Effects of Alternating Mild‐Cold and Hot Water Immersion on Lower‐Leg Blood Flow Following Immersion in Healthy Young Adults

Shear stress-mediated increases in blood flow are key physiological stimuli that regulate vascular endothelial function. This study assessed whether alternating lower-leg immersion in mild-cold (25°C) and hot water (42°C), with longer durations and repeated cycles, induces greater increases in post-immersion blood flow. Sixteen healthy adults (11 males, 5 females; age, 23 ± 6 years) completed four randomized trials involving unilateral immersion of the left lower leg in mild-cold and hot water: 3 min each ×2 (3CH2), 6 min each ×1 (6CH1), 3 min each ×4 (3CH4), and 6 min each ×2 (6CH2). After immersion, skin blood flow (SkBF) was significantly higher in 6CH2 than that in 3CH2 (p < 0.05), and popliteal artery (PA) blood flow was higher in 6CH2 than that in all other trials (p < 0.05), with significant time × trial interactions (p < 0.001). The area under the curve (AUC) for post-immersion SkBF was higher in 6CH2 than that in 3CH2 (p < 0.05), and PA AUC was greater in 6CH2 than that in all other trials (p < 0.05). Core temperature remained unchanged, and cardiovascular variables exhibited only minor changes within the resting physiological range across all trials. Therefore, alternating mild-cold and hot water immersions with longer and repeated cycles significantly increased lower-leg blood flow following immersion. This strategy may represent a safe and well-tolerated local thermal intervention for increasing peripheral circulation, including in individuals unable to engage in exercise or tolerate whole-body heating.