Effects of Menthol Mouth Rinsing on Performance and Surface EMG Activity During Heat-Stressed Cycling

Objective: This study investigated the effects of menthol (MEN) mouth rinsing (MR) on cycling performance, neuromuscular activation, and perceptual responses during high-intensity exercise in the heat. Methods: A total of 10 trained adolescent male cyclists (16.7 ± 1.3 yrs; VO_2peak: 62.3 ± 7.6 mL·kg^-1·min^-1) completed a familiarization and two randomized, single-blind trials using a modified variable cycling test (M-VCT) in the heat (31.45 ± 0.59 °C; 23.40 ± 2.55% RH). The participants rinsed with 0.01% L-menthol or a placebo every 6 min during exercise. Power output (PO), cadence (RPM), rating of fatigue (ROF), affective feeling (FS), and surface electromyography (sEMG) were recorded. Results: Menthol MR significantly increased mean PO by 1.67 ± 1.59% (MEN: 177.1 ± 33.0 W; PLA: 174.1 ± 32.1 W; p = 0.002; d = 1.42) and enhanced cadence (MEN: 87.4 ± 5.1 RPM; PLA: 84.5 ± 5.2 RPM; p = 0.027; d = 0.84), particularly during high-intensity intervals. No significant differences were observed in ROF or FS between conditions (p > 0.05). Five muscles were monitored for activation (RF, VM, VL, TA, Gast). A significant main effect of time demonstrated decreased activation in VM, TA, RF, and Gast. VL showed a trend toward a main effect of condition (p = 0.057), with lower activation in MEN. Both VL and RF exhibited significant condition × lap interactions (p = 0.007 and p = 0.017), with progressively lower activation in MEN as fatigue progressed. Conclusions: Menthol MR significantly improved cycling performance in the heat without altering perceptual or physiological strain. Some muscles demonstrated reduced activation with menthol MR, and further study is needed to confirm the magnitude of ergogenic effects and elucidate the physiological mechanism.