Head impact exposure in youth football has not been well-documented, despite children under the age of 14 accounting for 70% of all football players in the United States. The objective of this study was to quantify the head impact exposure of youth football players, age 9–12, for all practices and games over the course of single season. A total of 50 players (age = 11.0 ± 1.1 years) on three teams were equipped with helmet mounted accelerometer arrays, which monitored each impact players sustained during practices and games. During the season, 11,978 impacts were recorded for this age group. Players averaged 240 ± 147 impacts for the season with linear and rotational 95th percentile magnitudes of 43 ± 7 g and 2034 ± 361 rad/s2. Overall, practice and game sessions involved similar impact frequencies and magnitudes. One of the three teams however, had substantially fewer impacts per practice and lower 95th percentile magnitudes in practices due to a concerted effort to limit contact in practices. The same team also participated in fewer practices, further reducing the number of impacts each player experienced in practice. Head impact exposures in games showed no statistical difference. While the acceleration magnitudes among 9–12 year old players tended to be lower than those reported for older players, some recorded high magnitude impacts were similar to those seen at the high school and college level. Head impact exposure in youth football may be appreciably reduced by limiting contact in practices. Further research is required to assess whether such a reduction in head impact exposure will result in a reduction in concussion incidence.

Laboratory tests in which dummy headforms are used to evaluate helmet performance must be representative of real-world conditions to ensure helmets perform well in the field. The objective of this study was to quantify shape differences that may affect helmet fit between two dummy headforms commonly used for football helmet testing. Point-cloud models of a 50th percentile male Hybrid III headform and a medium NOCSAE headform were generated using a coordinate measuring machine. The headforms were optimally aligned and shape comparisons were made in the mid-sagittal plane, three coronal planes, and 3D. Planar and 3D differences were quantified by comparing maximum (MRD) and root-mean-square (RMSD) radial deviations. Minor differences were observed in the upper skull contours of all planar cross-sections, where MRDs were less than 3.5 mm and RMSDs were less than 1.7 mm. Larger deviations were observed in other regions including the jaw in the anterior coronal plane, where the MRD was 6.6 mm and the RMSD deviation was 4.5 mm. Substantial differences were noted between the Hybrid III and NOCSAE at the base of the skull, cheeks, jaw and chin. The headforms were also compared to a head model based on medical imaging of a human subject, which the NOCSAE matched more closely than the Hybrid III. The data presented in this study show that the Hybrid III and NOCSAE headforms have substantial shape differences in several regions that are important for helmet fit, possibly making the NOCSAE a better option for realistic helmet fit.

As advanced helmet testing methodologies are developed, the effect headform selection may have on the biomechanical impact response must be considered. This study sought to assess response differences between two of the most commonly used headforms, the Hybrid III and National Operating Committee on Standards for Athletic Equipment headforms, through a series of helmeted impact tests. A total of 180 pendulum impact tests were conducted with three impactor velocities and six impact locations. Test condition-specific significant differences were found between the two headforms for peak linear and angular accelerations (a = 0.05), although differences tended to be small. On average, the National Operating Committee on Standards for Athletic Equipment headform experienced higher peak linear (3.7 6 7.8%) and angular (12.0 6 21.6%) accelerations, with some of the largest differences associated with impacts to the facemask. Without the facemask impacts, the average differences in linear (1.8 6 6.0%) and angular (9.6 6 15.9%) acceleration would be lower. No significant differences were found in coefficient of variation values for linear (Hybrid III: 2.6 6 2.3%, National Operating Committee on Standards for Athletic Equipment: 2.0 6 1.4%) or angular (Hybrid III: 4.9 6 4.0%; National Operating Committee on Standards for Athletic Equipment: 5.2 6 5.8%) acceleration. These data have application toward development and validation of future helmet evaluation protocols and standards.

This study sought to evaluate the suitability of angular rate sensors for quantifying angular acceleration in helmeted headform impacts. A helmeted Hybrid III headform, instrumented with a 3-2-2-2 nine accelerometer array and angular rate sensors, was impacted (n = 90) at six locations and three velocities (3.1, 4.9, and 6.4 m/s). Data were low-pass filtered using Butterworth four-pole phaseless digital filters which conform to the specifications described in the Society of Automotive Engineers J211 standard on instrumentation for impact tests. Nine accelerometer array data were filtered using channel frequency class 180, which corresponds to a −3 db cutoff frequency of 300 Hz. Angular rate sensor data were filtered using channel frequency class values ranging from 5 to 1000 Hz in increments of 5 Hz, which correspond to −3 db cutoff frequencies of 8 to 1650 Hz. Root-mean-square differences in peak angular acceleration between the two instrumentation schemes were assessed for each channel frequency class value. Filtering angular rate sensor data with channel frequency class values between 120 and 205 all produced mean differences within ±5%. The minimum root-mean-square difference of 297 rad/s2 was found when the angular rate sensor data were filtered using channel frequency class 175. This filter specification resulted in a mean difference of 28 ± 297 rad/s2 (1.8% ± 8.6%). Condition-specific differences (α=0.05) were observed for 11 of 18 test conditions. A total of 4 of those 11 conditions were within ±5%, and 10 were within ±10%. Furthermore, the nine accelerometer array and angular rate sensor methods demonstrated similar levels of repeatability. These data suggest that angular rate sensor may be an appropriate alternative to the nine accelerometer array for measuring angular head acceleration in helmeted head impact tests with impactor velocities of 3.1–6.4 m/s and impact durations of approximately 10 ms.