Effects of menthol-based counterirritant on quadriceps motoneuron-pool excitability

Context: Motoneuron-pool facilitation after cryotherapy may be mediated by stimulation of thermoreceptors surrounding a joint. It is unknown whether menthol counterirritants, which also stimulate thermoreceptors, have the same effect on motoneuron-pool excitability (MNPE). Objective: To compare quadriceps MNPE after a menthol-counterirritant application to the anterior knee, a sham counterirritant application, and a control treatment in healthy subjects. Design: A blinded, randomized controlled laboratory study. Setting: Laboratory. Participants: Thirty healthy subjects (16 m, 14 f; 24.1 ± 3.9 y, 170.6 ± 11.4 cm, 72.1 ± 15.6 kg) with no history of lower extremity surgery volunteered for this study. Intervention: Two milliliters of menthol or sham counterirritant was applied to the anterior knee; control subjects received no intervention. Main Outcome Measures: The average vastus medialis normalized Hoffmann reflex (H_max:M_max ratio) was used to measure MNPE. Measurements were recorded at 5, 15, 25, and 35 minutes postintervention and compared with baseline measures. Results: H_max:M_max ratios for all groups significantly decreased over time (F_4,108 = 10.52, P <.001; menthol: baseline = .32 ± .20, 5 min = .29 ± .18, 15 min = .27 ± .18, 25 min = .28 ± .19, 35 min = .27 ± .18; sham: baseline = .46 ± .26, 5 min = .36 ± .20, 15 min = .35 ± .19, 25 min = .35 ± .20, 35 min = .34 ± .18; control: baseline = .48 ± .32, 5 min = .37 ± .27, 15 min = .37 ± .27, 25 min = .37 ± .29, 35 min = .35 ±.28). No significant Group × Time interaction or group differences in H_max:M_max were found. Conclusions: Menthol did not affect quadriceps MNPE in healthy subjects.