Measuring head kinematics in football: correlation between the head impact telemetry system and Hybrid III headform

Abstract

Over the last decade, advances in technology have enabled researchers to evaluate concussion biomechanics through measurement of head impacts sustained during play using two primary methods: (1) laboratory reconstruction of open-field head contact, and (2) instrumented helmets. The purpose of this study was to correlate measures of head kinematics recorded by the Head Impact Telemetry (HIT) System (Simbex, NH) with those obtained from a Hybrid III (HIII) anthropometric headform under conditions that mimicked impacts occurring in the NFL. Linear regression analysis was performed to correlate peak linear acceleration, peak rotational acceleration, Gadd Severity Index (GSI), and Head Injury Criterion (HIC(15)) obtained from the instrumented helmet and HIII. The average absolute location error between instrumented helmet impact location and the direction of HIII head linear acceleration were also calculated. The HIT System overestimated Hybrid III peak linear acceleration by 0.9% and underestimated peak rotational acceleration by 6.1% for impact sites and velocities previously identified by the NFL as occurring during play. Acceleration measures for all impacts were correlated; however, linear was higher (r(2) = 0.903) than rotational (r(2) = 0.528) primarily due to lower HIT System rotational acceleration estimates at the frontal facemask test site. Severity measures GSI and HIC were also found to be correlated, albeit less than peak linear acceleration, with the overall difference between the two systems being less than 6.1% for either measure. Mean absolute impact location difference between systems was 31.2 ± 46.3° (approximately 0.038 ± 0.050 m), which was less than the diameter of the impactor surface in the test. In instances of severe helmet deflection (2.54-7.62 cm off the head), the instrumented helmet accurately measured impact location but overpredicted all severity metrics recorded by the HIII. Results from this study indicate that measurements from the two methods of study are correlated and provide a link that can be used to better interpret findings from future study using either technology.

FIGURE 1

A pneumatic linear impactor (Biokinetics, Inc) was employed to replicate on-field reconstruction of head impacts. The ram mass, impactor surface, and target velocities were selected to best simulate on-field head impacts occurring in the National Football League.

FIGURE 2

Four primary impact sites were (A, B, C, and D) were identified as points of contact that most frequently result in mTBI for NFL athletes. Each site was impacted at four target speeds: 4.4, 7.4, 9.3, and 11.2 m/s.

FIGURE 3

The HIT System overestimated Hybrid III peak linear acceleration by 0.9% and underestimated peak rotational acceleration by 6.1% for all primary impact sites. Acceleration measures for all impacts were correlated; however, linear acceleration was higher (r² = 0.903) than rotational (r² = 0.528) primarily due lower HIT System rotational acceleration estimates at impact site A.

FIGURE 4

The average absolute difference between impact location estimated by the instrumented helmet and HIII for all impacts was 31.2 ± 46.3° for all primary impact sites (A, B, C, and D). Arrows shown on the headforms above, identify the average impact location by both systems for each impact sites. A * is placed to the left of the arrow indicating impact location estimated by the instrumented helmet. Six of the twelve impacts to site A were identified by the instrumented helmet as being symmetrically opposite of the actual location, and so two arrows are provided at that location for the instrumented helmet to differentiate those events.

FIGURE 5

The HIT System’s processing algorithm to estimate impact location best approximates the impact site on the helmet. The average location difference between the HIT System and HIII for sites A’ and A” was 42.1 ± 13.4°; however, the HIT System correctly identified the impact location as being low on the facemask, while the estimate of impact location from HIII was directly through the nose.

FIGURE 6

High speed video of a 9 m/s impact at the A” site. Top Row: (1) Initial contact occurs, (2) 1 ms after contact—facemask compresses, (3) 6 ms after contact—facemask is bent, helmet has lifted off the head, and the head begins to accelerate. Bottom Row: (4) 10 ms after contact—impactor surface pushes the facemask into the chin, (5) 16 ms after contact—the helmet experiences maximum deflection off the head.