Head orientation modifies the contribution of vestibular information to trunk control during walking

Electrical vestibular stimulation (EVS), which produces illusions of self-movement, is<br/>widely used to probe the vestibular system. Previously, we found EVS induced phasic,<br/>vertebral level specific paraspinal muscle activity during walking. This indicates that<br/>vestibulospinal reflexes contribute to trunk stabilization during walking. Head<br/>orientation changes the direction of the EVS induced illusion (e.g., in the frontal plane<br/>when facing forward and in the sagittal plane when the head is turned 90 degrees to<br/>the side) and modifies the vestibulospinal reflexes during standing. We investigated<br/>whether and how head orientation changes the contribution of vestibular information<br/>to paraspinal muscle activity during walking. Sixteen participants walked on a treadmill<br/>for 8 minutes at 78 steps/min and 2.8 km/h, in four conditions defined by the presence<br/>of EVS and head orientation (facing forward or leftward). Bipolar electromyography<br/>(EMG) was recorded bilaterally from the paraspinal muscles from the C7 to L4 vertebral<br/>level. Coherence, gain and delay between EMG and EVS were characterized.<br/>Significant EVS-EMG coherence in the paraspinal muscles was observed at heel<br/>strikes in both head orientations. The leftward head orientation significantly decreased<br/>the EVS-EMG coherence and gain compared to walking with head forward. This<br/>overall decrease may be explained by the effectiveness of passive stabilizing control<br/>in the sagittal plane during walking. However, the decrease in coherence was only<br/>significant at the left-lower and the right-upper vertebral levels around left heel strikes.<br/>This is possibly because the head yaw rotation pivot is located posterior to the origin<br/>of vestibular reference frame. Therefore, a leftward head orientation will induce an<br/>offset of the vestibular axis to the left relative to the trunk reference frame.<br/>Consequently, motion to the left encoded by the vestibular signal would be smaller<br/>than motion to the right, causing an asymmetric response to vestibular afference. This<br/>study confirmed the contribution of vestibular information in trunk stabilization while<br/>highlighting that this contribution is impacted by head orientation during walking.