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).

Teeth clenching has a significant immediate impact on exercise performance,^5,10,35 but the optimal clenching strength for peak performance during various exercises is unknown. Clenching has been observed with strenuous exercise, such as sports, and its effects on exercise and muscle performance have been reported.^37,39 Contraction of the upper limb and clenching has a remote facilitation effect that enhances the tendon reflex and H-reflex in distant muscles,^9,15 which is that the H-reflex (excitability of the spinal anterior horn cells) of the lower limbs increases with increase in clenching strength.^30,31 The mechanism of this effect involves presynaptic inhibition by descending inputs from the mechanoreceptors of the trigeminal nervous system, including periodontal receptors and masseter muscle spindles. ¹⁵

Spinal reciprocal inhibition (RI) plays a key role in smooth joint movement, coordination, gait, and running. The process involves disynaptic RI (reciprocal Ia inhibition) restricted by direct synaptic coupling, which originates from the afferent Ia fibers in the primary operating muscle to the spinal anterior horn cells of the antagonist muscle through the Ia-inhibitory interneurons.^32,38 Furthermore, long- (D2 inhibition) and short-latency inhibition (D1 inhibition) occur presynaptically via binding of the afferent Ia fibers in the primary operating muscle to the afferent Ia fiber terminals in the antagonist muscle through the primary afferent depolarization interneurons. ³² RI inhibits excessive muscle contraction in antagonist muscles via the aforementioned inhibitory pathways, enabling coordinated movement.^4,33,34 RI strength decreases with clenching ⁴⁰ and is believed to cause excessive simultaneous activation of antagonistic muscles and impede smooth joint movement.^3,17 Similarly, simultaneous activation of antagonistic muscles caused by the remote facilitation effect of clenching has been reported. ⁹ High-strength clenching contributes to joint immobilization by increasing the excitability and RI of spinal anterior horn cells, and clenching during exercise facilitates strong muscle contraction between antagonistic muscles (eg, weight lifting). Conversely, excessive simultaneous activation of the antagonist muscles during joint movement interferes with joint movements and reduces exercise performance when agility is required. ⁴ Clenching strength differs depending on the various exercises and phases of movement. ³⁷ However, previous studies^30,31,40 only examined the excitability of RI and spinal anterior horn cells caused by high-strength clenching. We clarified the optimal clenching strength for various exercises by examining the relationship between exercise performance and spinal function at various clenching strengths.

Therefore, we aimed to analyze the relationship between exercise performance and spinal function associated with clenching strength based on the hypothesis that high-strength clenching exhibits a strong remote facilitation effect, reduces RI, and activates antagonistic muscles. Contrarily, we hypothesized that low-strength clenching does not excessively activate the antagonist muscles as RI function remains intact despite the remote facilitation effect.

Methods

Two experiments were conducted to examine the relationships of exercise performance and spinal function with clenching strength. In experiment 1, spinal function (RI and excitability of spinal anterior horn cells) and exercise performance (ankle dorsiflexion maximum voluntary contraction [MVC]) were examined for different clenching strengths (0%, 12.5%, 25%, and 50% of the masseter MVC). In experiment 2, exercise performance (ankle dorsiflexion MVC) at each clenching strength (no contraction of the masseter [no-bite condition], moderate clenching strength [moderate condition], and maximal clenching strength [max condition]) was examined without adjusting the clenching strength for each subject.

Study Participants

For experiment 1, 20 healthy men (age, 21.1 ± 0.7 years; height, 171.3 ± 6.2 cm; body mass, 62.6 ± 6.5 kg) were analyzed. For experiment 2, a separate group of 20 healthy men (age, 21.1 ± 0.4 years; height, 170.5 ± 5.6 cm; body mass, 63.1 ± 9.1 kg) was evaluated. The subjects had no stomatognathic abnormalities. Written informed consent was obtained from all participants before the study. Ethics committee of our university provided approval for this study, which was conducted according to the ethical standards of the university and the 1964 Declaration of Helsinki and its later amendments.

