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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.
368 SUMITA, SASAKI, UENO, TANIGUCHI, OHYAMA
Table 1. Peak torque-velocity relationships during maximum
voluntary clenching of the teeth and the rest position.
Values are means•}S. D.
369TEETH CLENCHING AND KNEE EXTENSION
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
370 SUMITA, SASAKI, UENO, TANIGUCHI, OHYAMA
Table 2. Average power-velocity relationships during maximum
voluntary clenching of the teeth and the rest position.
Values are means•}S. D.
371TEETH CLENCHING AND KNEE EXTENSION
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
372 SUMITA, SASAKI, UENO, TANIGUCHI, OHYAMA
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
373TEETH CLENCHING AND KNEE EXTENSION
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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