Effects of Hot Water Immersion at Different Temperatures during a Transition Period on Vertical Jump Performance

Increased muscle temperature (Tm) enhances exercise performance but can decrease during the transition period between the warm-up and the competition. Although passive warming strategies can maintain Tm, the optimal heating temperatures and their effects on performance are unclear. Seventeen healthy males participated in this study with four experimental conditions: no intervention (CON), hot water immersion at 36°C (HWI36), 39°C (HWI39), and 42°C (HWI42) during a 20-min transition period. Vertical jump (VJ) performance, core temperature (Tc), the heart rate (HR), and perceived thermal comfort were measured across eight VJ sets (VJ1−8) performed over 1 h, and biomechanical analyses were conducted to understand the changes in jump performance. Significant improvements in VJ performance were observed immediately after the transition period under all hot water immersion (HWI) conditions compared to CON (CON 49.2 ± 4.7 vs. HWI36 50.9 ± 4.8 vs. HWI39 52.2 ± 3.8 vs. HWI42 53.5 ± 4.4 cm, all p < 0.05 for VJ2). HWI39 and HWI42 sustained greater VJ performance up to 50 min post-immersion than did CON and HWI36 (all p < 0.05 for VJ2–7). HWI39 and HWI42 increased knee range of motion and peak knee angular acceleration in the concentric phase during jumps (p < 0.05). Tc and the HR were significantly higher under the HWI42 condition (p < 0.05), with participants reporting greater thermal discomfort. HWI39 and HWI42 during the transition period enhanced and sustained VJ performance, with HWI39 being more tolerable. The enhanced VJ performance was primarily due to improved range of motion and concentric muscle contraction. These findings provide valuable insights for the optimization of passive warming strategies in competitive sports.