Limb Position for Measurements

The right lower limb location was measured at the hip (100°), knee (120°), and ankle (110°) joints. Participants’ posture during the experiment was maintained by fixing their thigh and foot to the seat and foot plate (Takei Scientific Instruments, Niigata, Japan), respectively (Figure 1).

Figure 1.

Limb position for measurements. The right lower limb position was measured at the hip (100°), knee (120°), and ankle joints (110°). Participants’ posture during the experiment was maintained by fixing their thigh and foot to the seat and foot plate, respectively. Electrodes were placed on the masseter, tibialis anterior muscle (TA), and soleus muscle. Between the electrical stimulation and surface electromyography electrodes of TA, a ground electrode was positioned.

Electromyography

Ag/AgCl electrodes (Blue Sensor, METS) for surface electromyography (EMG) were set 20 mm apart. For noninvasive assessment of muscles, they were positioned on the right masseter, tibialis anterior (TA), and soleus (Sol) muscles. ¹⁸ Then, a ground electrode was positioned between the electrical stimulation and electrodes of TA.^6,40 A band-pass filter (10-1000 Hz; amplified 100×) (FA-DL-720-140; 4Assist) was used to filter EMG activity before digital storage (10 kHz sampling rate) for offline analyses. Data analyses were performed with PowerLab 8/30 and LabChart 7 (both AD Instruments).

Clenching Conditions

In experiment 1, masseter muscle smoothing waveforms were displayed in real time on a monitor in front of the subject, and individual clenching strength (0%, 12.5%, 25%, and 50% MVC) was marked on the monitor and matched to ±5% MVC of the target clenching strength via visual feedback. Trials exceeding ±5% MVC were excluded. ⁴⁰ In experiment 2, moderate condition was set as the strength at which subjects felt most comfortable chewing and performing ankle dorsiflexion. We also instructed the subjects to avoid changing their facial expressions as much as possible to prevent facial muscle contraction during clenching. MVC of the masseter was measured thrice for 3 seconds, both before and after the experiment, and the stable 1-second value was analyzed. The average of 3 measurements was taken as MVC.

Electrical Stimulation

We induced nerve stimulation via pulses of 1 ms (rectangular wave) with an electrical stimulation device (SEN-8203; Nihon Kohden) with SS-104J isolator (Nihon Kohden). Selective tibial nerve stimulation was performed in a monopolar manner for inducing the Sol H-reflex and M waves. The cathode and anode were placed on the popliteal area and upper patella, respectively, for the test stimulus. Bipolar stimulation was performed to induce M waves in the TA, and the conditioning stimulus application was performed across the common peroneal nerve below the fibula head.^{19,22,32,36,41}

RI Measurement

RI was measured as described previously.^19,22,32,41 The Sol dominant (tibial) nerve was subjected to a test stimulus after the conditioning stimulus application to the dominant (common peroneal) TA nerve. Next, the amplitude of Sol H-reflex was documented. Conditioning stimulation before common peroneal nerve stimulation restricts spinal anterior horn cell excitability of the Sol through inhibitory interneurons. Hence, the Sol H-reflex amplitude was expected to reduce when the test stimulus was later applied to the tibial nerve. The conditioning stimulus intensity was set to the TA’s M wave threshold (stimulus intensity inducing ≤100 µV).^{20,21,23,32,41} Furthermore, careful positioning of the conditioning stimulus was done to prevent peroneus muscle activation.^19,22,41 Because RI extent varies with the H-reflex size, ⁷ the test stimulus intensity was set to exhibit a 15% to 25% H-reflex of the Sol M wave maximum amplitude (Mmax). Sol Mmax was measured before RI for setting the stimulus intensity of the Sol H-reflex. The 3 conditions of stimulation comprised a 2- or 20-ms conditioning stimulation–test stimulation interval (CTI) with a test stimulus and no conditioning stimulus (single). Previous studies have demonstrated that a CTI of 2 ms elicits the highest reciprocal Ia inhibition,^32,36 whereas that of 20 ms elicits the highest D1 inhibition. ³² RI was measured during 15 seconds of masseter muscle contraction in a randomized set of 3 stimulus conditions. The number of stimuli was 10 sets × 4 clenching conditions. At least 1 minute of rest was permitted between each set. Frequency of the stimulation was set to 0.3 Hz. On reaching 0.3 Hz, because the H-reflex is known to stabilize after the third stimulus, at least 3 stimuli were applied before obtaining any measurements. ¹⁶

