Objective To examine the effect of repetitive contact sport-related head trauma on indices ofheart rate variability (HRV).

Design Prospective Cohort.

Setting Laboratory.

Participants Forty-ninemale contact-sport (hockey or football) and four non-contact control athletes (age range: 17–22).

Intervention Prior to (T1) and upon completion of (T2) the competitive athletic season, a three-lead electrocardiogram and Kubios software (Kuopio, Finland) were used to assess heart rate variability during 5-minutes of quiet stance and while actively squatting at 0.10 Hz. Independent variables included condition (rest-vs-active) and time (T1-vs-T2).

Outcome measures Time-Domain: square root of mean squared differences of successive R-R intervals (RMSSD), percentage of successive R-R intervals that differ by more than 50 milliseconds (pNN50); Non-Linear: approximateentropy (ApEn). In a subset of participants (n=29) biomechanical head-impact exposure data was estimated usingthe xPatch (X2 Biosystems), affixed to the right mastoid.

Main results RM-ANOVA indicateda significant main effect of timein contact sport but not control athletes (all p>0.133). HRV metrics were decreased post-season relative to baseline (95% CI:s for differences: MeanRR4–48 ms, p=0.045; RMSSD 1–9 ms, p=0.018; pNN50 0.689–5.98, p=0.010; ApEn 0.022–0.163, p=0.044). Significant correlations were observed between the average peak linear acceleration per hit experienced and the change in MeanRR(R²=0.1382), and RMSSD (R²=0.1525).

Conclusions Exposure to repetitive subconcussive impacts overone season of contact-sport leads to decreases in heart rate variability. The magnitude of HRV reduction is related to the average magnitude of the linear component of hits to the head experienced during play

Competing interests None.