Reflex Vasodilation-Induced Changes in Skin Temperature and Pain Threshold Following Combined Application of Infrared and TENS: A Preliminary Study

Purpose: This study examined the changes in skin temperature and pressure pain threshold (PPT) at the extremities following combined application of infrared rays (IR) and transcutaneous electrical nerve stimulation (TENS) to the posterior lower back. Methods: Ten university students completed a randomized within-subject crossover design. The conditions were assigned via computer-generated randomization: IR only (IR), IR combined with conventional TENS (C-TENS), IR combined with acupuncture-like TENS (A-TENS), and IR combined with brief-intense TENS (BI-TENS), with a minimum two-day washout between sessions. The skin temperature (DITI) and PPT (algometer) were measured at pre-intervention and 0, 5, 15, and 30 minutes post-intervention on the dominant upper and lower extremities. Two-way repeated measures ANOVA with Bonferroni correction was applied. Results: A significant intervention main effect was found for PPT (upper extremity: F=5.491, p=0.004; lower extremity: F=7.331, p=0.001); IR revealed significantly higher thresholds than C-TENS (upper) and both C-TENS and A-TENS (lower). The upper extremity skin temperature showed a significant intervention×time interaction (p=0.003). C-TENS decreased progressively, while BI-TENS transiently increased immediately post-intervention. The lower extremity skin temperature showed a significant intervention main effect (p<0.001), with BI-TENS significantly higher than IR (p=0.002); the interaction was not significant. Conclusion: TENS type modulated the temporal skin temperature pattern in the upper extremity; BI-TENS with IR produced a higher overall lower extremity skin temperature, suggesting a possible reflex vasodilatory response. These findings provide preliminary evidence for clinical decision-making in combined thermal-electrical therapy.