Measurement of Spinal Anterior Horn Cell Excitability

The Sol H-reflex was used as an indicator of spinal anterior horn cell excitability. In the Sol H-reflex, the subject was asked to perform each biting task every 11 ± 1 seconds ¹ for 4 seconds. The task was cued by a light stimulator (Takei Scientific Instruments) placed in front of the subject, and electrical stimulation was applied to the tibial nerve 3 s after stimulation. The stimulus intensity was set at a constant value of 20% of the resting Sol Mmax throughout the experiment. ²⁸ The number of measurements was randomly set at 20 for each clenching condition, and 3 sets were conducted. A minimum of 1-minute rest was included between each set.

Ankle Dorsiflexion Task

The task movement was performed in dorsiflexion MVC with isometric contraction at 20° of ankle plantar flexion for 3 seconds. Moreover, we instructed the subjects to perform the maximum effort quickly on light stimulus presentation. During the task, care was taken not to hold the hand or stop breathing. The task was practiced thoroughly before the experiment to avoid the influence of motor learning. In experiment 1, two light stimuli were presented. Each clenching condition was started in response to the first light stimulus, and the second light stimulus was presented 4 ± 0.5 seconds later to perform the task. In experiment 2, the interval between light stimuli presentation was 2.75 ± 0.5 seconds. This shortening of the interval was to account for masseter fatigue, as the clenching strength did not require time to adjust on the forward monitor and because of the max condition. The task motion was performed twice for each clenching condition. The measurement items during task movement were reaction time, ankle dorsiflexion peak torque, and Sol/TA EMG ratio.

Experimental Protocol

The experimental protocol is presented in Figure 2. In experiment 1, the masseter MVC was initially measured to determine each clenching condition. Next, RI, spinal anterior horn cell excitability, and ankle dorsiflexion tasks were performed, and finally, the masseter MVC was measured to evaluate muscle fatigue. RI, spinal anterior horn cell excitability, and the clenching condition during the ankle dorsiflexion task were all performed randomly. In experiment 2, the masseter MVC was measured before and after the experiment, and only the ankle dorsiflexion task was performed.

Figure 2.

Experimental protocol. In experiment 1, four clenching conditions were employed (0%, 12.5%, 25%, and 50% MVC). RI was measured under 3 conditions (single, CTI 2 ms, and CTI 20 ms). The Sol H-reflex was used to measure spinal anterior horn cell excitability. For the ankle dorsiflexion task, ankle dorsiflexion at MVC was performed twice. In experiment 2, three clenching conditions were used (no-bite, moderate, and max conditions). The ankle dorsiflexion task was the same as that in experiment 1. The masseter MVC was measured before and after both experiments. The clenching conditions in each measurement were randomly assigned. Additionally, a rest interval of at least 1 min was included after each measurement. CTI, conditioning stimulation–test stimulation interval; MVC, maximal voluntary contraction; RI, reciprocal inhibition; Sol, soleus muscle.

Data Processing

For RI analysis, the Sol H-reflex and M wave amplitudes were calculated by averaging the waveform amplitude (10 waveforms) peak-to-peak values per stimulation. Subsequently, RI calculation was performed in percentage by dividing the Sol H-reflex amplitude by the Mmax amplitude (Sol H-reflex amplitude in %Mmax). Moreover, in the comparisons among the clenching conditions, the test stimulus’ H-reflex amplitude subjected to the conditional stimulus was divided by that of the test stimulus alone to calculate the percent notation ([amplitude of conditioned H-reflex amplitude/test H-reflex amplitude] × 100).

For H-reflex analysis, peak-to-peak amplitudes of the Sol H-reflex were calculated as amplitude values, and the H-reflex amplitudes (10 waveforms) obtained under each clenching condition were calculated as the additive average. To compare each clenching condition, the Sol H-reflex amplitude was calculated as a percentage by dividing the Sol H-reflex amplitude by the Mmax amplitude (Sol H-reflex amplitude in %Mmax).

