Context Football continues to demonstrate the highest rate of sport-related concussion (SRC) in high school athletics. To mitigate the SRC risk, the Michigan High School Athletic Association (MHSAA) implemented rules aimed at reducing the number of collisions occurring in practices. Objective To estimate the rates of SRC in MHSAA football programs and evaluate progressive limitations to collision practices over 5 consecutive seasons. Design Retrospective cohort study. Setting Michigan high school football. Patients or Other Participants High school (9th-12th grade) football athletes (>99% male) participating in MHSAA-sanctioned events. Main Outcome Measure(s) Designated administrators at each school recorded the total number of participating athletes and SRCs (defined as head injuries resulting from athletic participation that required the student-athletes to be withheld from activity after exhibiting signs, symptoms, or behaviors consistent with an SRC) in the MHSAA injury-surveillance system each season (2015-2016 through 2019-2020). Progressive limitations to collision practices occurred across the study period. We estimated athlete-exposures (AEs) as the total number of players multiplied by the total number of possible practices (11 weeks, 4 days each) or competitions (9 weeks, 1 day each) during each season. Incidence rates and rate ratios (RRs) with 95% CIs were used to compare practice and competition SRCs and each season with the most recent season. Results A total of 7755 football SRCs were diagnosed across the 5-year period. The overall SRC rate was 8.03 per 10 000 AEs (95% CI = 7.85, 8.21). The competition SRC rate (30.13/10 000 AEs) was higher than the practice rate (3.51/10 000 AEs; RR = 8.58; 95% CI = 8.19, 9.00). The practice SRC rate was lower in 2017-2018 (RR = 0.86; 95% CI = 0.77, 0.97), 2018-2019 (RR = 0.89; 95% CI = 0.79, 1.0), and 2019-2020 (RR = 0.83; 95% CI = 0.74, 0.94) relative to 2015-2016. Conclusions We found that the progressive limitations to collision practices were protective against SRCs, as the rate of SRC was lower in the 3 most recent seasons relative to 2015-2016. Key Points: - The overall estimated sport-related concussion (SRC) rate in high school football was 8.03 per 10 000 athlete-exposures, with higher rates occurring during competitions compared with practices. - Estimated practice SRC rates decreased after the implementation of rules restricting the amount of allowed collision time. - Estimated competition SRC rates also decreased across the study period, suggesting that risk mitigation occurred in competition when collision limitations in practice were imposed.
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