Ankle somatosensation in novel environments: A simulation of lunar gravity

Abstract

Exposure to microgravity (characterised by a sense of weightlessness) has shown to negatively impact human physiology. However, the effects of hypogravity (gravity less than that on Earth, but greater than microgravity, e.g., on the Moon) is less understood. Somatosensation, which includes several sensory modalities, plays an important role within the sensorimotor system to help identify body position and movement. Understanding the impact of hypogravity on somatosensation is crucial to assist astronauts in maintaining stable balance and gait, particularly as international space agencies prepare for extended missions to the Moon and Mars. However, conducting research in hypogravity is challenging due to high resource costs and small sample sizes. One method to simulate hypogravity is lying in a head-elevated supine position to reduce axial loading. This method affords research to be conducted on larger sample sizes and allows researchers to explore interventions for the purposes of supporting space travel.
Broadly, this research aims to assess how reduced axial loading (simulating Moon gravity) affects ankle somatosensory acuity and lower limb neuromuscular characteristics. It also examines whether increasing tactile stimulation to the foot, ankle and lower leg can improve ankle somatosensory acuity and postural stability compared to being barefoot. First, the effectiveness of a ‘compression-tactile sock’ is tested under normal gravity (1g) conditions as a proof of concept. Then, it is reevaluated under simulated hypogravity to assess whether the intervention is effective across different gravity conditions. Additionally, the relationship between ankle somatosensory acuity and postural stability is also explored under normal (1g) gravity to understand how somatosensation contributes to posture and balance.
The first chapter provides a general introduction to the anatomy of the somatosensory system and explores the effects of reduced gravity to somatosensation. It reviews micro- and hypogravity simulation techniques, and somatosensation assessment methods. Chapters two through five present a series of studies that address the aims of this research. Throughout these chapters, ankle somatosensory acuity was assessed using an Active Movement Extent Discrimination Assessment (AMEDA) protocol. Simulated Moon gravity was achieved using a purpose-built lunar wedge-bed which required participants to lie supine with their head elevated 9.2 degrees. administering approximately 16% of their body weight through their lower limbs. Postural stability was assessed using a Sensory Organisation Test (SOT) in upright, normal (1g) gravity only. All studies included healthy adults with unrestricted movement ability. The final chapter discusses the collective results of the studies and offers recommendations for future research.
In brief, ankle somatosensory acuity and neuromuscular characteristics (lower limb muscle activity, tone, and tension) were found to be reduced when on the lunar wedge-bed compared to when in upright standing. However, some lower limb muscle activity (namely in the Gastrocnemius and Vastus Medialis muscles) remained consistent between positions. This demonstrates how the somatosensory and neuromuscular systems may respond to a Moon gravity environment. When upright (normal 1g gravity) ankle somatosensation scores (AMEDA scores) were positively correlated with the most demanding postural stability conditions (SOT scores), highlighting the complementary nature of the tests in assessing somatosensation. Compared to barefoot, wearing a standard compression sock or a compression-tactile sock improved ankle somatosensory acuity when upright standing and when on the lunar wedge-bed. Notably, these improvements were observed only in individuals with below-average somatosensory acuity and had no effect to postural stability (SOT scores in normal 1g gravity). These findings suggest that responses to external stimuli vary among individuals and improvements in somatosensation in both Earth and simulated Moon gravity do not necessarily translate to enhanced postural stability.
This work expands our understanding of human physiology by revealing how the somatosensory and the neuromuscular systems are dampened under simulated hypogravity compared to when upright standing. It also emphasises the role of ankle somatosensation within postural control when in normal (1g) gravity and explored possible short-term interventions to improve somatosensory acuity in normal (1g) and simulated hypogravity.

Date of Award	2025
Original language	English
Supervisor	Gordon WADDINGTON (Supervisor), Nick BALL (Supervisor), Jeremy WITCHALLS (Supervisor) & Sarah WALLWORK (Supervisor)