Stress-Induced Changes in Autonomic Activity and Fascia Hydration: A Cross-Recurrence Quantification Analysis in a Naturalistic Exam Paradigm

Background: The autonomic nervous system is increasingly recognized as a key regulator of fascial physiology. Interventions that modulate autonomic activity, including breathing techniques, relaxation strategies, and movement-based therapies, have therefore been proposed to influence tissue hydration and mechanics, as well as functional outcomes, yet evidence under real-life stress conditions remains limited. This study examined the effects of acute exam stress on autonomic activity, lumbar tissue hydration and their dynamic interaction. Methods: Eighteen students (9 female, 9 male; 36.1 ± 11.6 years) were assessed in front of peers and an examiner as part of an osteopathic medical school training (exam condition) and again after a four-week recovery period (control condition). Heart rate variability (HRV) and local bioelectrical impedance analysis (BIA) of the lumbar region were recorded. Subjective stress over the past four weeks was measured using the Perceived Stress Scale (PSS-10). Interactions between HRV and BIA were analyzed using cross-recurrence quantification analysis (CRQA). Results: Compared to the control condition, the exam condition showed significantly increased CRQA values (0.38 ± 0.44 vs. −0.33 ± 1.27; p = 0.037), indicating stronger system coupling. HRV was significantly reduced (716 ± 141 ms vs. 813 ± 105 ms; p < 0.001), reflecting increased autonomic activation. BIA values were also significantly lower (24.55 ± 9.28 Ω vs. 30.25 ± 9.77 Ω; p = 0.015), suggesting altered tissue hydration. PSS-10 scores were significantly higher during the exam condition (22.63 ± 5.46). Conclusions: Acute psychosocial stress appears to induce measurable changes in autonomic regulation and lumbar tissue properties and enhances their dynamic interaction. These findings provide initial indication for the concept of autonomic-fascial coupling and emphasize the relevance of integrative, biopsychosocial approaches in clinical physical therapy.