Contrast Heat–Cold Versus Thermoneutral Showering in Trained Combat Sport Athletes: A Randomized Field Trial of Recovery Outcomes

<p>Contrast heat-cold showering is popular for recovery, but multisystem evidence and persistence after cessation remain unclear. To evaluate whether a 4-week post-training contrast heat-cold shower intervention produces an integrated recovery profile across four domains (autonomic, endocrine, perceptual, and microvascular) rather than testing a single isolated physiological pathway, across baseline (T0), post-intervention (T1), and 2-week wash-out (T2). Sixty combat-sport athletes were randomized to contrast showers (10 min alternating warm 38-40°C and cold 13-15°C) or an active thermoneutral-shower comparator (10 min, 32-34°C) after training for 4 weeks. Primary analyses were per-protocol (≥ 75% compliance; n = 57), with intention-to-treat sensitivity analyses. Outcomes were Total Quality Recovery (TQR), morning salivary cortisol (two mornings averaged), resting HRV, and post-occlusive reactive hyperemia (PORH). From T0 to T1, favorable between-group changes were observed for resting HR (ΔΔ -2.10 bpm, 95% CI -2.24 to -1.96; g -0.65; p < 0.001), lnRMSSD (ΔΔ 0.123 log units, 95% CI 0.108 to 0.137; g 0.74; p < 0.001), with similar T1 effects for RMSSD and SDNN, and TQR (ΔΔ 0.61 points, 95% CI 0.27 to 0.95; g 0.80; p < 0.001). These T1 autonomic and perceptual changes were not maintained at T2 (all T2-T0 ΔΔ p > 0.05). Cortisol and PORH-derived outcomes showed no statistically clear between-group differences at T1 or T2 (all p > 0.05). Compared with thermoneutral showering, 4 weeks of post-training contrast heat-cold showering produced short-term favorable between-group changes in autonomic regulation and perceived recovery, but not in morning cortisol or PORH-derived microvascular reactivity. These effects were not maintained after wash-out; therefore, causal attribution to the shower intervention alone and claims of persistent physiological adaptation should be made cautiously. Trial registration: ISRCTN15418049.</p>