The TA reaction time was the time at which EMG exceeded the mean EMG ± 3 × standard deviation (SD) at rest from the second light stimulation. The reaction time of the joint torque was the time at which the joint torque exceeded the baseline average torque ± 3SD from the second optical stimulation. ²⁵

Peak torque was calculated as the average of 2 dorsiflexion torques after analyzing the maximum dorsiflexion torque of the ankle dorsiflexion task for 3 seconds.

Sol/TA EMG ratio was calculated by calculating the total integrated value of TA and Sol EMG from the start of joint torque to 50 ms and then dividing Sol EMG by TA EMG and expressing it as a percentage.

Statistical Processing

RI was assessed using repeated-measures 2-way analysis of variance (ANOVA) to compare the clenching and stimulus conditions. For post hoc analysis, stimulation conditions (single and 2 stimulation conditions) for each clenching condition were compared using paired t tests with Bonferroni’s correction. Comparisons of clenching conditions for each stimulus condition were performed using multiple-comparison tests with Bonferroni’s correction.

Repeated-measures 1-way ANOVA for 1 factor of the clenching condition was used to compare spinal anterior horn cell excitability and exercise performance by clenching condition, and multiple-comparison tests using the Bonferroni correction were used as a post hoc analysis.

Pre- and postexperiment masseter MVC comparisons were performed using the corresponding t test. The level of statistical significance was set at P < 0.05.

Results

Experiment 1

RI Measurement

Repeated-measures 2-way ANOVA (clenching condition × stimulation condition) revealed main effects of the clenching (F [3, 57] = 4.502, P = 0.007, η² = 0.192) and stimulation conditions (F [2, 38] = 91.477, P < 0.001, η² = 0.828). Additionally, a significant interaction was observed between the 2 factors (F [6, 114] = 10.563, P < 0.001, η² = 0.357).

The Sol background EMG, masseter background EMG, and M wave amplitude of the TA are presented in Tables 1 and 2.

Table 1.

Background EMG (RI measurement) ^a

	0% MVC	12.5% MVC	25% MVC	50% MVC
Sol	3.7 ± 0.4	3.6 ± 0.4	3.5 ± 0.4	3.5 ± 0.3
Masseter muscle	1.2 ± 0.2	12.7 ± 0.4	25.1 ± 0.3	50.2 ± 0.7

EMG, electromyography; MVC, maximal voluntary contraction; RI, reciprocal inhibition; Sol, soleus muscle

^aData are presented as mean ± standard error. Sol background EMG (μV) (EMG 30-50 ms before the test stimulus). Masseter muscle background EMG (%MVC) (EMG 30-50 ms before the test stimulus).

Table 2.

TA M wave amplitude values (RI measurement) ^a

0% MVC	12.5% MVC	25% MVC	50% MVC
88.5 ± 4.2	90.0 ± 1.9	87.3 ± 2.6	90.0 ± 2.4

MVC, maximum voluntary contraction; RI, reciprocal inhibition; TA, tibialis anterior muscle.

^aData are presented as mean ± standard error (μV).

The Sol H-reflex amplitude did not differ among the clenching conditions for the single stimulus condition (Table 3). This result validated the observation that changes in the Sol H-reflex amplitude concerning the conditioning stimuli were independent of the test stimulus intensity.

Table 3.

Sol H-reflex amplitude (% Mmax) ^a

	0% MVC	12.5% MVC	25% MVC	50% MVC
Single	20.6 ± 0.3	21.3 ± 0.4	20.4 ± 0.4	20.8 ± 0.5
CTI 2 ms	16.7 ± 0.4 ‡	17.9 ± 0.5 ‡	18.2 ± 0.6 ‡	20.1 ± 0.7
CTI 20 ms	15.0 ± 0.4 ‡	15.6 ± 0.4 ‡	16.0 ± 0.5 ‡	17.2 ± 0.8 ‡

CTI, conditioning stimulation–test stimulation interval; Mmax, maximum amplitude of the M wave; MVC, maximal voluntary contraction; Sol, soleus muscle.

