Objective:​ Impacts causing concussion in professional football were simulated in laboratory tests to determine collision mechanics. This study focuses on the biomechanics of concussion in the struck player.

Methods:​ Twenty-five helmet impacts were reconstructed using Hybrid III dummies. Head impact velocity, direction, and helmet kinematics-matched game video. Translational and rotational accelerations were measured in both players’ heads;​ 6-axis upper neck responses were measured in all striking and five struck players. Head kinematics and biomechanics were determined for concussed players. Head displacement, rotation, and neck loads were determined because finite element analysis showed maximum strains occurring in the midbrain after the high impact forces. A model was developed of the helmet impact to study the influence of neck strength and other parameters on head responses.

Results:​ The impact response of the concussed player’s head includes peak accelerations of 94 ± 28 g and 6432 ± 1813 r/s², and velocity changes of 7.2 ± 1.8 m/s and 34.8 ± 15.2 r/s. Near the end of impact (10 ms), head movement is only 20.2 ± 6.8 mm and 6.9 ± 2.5 degrees. After impact, there is rapid head displacement involving a fourfold increase to 87.6 ± 21.2 mm and 29.9 ± 9.5 degrees with neck tension and bending at 20 ms. Impacts to the front of the helmet, the source of the majority of National Football League concussions, cause rotation primarily around the z axis (superior-inferior axis) because the force is forward of the neck centerline. This twists the head to the right or left an average of 17.6 ± 12.7 degrees, causing a moment of 17.7 ± 3.3 Nm and neck tension of 1704 ± 432 N at 20 ms. The head injury criterion correlates with concussion risk and is proportional to ∆V4/d1.5 for half-sine acceleration. Stronger necks reduce head acceleration, ∆V, and displacement. Even relatively small reductions in ∆V have a large effect on head injury criterion that may reduce concussion risks because changes in ∆V change head injury criterion through the 4th power.

Conclusion:​ This study addresses head responses causing concussion in National Football League players. Although efforts are underway to reduce impact acceleration through helmet padding, further study is needed of head kinematics after impact and their contribution to concussion, including rapid head displacement, z-axis rotation, and neck tension up to the time of maximum strain in the midbrain. Neck strength influences head ∆V and head injury criterion and may help explain different concussion risks in professional and youth athletes, women, and children.