EFFECT OF TEETH CLENCHING ON FORCE-VELOCITY …

Jpn. J. Phys. Fitness Sports Med. 1999, 48: 365•`374

EFFECT OF TEETH CLENCHING ON FORCE-VELOCITY

RELATIONSHIPS IN ISOKINETIC KNEE EXTENSION

YOSUKE SUMITA1) , YUKIO SASAKI2) , TOSHIAKI UENO3),

HISASHI TANIGUCHI2) and TAKASHI OHYAMA1,2,3)

Abstract

To investigate the effect of teeth clenching on isokinetic knee extension at various

velocities, isokinetic muscle strength during knee extension was measured in association

with teeth clenching at 30, 60, 150, 300 and 450 degrees per second (deg/s) using the

Cybex 6000 isokinetic dynamometer. The volunteer subjects were 9 healthy males (26.2•}

0.97 years). The peak torque per body weight and average power per body weight were

statistically analyzed. Our results demonstrated that the peak torque per body weight

with teeth clenching at 30, 60 and 150 deg/s significantly increased by 7.0% , 7.4% and

4.9% , respectively (p•ƒ0.05), but no significant differences were found at 300 and 450

deg/s. While the average power per body weight with teeth clenching at 30, 60 and 150

deg/s significantly increased by 6.5% , 6.1% and 6.9% , respectively (p•ƒ0.05), no sig-

nificant differences were found at 300 and 450 deg/s. A significant negative correlation

was shown between the isokinetic angular velocity and the difference in peak torque per

body weight derived from with and without teeth clenching (r=•|0.699; p•ƒ0.05).

These findings suggested that the effect of teeth clenching on isokinetic muscle strength of

knee extension was dependent on the angular velocity, and at lower angular velocities

teeth clenching had the effect of increasing the isokinetic muscle strength during knee ex-

tension.

(Jpn. J. Phys. Sports Med. 1999, 48 : 365•`374)

Key words : teeth clenching, isokinetic muscle strength, knee extension, peak torque, aver-age power

I . Introduction

It has been observed that some athletes clench

their teeth hard enough to fracture them during

weight lifting, shotputting and discus throwingl).

In recent years, a number of attempts have been

made to clarify the contribution of teeth clenching

to general motor functions. In the neurophysiolo-

gical field, Miyahara et al.2) examined the modula-

tion of the soleus H reflex in association with

voluntary clenching of the teeth in healthy adults

volunteers. The authors demonstrated that the am-

plitude of the H reflex increased significantly dur-

ing teeth clenching, and the increase in amplitude

of the H reflex showed a positive correlation with

the force of teeth clenching in humans. They con-

cluded that oral motor activity could exert a

strong influence on the motor activity of other

1)東京医科歯科大学大学院歯学研究科生体機能

制御歯科学系顎顔面補綴学講座

〒113-8549東京都文京区湯島1-5-45

Department of Maxillo-Facial Prosthetics, Life Science of

Maxillo-Facial Systems, Dental Research Division, Graduate

School, Tokyo Medical and Dental University, 1-5-45, Yushima,

Bunkyo-ku, Tokyo, 113-8549, Japan2)東京医科歯科大学歯学部附属病院顎口腔機能

治療部

〒113-8549東京都文京区湯島1-5-45

Clinic for Stomatognathic Dysfunction, University Hospital, Faculty

of Dentistry, Tokyo Medical and Dental University, 1-5-45, Yushima,

Bunkyo-ku, Tokyo, 113-8549, Japan3)東京医科歯科大学歯学部障害者歯科学講座

〒113-8549東京都文京区湯島1-5-45

Department for Stomatognathic Dysfunction, Faculty of Dentistry,

Tokyo Medical and Dental University, 1-5-45, Yushima,

Bunkyo-ku, Tokyo, 113-8549, Japan

366 SUMITA, SASAKI, UENO, TANIGUCHI, OHYAMA

parts of the body. Takada3) confirmed that for the

tibialis anterior, which was antagonist to the

soleus, the H reflex was facilitated during volun-

tary teeth clenching, and there was a positive cor-

relation between the level of teeth clenching force

and the degree of facilitation. Furthermore he

showed that the reciprocal Ia inhibition of the

soleus H reflex was reduced during teeth clen-

ching. He suggested that clenching of the teeth

strongly influenced reciprocal innervation.

