Head Impact Burden Differs Between Seasons in Youth and High School US Football Players.

Retrospective estimates of head impact burden in contact sports are often used in studies assessing for correlation of head impact exposure with neurocognitive outcomes later in life. These estimations assume a consistent impact burden from year to year. We monitored head impacts in youth tackle football players in the 6th-8th grades and high school varsity players using helmet-based sensors during two consecutive seasons. Helmet-based pressure sensors were used to measure head impacts, and estimates of peak linear acceleration (g's) were then grouped and summed for cumulative impact. After each season, players estimated whether they experienced an average, below average, or above average head impact burden in relation to their teammates. Of the 98 players who participated, 54 played in both seasons and sustained a median of 102.5 impacts (2206 cumulative g's) in season 1 and 161 impacts (3682 cumulative g's) in season 2. Using pairwise comparisons, number of impacts and cumulative g's increased in the youth players from season 1 to season 2 (p = 0.0021 and p = 0.0018, respectively), but there was a trend towards a decrease in number of impacts and g's at the high school level. In an exploratory analysis, perceived cumulative head impact and measured cumulative g's weakly correlated (Pearson's correlation coefficient = 0.26 for season 1 and 0.02 for season 2). In conclusion, cumulative head impacts differ significantly from one season to the next, and players have difficulty estimating their cumulative head impacts in relation to their team mates, especially at the youth level. Retrospective estimations of head impact burden should account for this variability.