Publications

Background/Objectives: The adverse effects of low gravity on human physiology are well documented; however, much of the literature is directed at changes which occur in microgravity (µg: weightlessness) with relatively less documented on changes in hypogravity (<1 g; >µg: gravity less than Earth’s but more than microgravity). Somatosensation and neuromuscular control may be of particular importance for astronauts as they prepare for future missions to walk on the Moon. This study aimed to explore the effect of reduced weight bearing (to simulate conditions of hypogravity) on ankle somatosensation, lower-limb muscle activity, tone, and stiffness, compared to full weight bearing.

Methods: Participants completed an ankle somatosensory acuity task (active movement extent discrimination assessment [AMEDA]) in two body positions: (1) upright standing (1 g), and (2) in a head-elevated supine, semi-weight bearing (0.16 g) position using a custom-built inclined “wedge bed”. The second position induced ~16% body weight on to the plantar aspect of the feet, simulating that of lunar gravity. We compared the AMEDA scores between the two positions. Lower-limb muscle activity was recorded via surface EMG throughout the AMEDA task for both positions. The ankle AMEDA has five depths of ankle inversion. We compared muscle activity between the body positions, and muscle activity between inversion depths “1” and “5” (within each position). Lower-limb muscle tone and muscle stiffness were assessed at rest in both body positions using the MyotonPRO.

Results: Fifty-five participants between the ages of 18 and 65 (28 females, 27 males; mean age of 40 years) completed the study. The AMEDA scores, muscle tone and stiffness were reduced when the participants were on the lunar wedge bed, compared to upright standing (p = 0.002; p < 0.001; p < 0.001). Some lower-limb muscles exhibited less activity in the lunar wedge-bed position compared to upright standing (tibialis anterior, peroneus longus, vastus lateralis, rectus femoris; p < 0.05) but others were unchanged (gastrocnemius, vastus medialis; p > 0.05). Muscle activity was unchanged between the AMEDA depths (p = 0.188).

Conclusions: The results provide insight into how the somatosensory and neuromuscular systems respond to reduced weight bearing and potentially lunar gravity conditions, thereby informing how to target interventions for future missions.

Keywords: hypogravity, active movement extent discrimination assessment (AMEDA), lower-limb muscle activity, electromyography (EMG), MyotonPRO, muscle tone, muscle stiffness, wedge bed

We found ankle somatosensation and some lower-limb neuromuscular function was reduced in the supine lying on the lunar wedge-bed position when compared to upright standing. These results suggest that somatosensory and neuromuscular function are negatively affected by a rapid change in posture, even with a reduced degree of body loading. This has important implications for astronauts taking their first steps on the Moon, suggesting that a reduction in lower-limb function could increase the risk of injuries and falls as they adapt to the challenging environment. However, it is important to note that while all other muscles assessed were negatively impacted, the activity of gastrocnemius and vastus medialis remained unchanged by the change in posture. This could be attributed to the specific roles of these muscles, or excessive variability within the data. Additionally, there was no change in muscle activity across the various AMEDA inversion depths, possibly due to the limited sensitivity of the EMG. These findings suggest that further research is warranted to better understand neuromuscular patterns and explore other sensory cues which might impact somatosensation, particularly under reduced body loading and within real hypogravity conditions. Such knowledge can aid the development of countermeasures to negate the side effects of reduced body loading to ankle somatosensation and neuromuscular function. As space agencies prepare for upcoming lunar exploration, interventions to address and mitigate decreases in ankle somatosensory and lower-limb muscle function are crucial for ensuring astronaut safety and the success of missions.