Effects of Clenching Strength on Exercise Performance: Verification Using Spinal Function Assessments

Abstract

Background: This study aimed to determine the relationship between exercise performance and spinal function based on clenching strength. Hypothesis: Low-intensity clenching contributes to joint movement, whereas high-intensity clenching contributes to joint fixation. Study Design: Randomized crossover trial. Level of Evidence: Level 3. Methods: Two experiments were conducted using 2 groups of 20 healthy adults. The 4 clenching conditions in experiment 1 were 0%, 12.5%, 25%, and 50% of the maximum voluntary contraction (MVC) of the masseter muscle. Experiment 2 consisted of 3 conditions: no-bite condition, moderate effort, and maximum effort (max condition). In experiment 1, spinal function and ankle dorsiflexion tasks were measured for each clenching condition, and the ankle dorsiflexion task was measured in experiment 2. Regarding spinal function, we measured spinal reciprocal inhibition (RI) and excitability of spinal anterior horn cells. For the ankle dorsiflexion task, ankle dorsiflexion MVC was performed for 3 seconds under each clenching condition. The items analyzed were reaction time, peak ankle dorsiflexion torque, and soleus (Sol)/tibialis anterior (TA) electromyography (EMG) ratio. Results: The results of experiment 1 illustrated that RI was significantly attenuated or eliminated with increasing clenching strength (>25% MVC). Spinal anterior horn cell excitability increased significantly with increasing clenching strength. The peak torque was significantly higher at 50% MVC than that at 0% MVC. In experiment 2, the peak torque was significantly higher under moderate and max conditions than no-bite condition, and the Sol/TA EMG ratio was significantly higher under max condition than that under moderate condition. Conclusion/Clinical Relevance: The results illustrated that during high-strength clenching (≥50% MVC), antagonist muscles are activated simultaneously to increase muscle strength. High-strength clenching improved kinetic performance (joint fixation), whereas low-strength clenching (<50% MVC) enhanced exercise performance (joint movement).

Effects of remote facilitation on ankle joint movement: Focusing on occlusal strength and balance

“…Therefore, the Moderate condition in this study was established to provide the optimum strength for joint movement since it activated the main action muscle, TA, as well as maintained RI toward the antagonist muscle, Sol. This consequence is supported by a previous study we conducted, which clarified that RI remained and worked sufficiently at occlusal strengths of less than 50% MVC 19 …”

“…Concerning the RJD improvement under Max conditions, the results contradict the hypothesis of this study that high‐intensity occlusion interferes with joint movement. RI reportedly ceased to function with a masseter muscle contraction strength of 50% MVC or more, 19 and it was expected that the Max conditions of the present study would promote the excessive activity of antagonist muscles. However, the EMG analysis results showed no significant difference in CI (ratio of the antagonist muscle activity to the agonist muscle) under the Max condition compared to those under the No‐bite condition, suggesting that the excessive activity of the antagonist muscle could not be observed and it did not interfere with joint movement under the Max condition.…”

“…This consequence is supported by a previous study we conducted, which clarified that RI remained and worked sufficiently at occlusal strengths of less than 50% MVC. 19 …”

“…Subjects felt the most comfortable chewing and performing motor tasks at the strength of the Moderate condition. 19 …”

“…Concerning occlusal strength, certain studies described that spinal excitability is enhanced with increasing occlusal strength. 16 , 17 , 18 Our research group also investigated teeth occlusion with low‐to‐high intensity and revealed a correlation between spinal cord excitability and occlusal strength 19 In addition, in line with enhanced spinal cord excitability, muscle strength would be activated immediately. 7 , 19 , 20 , 21 , 22 However, we described in our previous research that spinal reciprocal inhibition (RI), a key function in smooth joint movement, has been completely nonfunctional during high‐intensity occlusion (>50% of maximum voluntary contraction [MVC] of the masseter muscle).…”