Video Analysis for Injury Reconstructions

We performed a video analysis to obtain the impact velocity, impact location and direction, and type of event leading to the injury (helmet-to-helmet collision, shoulder-to-helmet collision, or fall). We calculated the impact velocity using Kinovea (version 0.8.20; Bordeaux, France) by determining the distance between the injured player's head and the impacting surface using a calibration grid placed on the field at a set time before the impact (0.04–0.20 seconds). We set the time before impact when we determined that no changes in the player's velocity before impact were observable (ie, not being slowed down by a tackle elsewhere on the body or not being slowed down in a fall by protecting himself with his arms). To measure velocity using this method, we had to be able to observe the head of the injured player at the time of the impact and at the required timeframe for distance measurement. We also had to observe known markers in the frame and on the field to apply a calibration grid and measure the distance before the impact. A calibration grid had to be placed on the playing surface in proximity to and in the general orientation of the impact. These methods yielded measurement errors of less than 10% in ice hockey.²¹ Given that the error is vastly influenced by the size and proximity of the calibration grid, it is likely to be similar or smaller in American football since there are more markings on the field to establish an optimal calibration grid.