Does vertical head shock remain stable during running while looking sideways?

Rationale/Objectives: The head is stabilized during running for navigation by modulations of lower limb shock. However, past studies had only tested the head stability when subjects were gazing forward. The modulation of vertical head shock (VHS) when looking sideways during running however remains unclear. The present study investigated whether VHS would remain stable when runners gazing at fixed-distance targets from multiple directions at eye level. Methods: Thirteen right-handed runners were asked to run on a self-paced treadmill with standard test shoes. Peak VHS and vertical tibial shock (VTS) were measured by two wireless accelerometers attached separately to the forehand and right distal tibia. Five small boxes with RGB LED lights were positioned two meters away from the center of the treadmill in front (i.e. at 0°), ±45° and ±90°. Each LED light was flashed once for 30 seconds during the running trial in a randomized sequence. Subjects were instructed to turn their gaze towards the flashing LED light and to keep their gaze at the light until the light was turned off. We compared peak VHS and VTS between gazing directions using repeated measures ANOVA. Results: We found stable VHS in general except that peak VHS was slightly smaller when runners were looking at the rightmost direction (p=0.04, Cohen's d=0.100) and VTS was higher in the same rightmost direction (p=0.016, Cohen's d=0.146). Conclusions: Head generally remains stable during running, except when gazing to an object 90° to the right. At such extreme gazing direction, we also observed changes in the lower limb kinetics for compensating the changes in VHS.