Oral l-menthol reduces thermal sensation, increases work-rate and extends time to exhaustion, in the heat at a fixed rating of perceived exertion

Purpose The study investigated the effect of a non-thermal cooling agent, l -menthol, on exercise at a fixed subjective rating of perceived exertion (RPE) in a hot environment. Method Eight male participants completed two trials at an exercise intensity between ‘hard’ and ‘very hard’, equating to 16 on the RPE scale at ~35 °C. Participants were instructed to continually adjust their power output to maintain an RPE of 16 throughout the exercise trial, stopping once power output had fallen by 30%. In a randomized crossover design, either l -menthol or placebo mouthwash was administered prior to exercise and at 10 min intervals. Power output, V ˙ O 2 , heart rate, core and skin temperature was monitored, alongside thermal sensation and thermal comfort. Isokinetic peak power sprints were conducted prior to and immediately after the fixed RPE trial. Results Exercise time was greater (23:23 ± 3:36 vs. 21:44 ± 2:32 min; P = 0.049) and average power output increased (173 ± 24 vs. 167 ± 24 W; P = 0.044) in the l -menthol condition. Peak isokinetic sprint power declined from pre-post trial in the l -menthol l (9.0%; P = 0.015) but not in the placebo condition (3.4%; P = 0.275). Thermal sensation was lower in the l -menthol condition ( P = 0.036), despite no changes in skin or core temperature ( P > 0.05). Conclusion These results indicate that a non-thermal cooling mouth rinse lowered thermal sensation, resulting in an elevated work rate, which extended exercise time in the heat at a fixed RPE.