In the kinesiological field, Ueno investigated

the relationship between teeth clenching and

isometric muscle strength of shoulder adduction in

normal volunteer subjects, and found a positive

correlation between muscle strength and the force

of teeth clenching. More recently, Sasaki et al.5)

showed that with respect to plantar flexion, teeth

clenching had no effect on isokinetic exercise,

whereas teeth clenching during isometric exercise

increased significantly muscle strength. Accor-

dingly, it seems clear that clenching of the teeth

can augment the static muscle strength of appen-

dages, but further studies are necessary to eluci-

date the effect of teeth clenching on dynamic mus-

cle strength. Sasaki et al.5) measured the isokine-

tic muscle strength of plantar flexion at 180 deg/s

angular velocity only, which may be considered to

be a relatively fast contractile velocity. Daily acti-

vities and sports performance need various kinds

and velocities of muscle contractions of the appen-

dages and body.

In the present study, we investigated the effect

of teeth clenching on isokinetic muscle strength of

knee extension exerted at various angular veloci-

ties ranging from slow to fast, and analyzed the

correlation between the effect of teeth clenching

on isokinetic muscle strength and the angular

velocity.

K. Materials and Methods

A. Subjects

Nine healthy male volunteers participated in the

present study. Their mean age, height and body

weight were 26.2•}0.97 years,170.4•}5.59 cm,

and 66.1•}6.60kg (mean•}S. D.). All subjects

gave their informed consent to the study. All of

them had all teeth except wisdom teeth. None of

them had malocclusioin and previous history of

injury to the lower extremities.

B. Isokinetic strength measurement

A Cybex 6000 Extremity Testing and Rehabi-

litation System (Cybex 6000, Lumex, Inc., NY,

USA.) was used to measure right knee extension

strength at 30, 60,150, 300, and 450 deg/s through

a 30-90•K knee angle (0•K= knee fully extended)

Fig. 1. Subject in testing position and block diagram.

367TEETH CLENCHING AND KNEE EXTENSION

(Fig. 1). Subjects were seated 10•K reclined on the

Cybex Extremity Testing Positioning Chair, and

firmly strapped at the thigh, pelvis, and torso to

minimize extraneous body movements. The knee

joint axis was aligned with the mechanical axis of

the dynamometer. A shin pad was placed just su-

perior to the medial malleolus. During knee exten-

sion, the subjects were instructed to keep their

heads on the headrest, to grasp the handles on

each side, and to hook the left leg underneath the

contralateral limb stabilizer. Prior to testing grav-

ity correction was obtained to eliminate the effect

of gravity from measured values. The torque sig-

nals, sampled at every half-degree, were stored in

the Cybex 6000 computer. The values of the peak

torque/body weight (PT/BW) and the average

power/body weight (AP/BW) were calculated on

the computer.

C. Bite conditions

The following bite conditions were instructed to

the subjects :1. Rest position (RP), We asked the

subjects to maintain their relaxed mandibular

position without teeth contacts during knee exten-sion under RP, and also keep his upper and lower

lips touch lightly. 2. Maximum voluntary clenching

of the teeth (MVCT). These bite conditions were

monitored with the amplitude of the right mas-seter integrated EMG, which was displayed on the

oscilloscope (CS-4025, Kenwood Co., Tokyo,

Japan) placed 1.5m ahead of the subjects. The EMG activity was recorded from the masseter

with bipolar Ag-AgCl surface electrodes (NE-121

J, Nihon Kohden Co., Tokyo, Japan :10 mm in diameter) that were placed 20 mm apart longitudi-

nally on the masseter.

The EMG was amplified using a High Gain Am-

plifier (AB-610J, Nihon Kohden Co., Tokyo, Japan.

Time constant: 0.03s ; high cut frequency: 10

kHz) . The signal was then full wave rectified and

integrated using an integration unit (EI-601G,

Nihon Kohden Co., Tokyo, Japan). In addition, the

amplified EMG was input to a desktop computer

through the Analog Digital Converter

(NB-MIO-16 X Data Acquisition Board and NB-DMA 2800 GPIB Interface Board, National In-

struments Co., Texas, USA.), and stored in an MO

disk unit with torque signal and rotational signal

from Cybex 6000 that had been amplified with a DC Amplifier (AD-610J, Nihon Kohden Co.,

Tokyo, Japan) (Fig. 1).