^aData are presented as mean ± standard error. This table presents the outcome for each time measurement and clenching condition. The Sol H-reflex and M wave amplitude were calculated as the mean ± standard error of the peak-to-peak values of the amplitude of each waveform. This represents H-reflex/Mmax × 100. The H-reflex amplitude of the single condition (divided by Mmax) was compared with that (divided by Mmax) of each CTI condition (2 and 20 ms).

^‡P < 0.001 (paired t test with Bonferroni’s correction).

The single stimulus, CTI 2 ms, and CTI 20 ms conditions were compared for each clenching condition (Table 3, Figures 3 and 4). Compared with that under the single condition, the H-reflex amplitude was significantly lower under the CTI 2 ms condition for 0%, 12.5%, and 25% MVC (P < 0.001), and reciprocal Ia inhibition was observed. No reciprocal Ia inhibition was observed at 50% MVC (P = 0.11). In comparisons between the clenching conditions, the H-reflex amplitude was significantly larger at 25% and 50% MVC than at 0% MVC (P < 0.001). The H-reflex amplitude was significantly larger at 50% MVC than at 12.5% MVC (P < 0.001).

Figure 3.

Raw data tracing in Sol. The figure presents the raw data trace of 1 representative subject. It shows reciprocal inhibition and spinal anterior horn cell excitability data. From the aforementioned data, the stimulation conditions were single, CTI 2 ms (reciprocal Ia inhibition), and CTI 20 ms (D1 inhibition), and 10 waveforms of the H-reflex of Sol were employed. The bold black lines are the summed averages ± standard errors of the 10 waveforms. The bottom figure presents 15 waveforms of the H-reflex of Sol, and the bold black lines are the average of the 15 waveforms. The horizontal axis shows the clenching condition (0%, 12.5%, 25%, and 50% MVC). CTI, conditioning stimulation–test stimulation interval; MVC, maximal voluntary contraction; Sol, soleus muscle.

Figure 4.

Reciprocal inhibition (RI) measurement. Panels a and b present CTI 2 ms and CTI 20 ms, respectively. The thin solid line shows the values for 20 subjects, and the thick solid line denotes the mean ± standard error. The vertical and horizontal axes show the amplitude of the conditioning H-reflex/amplitude of the test H-reflex × 100 and the clenching condition (0%, 12.5%, 25%, or 50% MVC), respectively. The comparison of the single condition with other 2 conditions (CTI 2 ms and CTI 20 ms) was conducted with paired t test accompanied by Bonferroni’s correction. Filled circles denote values that were not significantly different from those under the single condition. Open circles denote values significantly different from those under the single condition (P < 0.001). Multiple-comparison tests using the Bonferroni method were performed for comparisons between among the clenching condition. *P < 0.05, **P < 0.01, ^‡P < 0.001. CTI, conditioning stimulation–test stimulation interval; MVC, maximal voluntary contraction.

Compared with the results under the single condition, a significant decrease in the amplitude of H-reflex was observed under the CTI 20 ms condition for all clenching strengths; thus, D1 inhibition was observed. Among the clenching conditions, H-reflex amplitude was significantly larger at 25% (P = 0.003) and 50% MVC (P = 0.01) than at 0% MVC. Additionally, H-reflex amplitude was significantly larger at 50% MVC than at 12.5% MVC (P = 0.02).

Spinal Anterior Horn Cell Excitability

The results of the Sol and masseter muscle background EMG are presented in Table 4.

Table 4.

Background EMG (spinal anterior horn cell excitability) ^a

	0% MVC	12.5% MVC	25% MVC	50% MVC
Sol	2.7 ± 0.1	2.7 ± 0.1	2.7 ± 0.1	2.8 ± 0.1
Masseter muscle	1.2 ± 0.2	13.1 ± 0.9	23.5 ± 0.5	48.5 ± 0.8

EMG, electromyography; MVC, maximal voluntary contraction; Sol, soleus muscle.

^aData are presented as mean ± standard error. Sol background EMG (μV) (EMG 30-50 ms before the test stimulus). Masseter muscle background EMG (%MVC) (EMG 30-50 ms before the test stimulus).

