Exposure assessment for repeated sub-concussive head impacts in soccer: The HEalth and Ageing Data IN the Game of football (HEADING) study

The purpose of this paper was to develop exposure estimates for repetitive sub-concussive head impacts (RSHI) for use in epidemiological analyses. We used a questionnaire to collect lifetime history of heading and other head contacts associated with training and playing football from 159 former footballers all members of the English professional football association. We used linear mixed effect regression with player as the random effect, to model the number of headers, blows to the head and head-to-head impacts as a function of potential exposure affecting factors, which were treated as the fixed effects. Exposure affecting factors included playing position, league, context of play (game vs training) and decade of play. Age at time of response to the questionnaire was also included in the models. In model results, playing position was important, with RSHIs being highest among defenders and lowest among goalkeepers. Players headed the ball more during games than in training, and when playing in amateur or youth leagues compared with semi-professional or professional leagues. The average number of reported head impacts declined linearly throughout the observation period (1949-2015). The derived final model for headers explained 43%, 9% and 36% of the between player, within player and total variance in exposure, respectively with good precision and predictive performance. These findings are generally in agreement with previously published results pointing towards the models forming a valid method for estimating exposure to RSHI among former footballers although some further external validation is still warranted.

Automated summary

The purpose of this study was to estimate exposure to repetitive sub-concussive head impacts (RSHI) for epidemiological analyses. A questionnaire was used to collect data on lifetime history of heading and other head contacts from 159 former professional English footballers. Linear mixed effect regression models were used to examine the factors influencing the number of headers, blows to the head, and head-to-head impacts. The results showed that playing position, league, context of play, and decade of play were important factors affecting RSHI exposure. The final model explained a significant amount of variance in exposure and demonstrated good precision and predictive performance. These findings suggest that the model is a valid method for estimating RSHI exposure among former footballers, although further validation is still needed.