D. Test sequence

Before the test, each subject was asked to per-

form 3 submaximal repetitions of isokinetic knee

extension at each angular velocity as a warm-up.

Ten minutes after the warm-up, each subject per-

formed 20 testing trials in one daily session of 10

trials for each bite condition at each angular

velocity. The order was chosen arbitrarily. More

3-minute rest periods were set between the test-

ing trials. During the rest periods, all straps were

loosened to allow the subjects to relax. The same

tests were repeated over 5-day periods at inter-

vals of more than 2 weeks. Two weeks before the

actual test the day was assigned for practice to

gain familiarity with the testing procedures.

All measurements were carried out in the exclu-

sive laboratory, where the temperature was con-

trolled at approximately 22•Ž The tests were per-

formed as near to the same time of day as possi-

ble. Subjects received no verbal encouragement

during testing.

E. Data Analysis

The mean values of the PT/BW and the

AP/BW in all subjects for each bite condition

were calculated at 5 angular velocities. The paired

t-test was used to analyze the difference of the

means between the two bite conditions. The prob-

ability level accepted for statistical significance

was set at p•ƒ0.05.


Table 1. Peak torque-velocity relationships during maximum

voluntary clenching of the teeth and the rest position.

Values are means•}S. D.


A Spearman's Correlation Coefficient by rank

test was calculated between the angular velocities

and the, difference in PT/BW derived from the

two bite conditions. To determine if a correlation

was statistically different from zero at the 5%

significance level, a Fisher's r to z transformation

was carried out on the correlation. In addition, the

correlation between angular velocities and the dif-

ference of the AP/BW between the two bite con-

ditions were analyzed using the same methods.

III. Results

As for individual fluctuation of PT/BW and

AP/BW, no significant differences were found

among 5 testing days in each subject (0.210•ƒ

p•ƒ0.961 and 0.060•ƒp•ƒ0.972, respectively).

The results of the PT/BW in all subjects are

shown in Table 1 and Fig. 2. Significant dif-

ferences were found between the peak torque dur-

Fig. 2. Comparison of peak torque-veloctiy

relationships during maximum clenching of the

teeth and rest position.* : p•ƒ0.05, this probability indicate that peak

torque during maximum clenching of the teeth is signi-

ficantly greater than that during rest position. Vertical

bars denote•}S. D. Note : [Square]=peak torque per

body weight with maximum voluntary clenching of the

teeth ; [Empty cirele]=peak torque per body weight in

the rest position.

Fig. 3. Relationship between velocity and difference

in peak torque per body weight.

ing MVCT and RP at 30, 60, and 150 deg/s (p=

0.001, p•ƒ0.001 and p=0.001, respectively).

However there were no significant differences bet-

ween during MVCT and RP at 300 and 450 deg/s

(p=0.579 and 0.302, respectively). A significant

negative correlation was found between isokinetic

angular velocity and difference in PT/BW (r=•|

0.699; p•ƒ0.05) (Fig. 3) .

The results of the AP/BW in all subjects are

shown in Table 2 and Fig. 4. While at 300 and

450 deg/s there were no significant differences

between during MVCT and RP (p=0.867 and

0.939, respectively), significant differences were

evident between the average power during MVCT

and RP at 30, 60, and 150 deg/s (p•ƒ0.001, p•ƒ

0.001 and p=0.003, respectively). No significant

correlation was found between isokinetic angular

velocity and difference in AP/BW (r=•|0.083 ; p

=0 .582).

IV. Discussion

A. Factors involved in the experiment

Knee extension was chosen as the test task in

this study because the force-velocity relationships


Table 2. Average power-velocity relationships during maximum

voluntary clenching of the teeth and the rest position.

Values are means•}S. D.


Fig. 4. Comparison of average power-veloctiy

relationships during maximum clenching of the

teeth and rest position.

