Acute Oral Hydration and Thoracolumbar Fascia Stiffness in Healthy Adults

Objectives: To determine whether acute oral hydration alters thoracolumbar fascia stiffness in healthy adults and to characterize concurrent changes in urine-specific gravity and bioimpedance-derived fluid compartments. Methods: In this prospective single-arm repeated-measures study, 50 healthy adults were assessed at baseline (T0), 60 min after ingestion of 2 L of water (T1), and 6 h later (T2). Thoracolumbar fascia stiffness was measured by shear wave elastography at four standardized points (P1-P4). Hydration was assessed using urine specific gravity and bioimpedance-derived intracellular water, extracellular water, and the extracellular-to-intracellular water ratio. Repeated-measures analyses of variance and Holm-adjusted pairwise comparisons were used. Results: Urine specific gravity changed markedly over time (F(2,98) = 290.175, p < 0.001, ηp² = 0.856), decreasing from 1.016 ± 0.002 at T0 to 1.006 ± 0.002 at T1, with partial return at T2 (1.014 ± 0.002). Mean thoracolumbar fascia stiffness remained unchanged (45.88 ± 2.01, 45.87 ± 2.01, and 45.89 ± 2.02 kPa at T0, T1, and T2, respectively; F(2,98) = 0.671, p = 0.514, ηp² = 0.013). The two-factor shear wave elastography model showed no effect of point (F(3,147) = 2.147, p = 0.097) and no time-by-point interaction (F(6,294) = 0.195, p = 0.978). Bioimpedance parameters also showed no significant change (all p ≥ 0.381). An exploratory correlation between change in urine specific gravity and change in mean stiffness was observed from T0 to T1 (r = -0.346, p = 0.028). Conclusions: In this single-arm repeated-measures study, acute oral water ingestion was followed by a clear urinary hydration response but by no detectable short-term within-participant change in thoracolumbar fascia stiffness or bioimpedance-derived fluid distribution in healthy adults. Because no non-hydration or placebo control condition was included, these findings should be interpreted as evidence of temporal stability under the tested protocol rather than as a definitive controlled estimate of the causal effect of hydration. Clinical Registration: ISRCTN96046168 (31 March 2026).