Hot water immersion: Maintaining core body temperature above 38.5°C mitigates muscle fatigue.

Purpose: Hot water immersion (HWI) has gained popularity to promote muscle recovery, despite limited data on the optimal heat dose. The purpose of this study was to compare the responses of two exogenous heat strains on core body temperature, hemodynamic adjustments, and key functional markers of muscle recovery following exercise-induced muscle damage (EIMD).
Methods: Twenty-eight physically active males completed an individually tailored EIMD protocol immediately followed by one of the following recovery interventions: HWI (40°C, HWI ₄₀ ), HWI (41°C, HWI ₄₁ ) or warm water immersion (36°C, CON ₃₆ ). Gastrointestinal temperature (T _gi ), hemodynamic adjustments (cardiac output [CO], mean arterial pressure [MAP], and systemic vascular resistance [SVR]), pre-frontal cortex deoxyhemoglobin (HHb), ECG-derived respiratory frequency, and subjective perceptual measures were tracked throughout immersion. In addition, functional markers of muscle fatigue (maximal concentric peak torque [T _peak ]) and muscle damage (late-phase rate of force development [RFD _100-200 ]) were measured prior to EIMD (pre-), 24 h (post-24 h), and 48 h (post-48 h) post-EIMD.
Results: By the end of immersion, HWI ₄₁ led to significantly higher T _gi values than HWI ₄₀ (38.8 ± 0.1 vs. 38.0°C ± 0.6°C, p < 0.001). While MAP was well maintained throughout immersion, only HWI ₄₁ led to increased (HHb) (+4.2 ± 1.47 μM; p = 0.005) and respiratory frequency (+4.0 ± 1.21 breath.min ^-1 ; p = 0.032). Only HWI ₄₁ mitigated the decline in RFD _100-200 at post-24 h (-7.1 ± 31.8%; p = 0.63) and T _peak at post-48 h (-3.1 ± 4.3%, p = 1).
Conclusion: In physically active males, maintaining a core body temperature of ~25 min within the range of 38.5°C-39°C has been found to be effective in improving muscle recovery, while minimizing the risk of excessive physiological heat strain.
(© 2023 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)