* : p•ƒ0.05, this probability indicate that average

power during maximum clenching of the teeth is signi-

ficantly greater than that during rest position. Vertical

bars denote•}S. D. Note : [Square]=average power

per body weight with maximum voluntary clenching of

teeth ; [Empty cirele]=average power per body weight

in the rest position.

Table. 3. The masseter integrated EGM monitored

under each bite condition at 5 angular velocities.

The values were means•}S. D.(N=9). Before the

actual test, the right masseter EMG were measured 3

times during maximum voluntary clenching of the teeth

without knee extension every testing day. The mean

value per second was designated as 100, and right

masseter EMG activities per second were recorded

during knee extension under each bite condition and

each angular velocity.

of the knee extension have been investigated in

detail6•`12). The ranges of motion measured in

knee extension varied from study to study. Thor-

stensson et al.6) and Wyse et al.13) measured from

90 to 0 degrees, Perrine and Edgerton7) from 100

to 0 degrees, Marshall et al.11) from 110 to 10

degrees, and Li et al.14) from 90 to 5 degrees. However Taylor et al.15) showed that some normal

subjects were unable to maintain high angular

velocities (240, 300, and 400 deg/s) throughout the

full range of knee extension, and the leg deceler-

ated from 30 to fully extended position. They sug-

gested that, because the knee extension torque values collected at 30 degrees of flexion to fully

extended position contained possible errors, such

values should be excluded from the analysis.

Therefore, in this study, the range of motion was

set from 90 to 30 degrees for reasons of preven-

tion of knee joint hypertension.

As concerns reproducibility of the monitored

bite conditions, Table 3 shows the masseter inte-

grated EMGs which were obtained under each bite condition and each angular velocity in all subjects.

A significant difference of integrated EMG in

neither MVCT nor RP was found among the 5

groups of angular velocities (One-way analysis of variance, p=0.360 and 0.974, respectively). This

finding demonstrates that all subjects correctly

monitored each bite condition at each angular

velocity. The standard deviations of our study for

MVCT were smaller than that of Ueno's study4)

. This might be due to the difference in EMG waves

used for monitoring bite condition since this study

used the amplitude of integrated EMG, whereas

the force of teeth clenching was monitored by

means of the amplitude of raw EMG in Ueno's

study. Sasaki et who examined the effect of

teeth clenching on muscle strength of plantar fle-

xion in subjects in a spine position at only 180

deg/s, used the same amplitude of integrated EMG

for monitoring the bite condition. As compared

with the standard deviation of MVCT in their

study, those of our study were almost equal.

As for muscle fatigue, Bigland-Ritchie et al.16)

observed that the firing rates of brachial biceps


motor neurons fell during sustained maximum

voluntary contraction and the rates returned to

the control values within 3 minutes after blood

supply was restored by releasing the arterial cuff.

Many authors11,12,15,17,18) who measured iso-

kinetic muscle strength in knee extension gave the

subjects rest periods from 2 to 3 minutes, hence a

rest period of least 3 minute was assigned;

moreover, all straps were loosened between each

trial in this study.

Previous studies19,20) showed that the process

of measurement itself had a training effect which

resulted in augmented strength during subsequent

tests. Bohannon21) related that since training one

or more times per week resulted in strengthening

but training or testing once every two weeks did

not, strength testing more frequency than biweek-

ly might be inadvisable. Accordingly, we assigned

at least a 2 week interval between measurement

days to exclude the effect of training. The subjects

were also instructed to maintain their usual pat-

terns of activity.

•¬ strand and Rodahl22) suggested that it was

desirable to divide the torque value by the

subject's body height to compare data obtained on

subjects of different body size. However, Matsu-

moto23) recommended evaluating isokinetic muscle

strength by dividing the torque value by body

weight, and Hald et al.24) used this method, since

a very close correlation was found between iso-

kinetic torque and body weight25). In the Cybex

User's Guide, it was explained that torque data

normalized to body weight was useful for

inter-individual comparison. Consequently, the

value of the peak torque per body weight as well

as average power per body weight was analyzed

as a measure of muscle strength.

