Sex differences in regulatory mechanisms of the cardiovascular system during simulated hypokinesia

The study examined sex-specific differences in the autonomic response to simulated hypokinesia. Nine healthy males (23±3 years) and nine females (26±3 years) underwent 5‑day dry immersion. Heart rate variability (HRV) analysis and factor analysis (8 indices) were used to assess autonomic regulation. Factor analysis revealed three factors explaining >80% of variance. Factor 1 (SDNN, RMSSD) was universal for both sexes, but only in males did it also include heart rate with the opposite sign. Factor 2 comprised LF in females and VLF with HF, % (opposite signs) in males. Factor 3 comprised VLF in females and LF in males. Thus, adaptation to hypokinesia involves both universal (SDNN, RMSSD) and fundamentally different sex-specific regulatory strategies: modular organization in females (independent LF and VLF) versus integrated, reciprocal organization in males (VLF and HF coupling). Analysis of the dynamics of individual HRV parameters revealed that in women, the role of vagal modulations is initially more pronounced, and their weakening passively enhances the contribution of other regulatory branches; in men, by contrast, all regulatory levels are engaged simultaneously. These results highlight the importance of considering sex as a factor determining autonomic strategy. They also provide a basis for further research under conditions of reduced mobility using larger pooled cohorts and a longer follow-up period.