Repeated-measures 1-way ANOVA revealed a main effect of the clenching condition (F [3, 57] = 61.680, P < 0.001, η² = 0.765). As a post hoc test, H-reflex amplitude was significantly larger at 12.5%, 25%, and 50% MVC than at 0% MVC (P < 0.001), with the amplitude at 50% MVC being the largest among these (P < 0.001, Figures 3 and 5).

Figure 5.

Excitability of the spinal anterior horn cells. The thin and thick solid lines show the values for 20 subjects and the mean ± standard error, respectively. The vertical and horizontal axes present the H-reflex amplitude as percent Mmax and the clenching condition (0%, 12.5%, 25%, or 50% MVC), respectively. For comparisons among the clenching conditions, a multiple-comparison test was performed using Bonferroni’s correction. *P < 0.05, **P < 0.01, ^‡P < 0.001. Mmax, maximum amplitude of the M wave; MVC, maximal voluntary contraction.

Ankle Dorsiflexion Task

The masseter muscle background EMG results are presented in Table 5.

Table 5.

Background EMG ^a

	0% MVC	12.5% MVC	25% MVC	50% MVC
Masseter muscle	1.0 ± 0.2	12.0 ± 0.5	23.1 ± 0.7	52.1 ± 0.9

EMG, electromyography; MVC, maximal voluntary contraction.

^aData are presented as mean ± standard error. Masseter muscle background EMG (%MVC) (EMG from 200 ms before light stimulation to 100 ms after light stimulation).

Reaction Time

No main effect of the clenching condition was found on the reaction times of TA EMG (F [3, 57] = 2.489, P = 0.07, η² = 0.116, Figure 6a). However, a main effect of the clenching condition was identified on the reaction time of joint torque (F [3, 57] = 2.905, P = 0.04, η² = 0.133). No significant difference among the clenching conditions was observed using a post hoc test (Figure 6b).

Figure 6.

Ankle dorsiflexion task (experiment 1). Panel a presents TA EMG, and panel b presents the reaction time of ankle dorsiflexion torque. The vertical axis shows the reaction time (s). Panel c presents the peak torque. The vertical axis shows the peak torque (N·m). Panel d presents the total integrated values of TA EMG, and panel e presents the total integrated value of Sol EMG. The vertical axis shows the total integrated value (mV·ms) of EMG. Panel f presents the Sol/TA EMG ratio. The vertical axis presents the ratio in % notation (%). All bar graphs are presented as the mean ± standard error for 20 persons. The horizontal axis shows the clenching condition. Multiple-comparison tests using the Bonferroni correction were performed for comparisons among the clenching conditions. *P < 0.05. EMG, electromyography; MVC, maximal voluntary contraction; Sol, soleus muscle; TA, tibialis anterior muscle.

Peak Torque

Repeated-measures 1-way ANOVA revealed a main effect of clenching condition on peak torque (F [3, 57] = 3.236, P = 0.03, η² = 0.146). Comparisons of peak torque for each clenching condition illustrated that its value was significantly higher at 50% MVC than at 0% MVC (P = 0.01) (Figure 6c).

Sol/TA EMG Ratio

No main effect of the clenching condition was observed on TA EMG (F [3, 57] = 1.443, P = 0.24, η² = 0.071, Figure 6d), Sol EMG (F [3, 57] = 1.149, P = 0.34, η² = 0.057, Figure 6e), or Sol/TA EMG ratio (F [3, 57] = 0.884, P = 0.46, η² = 0.044, Figure 6f).

Masseter MVC

A comparison of the masseter MVCs before (0.352 ± 0.041 mV) and after (0.359 ± 0.038 mV) the experiment displayed no significant difference (P = 0.51). Therefore, the masseter muscle was not fatigued after the experiment.

Experiment 2

Ankle Dorsiflexion Task

The masseter muscle background EMG in the moderate condition was 25.6 ± 3.0% MVC.

Reaction Time

No main effect of clenching condition was found on reaction times of TA EMG (F [3, 57] = 0.300, P = 0.74, η² = 0.016, Figure 7a) or on those of joint torque (F [3, 57] = 0.461, P = 0.63, η² = 0.024, Figure 7b).

Figure 7.