B. Effect of teeth clenching on knee exten-

sion strength

This study showed that both torque (PT/BW)

and power (AP/BW) significantly increased in

association with teeth clenching at 30, 60, and 150

deg/s, but there were no significant differences at

300 and 450 deg/s. According to Wyatt and

Edwards26), the knee extended at a rate of 233

deg/s during normal walking. Cooper and Glas-

sow27) showed that the angular velocity of knee

extention of a trained athlete was 711 deg/s dur-

ing running. Considering these facts and our

results together with those of previous studies'

which investegated the effect of teeth clenching on

the isometric muscle strength of upper and lower

appendages, teeth clenching may contribute to

improvement of both static athletic performance

and dynamic performance exerted at less than

walking speed.

Caiozzo et al.8) suggested that the muscle train-

ing at lower speeds markedly influenced muscle

strength in the slow velocity-high force region.

They showed that the isokinetic training of knee

extension at 96 deg/s, which was assigned two

sets of 10 single maximal voluntary efforts three

times a week for 4 weeks, resulted in significant

muscle strength improvements at 0 to 240 deg/s.

The rates of increase of maximal knee extension

torques ranged between 14.7 and 5.5% . On the

other hand, the rates of increase of PT/BW at 30

to 150deg/s in our study ranged from 7.4 to

4.9% . Isokinetic training is considered to be

more effective than teeth clenching with respect to

the improvement of muscle strength. However, the

effect of teeth clenching is simultaneously

apparent, whereas the training effect appears gra-

dually after several days or weeks. Consequently,

it is considered that the instantaneous augmenta-

tion of muscle strength and power with teeth clen-

ching is quite useful in specific sports contexts.

Also there is a possibility that even greater

improvement of muscle strength and power might

be obtained by means of exertion of isokinetic

training at lower speed in association with teeth


clenching.

As mentioned in the introduction, Miyahara et

al.2) and Takada3) showed that teeth clenching in-

creased the amplitude of the H reflex of the soleus

and tibialis anterior in humans. Hagiya28) also

demonstrated that the monosynaptic reflex of cru-

ral muscles in anesthetized rabbits were

non-reciprocally facilitated in association with

rhythmical jaw movement. Moreover, Tanaka29)

and Takada3) found that reciprocal Ia inhibition

was reduced by rhythmical jaw movement in the

anesthetized rabbits and by teeth clenching in

humans, respectively. These findings suggested

that the oral motor functions involved in teeth

clenching had an influence on the motor activity

of other parts of the body via the following two

mechanisms : by elevating the excitability of motor

neuron pool ; and by reducing reciprocal Ia inhibi-

tion.

During the exertion of isometric muscle

strength, co-contraction of the agonist and the

antagonist is often observed30) . Accordingly it is

considered that the findings2,3,28) in the

neurophysiological field agree closely with pre-

vious findings4,5) for improvement of isometric

muscle strength in the kinesiological field. Chiefly

the former mechanism may be concerned with the

improvement of isometric muscle strength with

teeth clenching.

On the other hand, during exertion of dynamic

strength involved in isokinetic muscle strength

as investigated in the present study, reduction of

reciprocal Ia inhibition is unfavorable phenomena

because smooth rotational movement of joints is

necessary. We assume that the balance between

the above-described two mechanisms may change

as the angular velocity changes. Briefly, it is con-

cluded that at lower angular velocities,

co-contraction of the agonist and the antagonist

muscles may exert the stronger influence, accom-

panied by significant improvements due to the

effect of teeth clenching on this particular mechan-

ism ; while at the higher angular velocities, reduc-

tion of reciprocal Ia inhibition may exert the

stronger influence, resulting in no significant dif-

ferences being found.

Recently some authors31,32) have reported on

the modulation of the H reflex and reciprocal Ia

inhibition during walking and running in humans,

although in previous neurophysiological stu-

dies2,3) these modulation were recorded under

conditions not involving exertion of muscle

strength in humans. The simultaneous

measurement of the H reflex, the reciprocal Ia

inhibition, and the muscle strength will be

required in future.

Acknowledgements

The authors wish to thank all the members of the

staff at our department and all volunteer subjects for

their cooperation.

A part of this study was presented at the 53rd

Annual Meeting of the Japanese Society of Physical Fit-

ness and Sports Medicine, in September 1998, Kanagawa,

Japan.(Accepted Feb. 25, 1999)

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