Ankle Somatosensation and Lower-Limb Neuromuscular Function on a Lunar Gravity Analogue

Abstract

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: Myoton; active movement extent discrimination assessment (AMEDA); electromyography (EMG); hypogravity; lower-limb muscle activity; muscle stiffness; muscle tone; wedge bed.

Figure 1

The AMEDA was used to assess participants’ ankle somatosensory acuity. The test required participants to place both feet on the AMEDA, either in (a) upright standing or (b) while lying on the lunar wedge bed. On the lunar wedge bed, (b) participants lay supine with their head elevated at approximately 9.2 degrees from horizontal. This position provided approximately 16% of the participant’s body weight through the plantar aspect of their feet (i.e., the vertical load, 1 g, is still present).

Figure 2

Results of the ankle AMEDA demonstrate greater performance in the upright standing position (represented by blue circle) compared to the head-elevated supine on lunar wedge-bed position (represented by red triangle). Mean scores are represented by an AUC score between 0.5 (score achieved by chance) and 1.0 (perfect score). Error bars represent 95% confidence interval.

Figure 3

Results of muscle activity recorded via EMG demonstrate significantly less muscle activity in head-elevated supine on the lunar wedge bed (represented by triangles) compared to upright standing (represented by circles) for tibialis anterior, peroneus longus, vastus lateralis, and rectus femoris. Vastus medialis also had reduced activity on the lunar wedge bed compared to upright standing; however, it did not reach statistical significance. Activity of gastrocnemius increased when in upright standing compared to the lunar wedge bed but did not reach statistical significance. Results represented by the mean peak RMS values as measured in microvolts (µV). Error bars represent 95% confidence interval. * Denotes p < 0.05.

Figure 4

Results of biomechanical properties recorded via MyotonPRO demonstrate significantly less muscle tone and muscle stiffness in head-elevated supine on the lunar wedge bed (represented by triangles) compared to upright standing (represented by circles) for all muscles. Tone (left) is represented in Hertz (Hz) and stiffness (right) is represented in Newtons per meter (N/m). Error bars represent 95% confidence interval. * Denotes p < 0.05 and ** denotes p < 0.001.