Ankle dorsiflexion task (Experiment 2). Panels a and b present TA EMG and the reaction time of ankle dorsiflexion torque. The vertical axis shows the reaction time (s). Panel c presents the peak torque. The vertical axis shows the peak torque (N·m). Panel d presents the total integrated values of TA EMG, and Panel e presents the total integrated value of Sol EMG. The vertical axis shows the total integrated value (mV·ms) of EMG. Panel f presents the Sol/TA EMG ratio. The vertical axis presents the ratio in % notation (%). All bar graphs are presented as the mean ± standard error of 20 persons. The horizontal axis presents the clenching condition. Multiple-comparison tests using the Bonferroni correction were performed for comparisons among the clenching conditions. **P < 0.01. EMG, electromyography; MVC, maximal voluntary contraction; Sol, soleus muscle; TA, tibialis anterior muscle.

Peak Torque

A main effect of clenching condition was identified on peak torque in the second experiment (F [3, 57] = 13.906, P < 0.001, η² = 0.423). The peak torque was significantly higher under the moderate (P = 0.002) and max conditions (P = 0.001) than under the no-bite condition (Figure 7c).

Sol/TA EMG ratio

The clenching condition did not exert main effects on TA EMG (F [2, 38] = 0.185, P = 0.83, η² = 0.010, Figure 7d) or Sol EMG (F [2, 38] = 2.702, P = 0.08, η² = 0.125, Figure 7e), it had a main effect on the Sol/TA EMG ratio (F [2, 38] = 3.715, P = 0.03, η² = 0.164). Sol/TA EMG ratio was significantly higher under the max condition than under the moderate condition (P = 0.009) (Figure 7f).

Masseter MVC

A comparison of the masseter MVCs before (0.424 ± 0.047 mV) and after (0.414 ± 0.047 mV) the experiment was not significantly different (P = 0.27). Therefore, the masseter muscle was not fatigued after the experiment.

Discussion

The following were the main findings of the present study: RI decreased or disappeared, and spinal anterior horn cell excitability increased as the clenching strength increased. Concerning exercise performance associated with ankle dorsiflexion, peak torque was increased by clenching, and the Sol/TA EMG ratio was increased by high-strength clenching.

Based on the results of this study, spinal anterior horn cell excitability increased with increasing clenching strength. The increase in Sol H-reflexes with clenching could be a remote prompting effect of inhibitory descending inputs that suppress presynaptic inhibition. In previous studies,^30,31 the H-reflex was measured at a clenching strength of 20% MVC, whereas the Sol H-reflex amplitude was significantly increased by even lower-strength clenching in the present study. These results support previous findings,^30,31 and we further demonstrated that even lower contraction strengths have a remote facilitation effect. The Jendrassik maneuver increases spinal anterior horn cell excitability, even in palmar flexion with 10% MVC. ²⁶ Therefore, sufficient contraction strength was achieved during clenching to induce a remote facilitating effect even at 12.5% MVC.

Based on our RI results, reciprocal Ia inhibition remained repressed at clenching strengths of ≤25% MVC, but this inhibition disappeared at 50% MVC. Contrarily, D1 inhibition remained suppressed at all clenching strengths. RI decreased with clenching strengths exceeding 25% MVC. Previous studies found that RI is attenuated by high-strength clenching (70% MVC) and the Jendrassik maneuver.^8,40 However, in a previous study, ⁴⁰ Sol H-reflexes increased ³¹ with increasing clenching strength during RI measurements, but the measurements were made without adjusting the test stimulus intensity. Because the level of inhibition depends on the test stimulus intensity, ⁷ RI may not have been accurately measured. Therefore, in this study, the test stimulus intensity was constant at 20% of Sol Mmax, which enabled accurate RI measurement. Renshaw cell–mediated recurrent inhibition reportedly acts with increased spinal anterior horn cell excitability and inhibits Ia-inhibitory interneurons from the antagonist muscle,^2,24,27 resulting in the attenuation and disappearance of RI. ²⁹ Hence, the increased spinal anterior horn cell excitability of the Sol muscle with increased clenching strength may have resulted in enhanced recurrent inhibition via Renshaw cells, which suppressed input from inhibitory interneurons from the TA, thereby reducing and eliminating RI. During low-strength clenching (12.5% and 25% MVC), spinal anterior horn cell excitability associated with clenching increased, and RI function remained, suggesting that the agonist muscle during joint movement may be activated and have suitable clenching strength to inhibit antagonist muscles. Conversely, during high-strength clenching (≥50% MVC), spinal anterior horn cell excitability increased, and RI decreased or disappeared. In a previous study, ⁴⁰ clenching increased spinal anterior horn cell excitability in the TA and Sol. High-strength clenching might be suitable for increasing the simultaneous activation of antagonist muscles and boosting exercise performance to fix the joint.

