The impact of 14-Day Head-Down Bed Rest with or without an Exercise countermeasure on Standing Balance Control.

Introduction: Exposure to space environment has significant repercussions on the sensorimotor system due to the mechanisms of adaptation to microgravity. Astronauts experience spatial disorientation, inadequate coordination of their movements, and disrupted postural control upon return to Earth. Since these changes can compromise their ability to perform longer-duration missions, it is imperative to limit these adverse effects. The effects of microgravity were experimentally simulated through a six‑degree head‑down bed rest (HDBR) protocol. It is well recognized that exercise can mitigate some of the negative effects that come after extended periods of inactivity/microgravity. Therefore, the aim of this study was to determine whether physical exercise is an effective countermeasure to limit the decrease in standing balance control following 2 weeks of HDBR. Methods: Twenty-two participants were divided into two groups for a 14-day HDBR intervention. The first group engaged in a daily high-intensity interval training (HIIT) regimen combined with resistance exercise and continuous aerobic training, while the second group served as the control and refrained from any exercise activities. To evaluate participants' capacity to integrate visual, somatosensory, and vestibular systems for balance maintenance across six distinct conditions, the Sensory Organization Test (SOT) as well as the Head-Shake SOT from NeuroCom® was employed. The following outcomes were computed: Equilibrium score (ES), indicative of balance capabilities, the strategy analysis score (SA), reflective of the strategies employed by participants to sustain balance, and the components of the Head-Shake SOT, namely ES ratio fixed score, ES ratio sway score, HS-SOT 2 score and HS-SOT 5 score, which tests efficient processing of vestibular inputs in complex conditions. Results: Following the intervention, no difference between groups were observed in either score. However, pooling participants of the control and experimental group together showed a significant shift in their postural strategies after HDBR towards a hip strategy, rather than ankle strategy (SA score; p  = 0.027). This suggests a greater perceived destabilization caused by the perturbation. Furthermore, the HS-SOT 2 score was significantly decreased between pre and post HDBR ( p  < 0.001). This suggests that with eyes closed, vertical headshakes lead to increased instability after HDBR. Moreover, irrespective of the group allocation, higher pre-HDBR scores were associated with smaller changes in the scores post-HDBR. This negative correlation was mainly observed in the exercise group. No difference between men and women or with age (55 to 65) were observed within each group and when groups were pooled together. These results suggest that countermeasures might not prevent decline in balance abilities following 2 weeks of HDBR and that baseline individual balance capabilities might be indicative factor of the changes in balance responses after HDBR. Clinical Trial Registration Number: NCT04964999, 2021-07-16