Purpose: This study aimed to (a) examine the effects of hot-water immersion (HWI) on vertical-jump performance, lower-limb power, and stretch-shortening cycle responses and (b) evaluate temperature-dependent responses by comparing 2 HWI conditions (40 °C vs 42 °C). Methods: Twenty healthy, physically active participants (12 male and 8 female; age 25.1 (2.7) y; body mass index 22.6 [1.1] kg/m2) completed countermovement-jump (CMJ) and drop-jump assessments before, after, and 15 minutes after a 45-minute water-immersion session at either 34 °C (control), 40 °C, or 42 °C in a randomized order. CMJ outcomes included jump height, peak force, and peak power, while drop-jump outcomes included jump height, reactive strength index, and vertical stiffness. Results: CMJ height increased following a 45-minute HWI session at both 40 °C and 42 °C (P < .001, d = 1.09; P < .001, d = 1.77), with no differences between the 2 conditions (P = .515). Similar improvements were found for peak power and force. At 15 minutes post-water immersion, all performance measures returned to baseline and did not differ from the control condition. For the drop-jump tests, a main effect of temperature was observed. HWI at 42 °C significantly increased drop-jump height (P = .004; d = 0.71), whereas 40 °C did not (P = .205). Reactive strength index increased following both 40 °C (P = .002; d = 0.74) and 42 °C (P < .001; d = 0.91) conditions. No significant changes were observed in vertical stiffness. Conclusion: Both HWI conditions, 40 °C and 42 °C, improved CMJ performance (height, force, and power) and reactive strength index, while only HWI at 42 °C enhanced drop-jump height. These effects were transient, returning to baseline within 15 minutes postimmersion.
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