Peak torque, as a measure of exercise performance was increased at 50% MVC and under moderate and max conditions. In previous studies that examined the relationship between clenching and exercise performance, only maximal clenching was investigated,^11-14 and the effect of other clenching strengths on peak torque is unknown. The results of this study suggest that clenching may increase peak torque as spinal anterior horn cell excitability increases. However, although spinal anterior horn cell excitability increased and RI function remained under 12.5% and 25% MVC conditions, no increase in peak torque was observed. This could be because the subjects performed ankle dorsiflexion task in addition to the task of achieving the clenching strength displayed on the monitor, which resulted in a dual task. To exclude this factor, the moderate condition in experiment 2 allowed only ankle dorsiflexion task to be performed without clenching strength adjustment, and an increase in peak torque was observed. Thus, the results of exercise performance in experiment 1 suggest that the effects of the dual task should also be considered.

The Sol/TA EMG ratio was significantly increased under the max condition compared with that under the moderate condition. These results for exercise performance with those of spinal function in experiment 1 revealed that the moderate condition led to an increase in peak torque, residual RI function, and a low Sol/TA EMG ratio. In the moderate condition (~25% MVC), agonist muscle activity was induced by a remote facilitation effect and the function of antagonist muscle inhibition remained, suggesting that exercise performance, during which agonist muscles are mainly active (eg, joint movement), may be improved. The maximal condition induced the activity of the agonist muscles via a remote facilitation effect, antagonist muscle activity associated with increased excitability of the spinal anterior horn cells, and decreased RI function. Antagonist muscles may activate each other (eg, joint fixation) to improve exercise performance.

Our results provide the basic knowledge that athletic performance may be improved by achieving optimal clenching strength in athletes. In the future, it is necessary to examine the associations of clenching strength with athletic characteristics and clarify the relationship between optimal clenching strength and exercise performance.

This study had 3 limitations. First, it is possible that the spinal anterior horn cell excitability was not accurately measured because the H/Mmax recruitment curve was not used to measure this variable. However, previous studies^30,31 of clenching used a simple method to measure the excitability of spinal anterior horn cells at constant stimulus intensity. Because of the study design, the H/Mmax recruitment curve could be measured only at rest and not during clenching. Therefore, in this study, we used a simple measurement method with constant stimulus intensity as described in previous studies.^30,31

Second, the ankle dorsiflexion task was limited to measurements of only isometric contraction. Therefore, the appropriate clenching strength for joint movement was not determined. In the future, it will be necessary to examine the extent of antagonist muscle activity during joint movement and the smoothness of joint angles.

Third, this study is a basic study of the effects of clenching on spinal cord function and dorsiflexion movements, and it is unclear whether they can be adapted to dynamic movements such as sports movements. However, it is believed that the remote facilitation effect of clenching is applied to muscles lower than the masseter muscle and that not only single-joint but also multiple-joint movements are similarly affected. As for future studies, it is necessary to verify the results with multiple-joint movements and dynamic movements.

In conclusion, the peak torque and Sol/TA EMG ratio were increased during high-strength clenching (≥50% MVC) in addition to the decreased RI function, and this clenching strength was suitable for exercise performance (joint fixation) during which antagonist muscles were simultaneously activated to improve muscle strength. Contrarily, during low-strength clenching (<50% MVC), peak torque increased, Sol/TA EMG ratio decreased, and residual RI function remained. This clenching strength was suitable for exercise performances (joint movement) during which antagonist muscles are inhibited.