Menthol-induced cutaneous stimulation combined with self-paced walking training improves knee extension performance in untrained older healthy females.

[Purpose] The present study aimed to investigate whether self-paced walking training utilizing the facilitating effect of skin cooling with menthol gel application was effective in untrained older healthy females. [Participants and Methods] Forty-two untrained healthy older females (aged 60-69 years) were divided into the following three groups: (i) Walking training with menthol group: GM, (ii) Walking training group: GW, and (iii) Control group: GC. The participants in GM and GW performed self-paced walking for 30 minutes a day, 2 times a week, for 6 weeks. Menthol gel was applied to the front of the thigh of the participants in GM. Maximal voluntary contraction and rate of force development were measured pre- and post-training and walking speed was measured during the training. The number of steps taken and walking speed in daily activity were measured and the average of these parameters per day were calculated. [Results] The main findings were [1] knee extension muscle strength increased in GM and GW, and [2] rate of force development only improved in GM. [Conclusion] These results suggest that walking training utilizing the facilitating effect of skin cooling enhances muscle function in untrained older healthy females and that the present skin cooling method with menthol gel application may be recommended as a training strategy.

TRPM8-mediated cutaneous stimulation modulates motor neuron activity during treadmill stepping in mice.

Motor units are generally recruited from the smallest to the largest following the size principle, while cutaneous stimulation has the potential to affect spinal motor control. We aimed to examine the effects of stimulating transient receptor potential channel sub-family M8 (TRPM8) combined with exercise on the modulation of spinal motor neuron (MN) excitability. Mice were topically administrated 1.5% icilin on the hindlimbs, followed by treadmill stepping. Spinal cord sections were immunostained with antibodies against c-fos and choline acetyltransferase. Icilin stimulation did not change the number of c-fos+ MNs, but increased the average soma size of the c-fos+ MNs during low-speed treadmill stepping. Furthermore, icilin stimulation combined with stepping increased c-fos+ cholinergic interneurons near the central canal, which are thought to modulate MN excitability. These findings suggest that TRPM8-mediated cutaneous stimulation with low-load exercise promotes preferential recruitment of large MNs and is potentially useful as a new training method for rehabilitation.

Effect of stimulation of cold receptors with menthol on EMG activity of quadriceps muscle during low load contraction.

PURPOSE: Facilitatory and inhibitory responses of spinal motor neurons are influenced by somatosensory input from the skin. The purpose of this study, employing electromyography, was to examine the neuromuscular changes that occur with menthol applied to the skin over the quadriceps muscle. METHODS: Forty-two healthy volunteers performed isometric knee extensions at 35% maximum voluntary contraction (MVC) in three groups (Adult Placebo, Adult Menthol, Older Adult Menthol). Stimulation used was application of 5% menthol gel to the skin. Surface electromyography (sEMG) from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) was recorded using miniature pair electrodes. RESULTS: Root mean square electromyography (rmsEMG) in VL and VM significantly increased with menthol stimulation both in Adult and Older Adult, but no significant difference was observed between Adult Menthol and Older Adult Menthol. There was a significant decrease in mean power frequency (MPF) in VM with menthol stimulation in Older Adult, but no significant changes were observed in Adult Menthol. CONCLUSION: Neuromuscular modulation was observed with the application of menthol gel at low loads in the present study. These findings could lead to a new method of muscular training that targets the recruitment of fast type muscle safe for older adults.