The impact of 14-day head-down bed rest with or without an exercise countermeasure on standing balance control: a randomized controlled trial

Exposure to space environment disrupts the sensorimotor system due to adaptation to microgravity, leading to spatial disorientation, impaired coordination, and reduced postural control upon return to Earth. To simulate these effects, a six‑degree head‑down bed rest (HDBR) protocol was used. This randomized controlled trial examined whether exercise could mitigate declines in standing balance control following 14 days of HDBR. Twenty-two participants were assigned to either an exercise group, performing daily high-intensity interval training combined with resistance and aerobic exercise, or a non-exercising control group. Balance was assessed using the sensory organization test (SOT) and head-shake SOT, with outcomes including equilibrium score (ES), strategy analysis (SA), and vestibular-related measures (e.g., SOT-2M, SOT-5M). No between-group differences were observed in either ES or SA. However, when data were pooled, ES during SOT-2M was significantly decreased (p < 0.001), indicating greater instability under eyes-closed, head-shake conditions. Higher baseline ES during SOT-2M was associated with smaller declines post-HDBR, particularly in the exercise group. No effects of sex or age (55-65 years) were found. These findings suggest that exercise may not prevent balance declines after short-term HDBR and that baseline balance capacity may be associated with variability in individual responses. Clinical Trial Registration: NCT04964999 (2021-07-16).