Kinematic response to warned and unwarned perturbations in running humans

The incidence of traumatic brain injury during contact sports and recreation remains high despite a greater understanding of its lifelong debilitating effects. Athletes are more susceptible to injury during unexpected (blind-sided) impacts compared to perturbations in which they are able to prepare and brace themselves prior to impact. In this study, we monitored the kinematic response in running subjects to perturbations applied with and without warning.

Our previous work on this topic has demonstrated that a warning prior to a perturbation is associated with increased activity of core musculature that could provide protective bracing and stabilization. We hypothesized that the elevated muscle activity when participants were warned prior to the perturbation would result in decreased displacement and acceleration of the body and head.

Perturbations were applied to running participants in the fore-aft and lateral directions. The warning consisted of a directional audio-visual signal delivered 0.5 s prior to impact. Perturbations were given during the start of the stance phase or during the start of the flight phase. Kinematics were quantified utilizing a full body marker setup based on functional and anatomical landmarks on each participant.

When a warning is given prior to perturbation the pelvis is displaced significantly less in all directions (fore-aft and laterally) than in unwarned trials. The torso was displaced significantly less during forward, rightward, and leftward perturbations in warned trials compared to unwarned trials. Warned trials displayed lesser head displacements than unwarned trials. The presence of a warning often decreased the angle of the torso relative to the pelvis and the angle of the head relative to the torso. Angular acceleration of the head was decreased in warned trials compared to unwarned trials following forward, right, and left perturbations.

Results from this study support the hypothesis that knowledge of an impending perturbation is used to brace the body, resulting in perturbations that have less of a destabilizing effect compared to unwarned, "blind-side" impacts.