Muscle regeneration is improved by hot water immersion but unchanged by cold following a simulated musculoskeletal injury in humans.

Cryotherapy is a popular strategy for the treatment of skeletal muscle injuries. However, its effect on post-injury human muscle regeneration remains unclear. In contrast, promising results recently emerged using heat therapy to facilitate recovery from muscle injury. This study aimed to examine the effect of three different thermal treatments on muscle recovery and regeneration following a simulated injury in humans. Thirty-four participants underwent a muscle damage protocol induced by electrically stimulated eccentric contractions triggering regenerative processes following myofibre necrosis. Thereafter, participants were exposed to daily lower body water immersion for 10 days in cold (CWI, 15 min at 12°C), thermoneutral (TWI, 30 min at 32°C) or hot water immersion (HWI, 60 min at 42°C). Muscle biopsies were sampled before and at +5 (D5) and +11 (D11) days post-damage. None of the water immersions differed in recovery of force-generating capacity (P = 0.108). HWI induced a lower perceived muscle pain than TWI (P = 0.035) and lower levels of circulating creatine kinase (P ≤ 0.012) and myoglobin (P < 0.001) than TWI and CWI. Contrary to our hypothesis, CWI did not improve perceived muscle pain or reduce circulating markers of muscle damage (P ≥ 0.207). Expression of heat shock proteins 27 and 70 was significantly increased in HWI (P < 0.038) at D11 and appeared blunted using CWI. Furthermore, nuclear factor-κB expression significantly increased in all conditions except HWI, while interleukin-10 was upregulated only in HWI at D11 (P = 0.014). In conclusion, our results support the use of HWI but not cold, to improve muscle regeneration following an injury. KEY POINTS: Cryotherapy and heat therapy are popular strategies in the treatment of skeletal muscle injury; however, existing literature is equivocal, and their effects on human muscle regeneration remain unknown. We investigated the effect of three thermal treatments (cold water immersion (CWI): 15 min at 12°C; thermoneutral water immersion (TWI): 30 min at 32°C; or hot water immersion (HWI): 60 min at 42°C) performed daily for 10 days following electrically stimulated eccentric muscle damage inducing regenerative mechanisms. CWI did not improve chronic perceived muscle pain nor reduce circulating markers of muscle damage. HWI limited chronic perceived pain and circulating markers of muscle damage, potentially influenced inflammatory mechanisms, and increased the expression of heat shock proteins. HWI appears more beneficial than CWI in improving muscle regeneration after a muscle injury.