The Balance for Balance: Weighting of Vestibular and Proprioceptive Signals in Trunk Stabilization During Walking

Sensory signals from the visual, vestibular, and proprioceptive systems are integrated based on their reliability to stabilize upright posture, in a process known as ‘sensory reweighting’. For example, when the sensory signals become noisy, the central nervous system decreases the reliance on such signals (down-weighting), while up-weighting alternative sensory modalities. However, in some cases, such reweighting could lead to over-reliance on specific sensory systems, which may hamper adaptation across tasks and environments. It has been suggested that reduced weighting of proprioceptive signals from paraspinal muscles may explain the increased fall risk in individuals with low-back pain. The overall aim of this thesis was to investigate how vestibular and proprioceptive signals are weighted and how this process affects trunk stabilization especially during walking. The first two studies in this thesis showed that vestibular signals contribute to trunk stabilization by modulating paraspinal muscle activity during walking, and that this modulation depends on head orientation. Next, we showed that vestibular signals contribute to the estimation of centre of mass state, and as such to stabilizing upright posture during walking through coordinated foot placement. Finally, we showed how sensory weighting of vestibular and proprioceptive signals changes across different conditions, for example between standing on one or two legs and between different walking speeds. Preliminary results showed that prolonged vestibular stimulation induces down-weighting of vestibular signals and up-weighting of lumbar proprioceptive signals for trunk stabilization during walking. In sum, our findings show that vestibular and proprioceptive signals are dynamically weighted, varying between postural tasks, over walking speeds and over the gait cycle. We provide preliminary evidence that the weighting can be changed by sustained stimulation. These findings advance our knowledge of multisensory integration for control of upright posture, especially during walking. Moreover, our results suggests that rehabilitation interventions addressing sensory impairments that affect control of trunk posture and balance may need to include different tasks and walking speeds to facilitate transfer of functional improvements into real-world situations. Finally prolonged stimulation may offer possibilities to target changed sensory weighting in rehabilitation.