with heart rate recovery speed; recovery dynamics remained preserved at lower exposure levels (i.e., ≤200g) but were progressively attenuated at higher cumulative loads (i.e., >200g). Conclusions: Head impact exposure is associated with systematic alterations in heart rate recovery dynamics during live sport participation. These findings identify cardiovascular recovery as a sensitive, non-invasive physiological signal linked to head impacts, independent of measured locomotor workload variables. While clinical outcomes were not assessed, the results demonstrate the feasibility of integrated wearable monitoring to characterize physiological strain and motivate future studies linking impact history with functional and clinical endpoints.