Author(s): SUGAJIMA, Y., ISHIDA, N., OMORI, T., KATO, T., SERA, O., MURAOKA, R., YAMAMOTO, H., Institution: ASAHI UNIVERSITY, Country: JAPAN, Abstract-ID: 687

INTRODUCTION:
Mouthguards (MGs) are mandatory for injury prevention in contact sports such as rugby football. Recently, MGs have gained attention as ergogenic aids that may enhance muscle strength and power through "remote facilitation" produced by clenching. However, previous findings remain inconsistent, and the effects on specific upper-limb movement characteristics (flexion/extension) and bilateral strength balance (dominant vs. non-dominant) are still unclear. This study aimed to investigate the effects of clenching with a custom-made MG on isokinetic shoulder strength, bilateral strength ratio, and muscle activity in university rugby players.
METHODS:
Fourteen male university rugby players participated in the study. Isokinetic shoulder flexion and extension were measured in both the dominant and non-dominant limbs under two conditions: without a mouthguard (NMG) and with a custom-made mouthguard (MG) involving maximal clenching. Measurements were performed at angular velocities of 60, 120, and 180 °/s. Evaluated parameters included peak torque (body-weight ratio), the non-dominant-to-dominant (ND/D) ratio, and normalized electromyography (EMG) values of the masseter, deltoid, and pectoralis major muscles. Data were analyzed using paired t-tests.
RESULTS:
For isokinetic strength, the MG condition demonstrated a significant increase in peak torque in the dominant limb during both flexion and extension at the moderate velocity of 120 °/s compared with the NMG condition (flexion, p < 0.05; extension, p = 0.01). In the nondominant limb, peak torque increased significantly during extension at the low velocity of 60 °/s. EMG analysis revealed that during dominant-limb flexion with the MG, masseter activity increased significantly, with a tendency toward increased deltoid activity (p < 0.1). During dominant-limb extension, deltoid and pectoralis major muscles were significantly higher in the MG condition (p < 0.01).
Bilateral Balance: While the ND/D ratio during flexion at 60 °/s did not reach significance, it shifted from 1.06% to 0.95 in the MG, indicating a tendency toward reduced bilateral strength disparity (p < 0.1).
CONCLUSION:
These results suggest that wearing a custom-made MG and clenching significantly improves maximal strength, particularly in moderate-velocity movements of the dominant limb, through the remote facilitation effect (Jendrassik maneuver), which enhances central nervous system excitability. Therefore, mouthguards serve not only as protective devices but also as effective tools for maximizing athletic performance.