Abstract: INTRODUCTION Exposure to weightlessness and its ground-based model, Dry Immersion (DI), which induces support and motor unloading, leads to hyperreflexia due to increased spinal excitability. However, the integrated dynamics of both spinal and corticospinal (supraspinal) excitability during prolonged DI remain unexplored. A comparative analysis using transcranial magnetic stimulation, trans-spinal magnetic stimulation (MS), and the H-reflex can reveal adaptive changes in motor control at different levels of the nervous system. The aim of this study was to investigate the dynamics of corticospinal and spinal excitability during and after 14-day DI. METHODS The study involved eight healthy male volunteers. Spinal motoneuron pool excitability was assessed by analyzing the H-reflex parameters (threshold and peak amplitude) of the soleus and gastrocnemius muscles. Corticospinal excitability was evaluated by recording motor evoked potentials (MEPs) from the same muscles in response to transcranial and trans-spinal MS, analyzing the resting motor threshold (RMT) and MEP amplitude. H-reflex measurements were taken before DI, 5 hours after its onset, on days 3, 5, 7, 9, 11, 13, 15 of DI, and on recovery days 2 and 5. MS sessions were conducted before DI, after the 1st and 2nd weeks of DI, and on recovery days 2 and 4. RESULTS H-reflex: a decrease in the soleus H-reflex threshold was observed within 5 hours of DI onset, reaching a minimum by Day 3, while its peak amplitude was maximal on Day 7. Similarly, the gastrocnemius H-reflex amplitude increased by Day 5 and remained elevated until the end of immersion. Corticospinal excitability (transcranial MEPs): after 1 week of DI, RMT of transcranial MEPs slightly decreased, while peak amplitude increased significantly. These changes intensified after 2 weeks of exposure. By recovery day 4, neither parameter had returned to baseline. Spinal excitability (trans-spinal MEPs): RMT of trans-spinal MEPs decreased after 1 week but increased slightly after 2 weeks of DI. In contrast, MEP amplitudes increased progressively throughout the 14-day exposure. By recovery day 4, both RMT and amplitude for m. soleus remained above baseline, while values for m. gastrocnemius returned to pre-DI levels. DUSCUSSION A 14-day DI induced a progressive increase in corticospinal excitability that persisted for at least 4 days post-exposure. Spinal excitability also increased, although the slight rise in RMT after the second week suggests that compensatory spinal mechanisms may have mitigated a further increase. Both 7-day and 14-day unloading altered spinal reflexes, with dynamics indicating distinct acute and chronic phases. This pattern points to the development of adaptive spinal compensatory processes after the first 3–5 days of exposure. The study was supported by a grant from the Russian Science Foundation № 24-15-00309, https://rscf.ru/en/project/24-15-00309/.

Cite: Shigueva T.A. , Riabova A.M. , Artamonova N.M.A. , Bekreneva M.P. , Kurbanova O.E. , Gainutdinova E.A. , Zelenskaya I.S. , Ponomarev I.I. , Kitov V.V. , Rukavishikov I.V. , Tomilovskaya E.S.
Changes in corticospinal and spinal excitability during 14-day Dry Immersion
45th International Symposium on Gravitational Physiology (ISGP), Cologne, Germany, 25-29 May 2026 25-29 May 2026