Cold-water immersion (CWI) is among the most widely used post-exercise recovery modalities in elite sport and clinical rehabilitation, yet a fundamental paradox now emerges from the evidence: the same protocols that accelerate parasympathetic reactivation and reduce perceived muscle soreness also suppress the molecular and cellular machinery of skeletal-muscle hypertrophy when applied repeatedly across resistance-training mesocycles. This narrative review synthesises the multi-axis acute physiological response to CWI, the meta-analytic evidence supporting its acute recovery benefits, and the molecular, cellular and longitudinal evidence documenting its chronic attenuation of resistance-training adaptations. Acute CWI (10-15°C, 10-15 min) consistently augments parasympathetic reactivation, reduces soreness within 24-72 h (network meta-analytic SUCRA 88%) and modulates inflammatory time-courses. In parallel, equivalent protocols attenuate post-resistance-exercise mechanistic target of rapamycin complex 1 signalling, ribosomal biogenesis and satellite-cell-mediated myonuclear addition, translating into reduced type II fibre cross-sectional area and modest impairments of strength gain (effect size -0.23). Endurance-related microvascular and mitochondrial adaptations appear less affected and may be selectively enhanced. A protocol-by-goal decision framework links CWI parameters - water temperature, duration, body coverage and temporal placement - to expected adaptive outcomes across hypertrophy, strength, endurance and concurrent training contexts. CWI is neither uniformly beneficial nor uniformly harmful; its effects are predictable and depend on the training goal. CWI should be used when fast recovery between sessions matters most, and withheld after resistance training when muscle growth is the main goal of the training block.
