Abstract

Aim: Autonomic nervous system (ANS) imbalance may contribute to functional pelvic disorders such as overactive bladder (OAB). Transcutaneous tibial nerve stimulation (TTNS) is a non-invasive therapy for OAB; however, its autonomic modulation mechanisms remain unclear. This exploratory study evaluated the acute effects of TTNS on ANS activity in healthy volunteers using heart rate variability (HRV) to model neuroautonomic pathways relevant to OAB. Methods: In this open-label, prospective, exploratory, single-arm study, 20 healthy volunteers (11 women; median age 31 years) underwent three 10-min phases: baseline rest, continuous submotor TTNS via surface electrodes on the left tibial nerve, and post-stimulation recovery. HRV was recorded using a Polar H10 sensor and analyzed with Kubios software. Time-domain (SDNN, RMSSD, pNN50), frequency-domain (LF, HF, LF/HF), and additional indices (PNS, SNS, Stress indices) were assessed in standardized 5-min windows. Friedman and Wilcoxon tests with Bonferroni correction were applied. A vagal-oriented composite response score (z-delta mean) was correlated with age and BMI (Spearman, permutation-based p values). Results: Significant phase effects were observed for PNS index, SNS index, Stress index, SDNN, and RMSSD (p < 0.05). Post-hoc analyses confirmed increases in PNS index, SDNN, and RMSSD, and decreases in SNS and Stress indices during stimulation compared with baseline. Partial post-stimulation persistence was noted for SDNN and Stress index, although these changes did not remain significant after Bonferroni correction. The composite response score correlated negatively with age (rho = -0.52; p = 0.019; permutation p = 0.016) and showed a non-significant positive trend with BMI (rho = 0.38; p = 0.10). Conclusions: Acute TTNS enhances parasympathetic and suppresses sympathetic activity, with partially sustained effects after stimulation. Younger age predicts stronger vagal responsiveness, suggesting age-dependent neuromodulatory efficacy. Validation in OAB populations is warranted.