A labyrinth for stable walking
Authors:   , M Magnani Rina 
Published: August 2023

A labyrinth for stable walking: The role of head orientation and vestibular information in stabilizing gait

Summary
The central nervous system integrates a range of sensory inputs with motor outputs to ensure an essential function: the control of gait stability. The visual, somatosensory, and vestibular systems provide information to the central nervous system to keep the center of mass within the base of support while walking. The vestibular system plays a primary role in spatial orientation and head stabilization and has been related to mediolateral gait stability. This thesis aimed to study the role of vestibular information in the control of gait stability. Chapter 1 provides a brief overview of how the sensory systems contribute to the control of gait stability, and especially the findings of the vestibular system's role, it also covers the mechanisms of gait stability.
Chapter 2 entails an experiment using daily tasks that constrain head motion and the visual field by using a cell phone to observe the effects on gait stability in healthy young adults. The conditions in which head movement (head roll) and visual field were constrained such as talking and texting on a cell phone, impaired gait stability compared to the control condition. In chapter 3, the aim was to compare the effects of imposed head orientation only (forward, upward, downward, and yaw movement) on gait stability, and for that we studied different populations, which we assumed to have different levels of stability (dancers, young adults, older adults). Head orientation changes influenced the gait pattern and yaw motion reduced gait stability the most. Also, the findings showed that older adults had a less stable gait, but dancers were not more stable than young adults.
Chapter 4 addressed how the motor output evoked by the vestibular system varies according to the demands on the control of stability of walking. The results showed that vestibulomuscular coherence differed according to the gait stabilization demands. In comparison to the control condition, stabilized and wide-base walking conditions showed reduced coherence and narrow-base walking showed increased coherence overall. The results indicated that vestibulomuscular coupling varies systematically over the gait cycle and according to the need to control gait stability.
Given that vestibular information is used more in conditions with stronger stabilization demands, the destabilizing effects of a vestibular error signal, given by electrical vestibular stimulation, were expected to be larger in the conditions with higher vestibulomuscular coherence in chapter 4 (e.g. narrow-base walking). In chapter 5, we studied the effects of electrical vestibular stimulation as a vestibular error signal on gait stability. The vestibular error signal decreased gait stability, increased center of mass variability, and decreased coupling between the center of mass state and foot placement, the main mechanism to stabilize walking. The expected interactions of electrical vestibular stimulation and stabilization demands were not found. This indicates that although the use of vestibular information is dependent on stabilization demands, this is not directly reflected in gait stability.
Chapter 6 present a reflection on the findings to shed a light on the role of the vestibular system in the control of gait stability. It also discusses the limitations of the studies presented and the practical implications of the findings.
Taken together, this thesis revealed that the use of vestibular information is phase-dependent and varies with the stabilizing demands of gait. Head orientation changes, presumably through effects on vestibular output combined with effects on visual information, influence gait stability. Older adults were less stable than young adults, but dance training in young adults did not reduce the effects of head orientation changes.