Purpose The objective of this randomized, single-blind, crossover study was to determine whether patches re-emitting far-infrared radiation (FIR) affect the energy cost of running (ECR) and the maximal isometric force during and after a short trail run. Methods Sixteen highly trained male trail runners (age: 21.0 ± 5.8 years; peak oxygen consumption at 1850 m: 65.3 ± 7.4 mL min-1 kg-1) ran six laps of a 3.6 km course (elevation gain per lap: 110 m) while wearing FIR patches or placebo patches positioned at specific key points (i.e., points of myofascial chains convergence and acupuncture points). ECR, maximal isometric force of quadriceps, rest popliteal artery diameter and blood flow, tibial anterior oxygenation and perfusion, and perceived fatigue were assessed before, immediately after, and 48 h after the short trail run, while maximal isometric force was also assessed midway. Heart rate and speed were recorded continuously during the run. Results During the run, speed and perceived fatigue did not significantly differ between conditions. However, 48 h after the run, there was a significant difference (post hoc analysis from condition × time interaction) in perceived fatigue between the placebo and FIR conditions (+ 1.3 ± 2.1, p < 0.01, dz = 0.63). For the ECR, there was no significant condition × time interaction (p = 0.18) and no effect of time (p = 0.85) or condition effect (p = 0.13). For the maximal isometric force, there was no significant condition × time interaction (p = 0.09) or condition effect (p = 0.78), although there was an effect of time (p < 0.01). For the resting popliteal artery diameter, there was no significant time × condition interaction (p = 0.23), or effect of condition (p = 0.13), or an effect of time (p = 0.79). For the popliteal artery blood flow, there was no significant condition × time interaction (p = 0.35), or effect of condition (p = 0.59). However, a significant effect of time (p < 0.01) was found. For the tissue saturation index, the total hemoglobin, deoxyhemoglobin and oxyhemoglobin, we did not observe a significant condition × time interaction (p > 0.14) or significant effect of condition (p > 0.27) or time (p > 0.09). Conclusion Thus, neither the short trail nor the FIR patches had a significant effect on ECR. Furthermore, FIR did not significantly influence resting vascular function and tissue oxygenation, or attenuate maximal isometric force loss; however, our data showed a significant reduction in perceived fatigue 48 h after the short trail run. However, because the experimental protocol was not specifically designed to investigate this aspect, and due to the dissociation between the effect on perceived fatigue and unchanged physiological responses, the clinical relevance of these findings may be limited and warrants further investigations.
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