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).