Vestibular and proprioceptive contributions to trunk stabilization differ across postural tasks and walking speeds.

Stabilizing upright posture of the trunk in humans relies on vestibular and proprioceptive afference. Previous studies found that the feedback responses to sensory afference vary between postures and tasks. We investigated whether and how vestibular and proprioceptive afference contribute to trunk stabilization during different postural tasks, and during walking at different speeds. Twelve healthy adults performed tasks in a random order - sitting, standing on the right foot or both feet, and treadmill walking at five speeds (0.8, 2.0, 3.2, 4.3 and 5.5 km h-1) - while exposed to unilateral muscle vibration on the right paraspinal muscles at the level of the second lumbar vertebra, or to a step-like electrical vestibular stimulation (EVS) with the anode behind the left ear. The mediolateral displacements of markers at the sixth thoracic (T6) level and the sacrum in the global coordinate system were used to evaluate the responses to sensory stimulation. No significant responses to EVS at the T6 and sacrum levels were found in sitting and standing. Responses to muscle vibration were significant and differed between unipedal standing compared with sitting and bipedal standing. The latter suggests a different interpretation of the sensation of muscle lengthening in these postures. During walking, the magnitude of the responses to both stimuli increased from very slow speeds to moderate speeds. From moderate to higher speeds, responses to muscle vibration decreased, whereas responses to EVS plateaued. These findings suggest speed-dependent modulation of vestibular and proprioceptive contributions in trunk stabilization during walking.