Figure 2. Head impact reconstruction setup.

(A) 2D pixel location of 20 fiducial markers (10 on each helmet) were tracked in stereo high speed video to resolve (B) the position, orientation, and velocity of a helmet-fixed reference frame for each player. In the camera reference frame, video tracking and stereo triangulation produced average x-y-z errors of [1, 1.5, 1] mm in tracking fiducial marker locations, over the 10 frames/4 ms before helmet contact, with respect to their (true) digitized locations. (C) Drop test reconstruction was set to match initial conditions for the player in the right. From the available discrete head orientations in twin-wire drop testing (Fig. 1C), the closest helmet impact location was within 5 cm of the impact location in the field. In the field frame, the x-y-z translational velocity of the player’s helmet was [0.3, 0.8, −1.3] m/s while the velocity of the opposing player’s helmet was [−0.3, −1.4, −0.3] m/s. The magnitude of the vector difference between these values (2.4 m/s) corresponded to the relative impact speed of the players and was reconstructed by a 0.3 m drop. Due to physical limitations of twin-wire drop testing, reconstructing the helmet contact location produced a velocity orientation error of 28°