TITLE: Changes in the thickness of the cervical flexor depending on the contraction
level of the masticatory muscle during deep cervical flexor training

  ABSTRACT:
[Purpose] This study examined the changes in the thickness of the deep cervical flexor
according to the contraction intensity of the masticatory muscle during deep cervical
flexor training. [Subjects and Methods] Twenty healthy adults were randomly selected and
the thicknesses of their longus colli and sternocleidomastoid were measured with
ultrasound when the masticatory muscle contracted during deep cervical flexor training.
[Results] The thickness of the longus colli tended to increase in proportion to the
contraction intensity of the masticatory muscle, with a significant difference. However,
the thickness of the sternocleidomastoid did not significantly differ with the contraction
intensity of the masticatory muscle. [Conclusion] During deep cervical flexor training,
when co-contraction of the masticatory muscle occures, changes in the thickness of the
longus colli may be selectively increased. Deep cervical flexor training was most
effective during contractions of a submaximal intensity.

BODY.INTRODUCTION:
The longus colli and longus capitus are representative deep cervical flexors. These muscles
have many proprioceptors for postural control and therefore play a role in supporting
position sense and balance of the head and neck1). However, chronic neck pain patients undergo relatively abnormal
hypertension of the superficial cervical flexor2) and exhibit shortening of the subocciput region due to the
weaknesses of the longus colli and longus capitus during movement of the cervical area3). Therefore, deep cervical flexor training is
conducted for selective strengthening and retraining of the deep cervical flexor4).  The detailed method of this training is to draw the chin in and make the curve of the
cervical area flat5). Here, chin-in motion
affects the contraction of the muscles connecting the mandible and cervical area. For
example, elevation (mouth closed) of the mandible triggers a contraction of the cervical
flexor, while depression (mouth open) of the mandible brings about a contraction of the
cervical extensor6). In addition, the
masticatory muscle attached to the mandible plays the role of propioceptor, as well as
performing the functions of mastication, and therefore is involved in conducts postural
control of the head, cervical area, and upper limb in co-ordination with the deep cervical
flexor7). Previous research has reported
that during the motion of clenching, the direction and intensity of mastication resulted in
significantly different activations of the cervical muscles. One study stated that changes
in the location of the lower jaw, such as opening or clenching the mouth, lead to
significant differences in the cervical muscles8). This is because the fascia covering the masticatory muscle has
continuity with the fascia of the cervical muscles and has the same functional unit;
therefore, it affects the activity of the cervical muscles, and the contraction of the
masticatory muscle in coordination with the cervical muscles increases the contraction of
the cervical muscles9). In previous
research, changes in the thickness of the deep cervical flexor were observed when clenching
at a submaximal intensity, but the degree of coordination of the cervical flexor will become
different according to the contraction intensity of the masticatory muscle. Therefore, this
study compared the thickness of the cervical flexor according to the contraction intensity
of the masticatory muscle during deep cervical flexor training.

BODY.SUBJECTS AND METHODS:
This study recruited 20 healthy adults who voluntarily consented to participate in this
study and signed a written consent. This experiment was approved by the Bioethics Committee
of the Catholic University of Pusan. The subjects were those who had no neck pain,
temporomandibular joint pain, or headache within the last six months. During the
familiarization session, the subjects were taught how to perform deep cervical flexor
training, and maximal voluntary isometric contraction (MVIC) of the masseter was measured;
then, clenching was performed at 10, 50, and 80% MVIC. The clenching intensity was adjusted
by the subjects who were given feedback of the root mean squares values of the
electromyography trace on a monitor (Myo-System DTS, Noraxon U.S.A. Inc., Scottsdale, CA,
USA). In this experiment, the subjects lay in a supine position on a bed. First, a pressure
biofeedback unit was placed on the cervical posterior surface of the subjects and the
posterior surface of the neck was pressured to 20 mmHg with the pressure biofeedback unit.
The subjects then performed chin-in with the mouth lightly closed without contraction of the
masticatory muscle. In this position, the experimenter measured the thicknesses of the
longus colli and sternocleidomastoid of each subject with ultrasound. Then, both masseters
of the subjects were cleaned with alcohol cotton and dried. Ag-AgCl electrodes were attached
to the subjects who were then instructed to conduct chin-in in the same way, while
simultaneously clenching the masseter muscle at intensities of 10, 50, and 80% MVIC. The
ultrasound measurement location was 2 cm downward from the thyroid cartilage. The area was
marked with a marker and the measurement was taken with the ultrasound transducer placed at
a right angle, 1 cm from the right side. The longest front and rear distances among the
images of the muscular cross-section were measured10). Each motion was measured three times and the average value was
used for the analysis. To prevent damage to the subjects' teeth and temporomandibular
joints, the subjects conducted clenching with a mouthpiece placed in the mouth. To prevent
muscle fatigue, each motion was maintained for 5 s and 2 min of resting time was given.
Ultrasound (SONOACE X4, MEDISON, Seoul, South Korea) was measured using a 7.5 MHz linear
transducer in B-Mode. SPSS 19.0 was used for the statistical analysis. Descriptive
statistics of the subjects' general characteristics were calculated and repeated measures
analysis of variance was used to examine changes in the thicknesses of the longus colli and
sternocleidomastoid according to the contraction intensity of the subjects' masticatory
muscles. A significance level of α = 0.05 was used.

BODY.RESULTS:
The general characteristics of the subjects are shown in Table 1Table 1.Characteristics of the subjects (N=20)VariablesValueGender (M/F)15/5Age (yrs)21.4 (1.8)Height (cm)172.7 (4.4)Weight (kg)68.8 (7.76).  The thickness of the longus colli tended to increase in proportion to the contraction
intensity of the masticatory muscle when the masticatory muscle was concurrently contracted,
compared to when only deep cervical flexor training was conducted, with a significant
difference (p<0.05). However, there was no significant difference in the muscle thickness
of the sternocleidomastoid according to contraction intensity (p>0.05) (Table 2Table 2.Difference in the longus colli and sternocleidomastoid thickness according to the
contraction intensity of masticatory muscle (units: cm)MusclesIntensityMean±SDPost-hocLC80%MVIC d0.60±0.14*a·b·c<d50%MVIC c0.55±0.1310%MVIC b0.51±0.12Control a0.50±0.15SCM80%MVIC d0.77±0.21a·b·c·d50%MVIC c0.74±0.2510%MVIC b0.77±0.26Control a0.71±0.19).

BODY.DISCUSSION:
This study measured changes in the thickness of cervical flexors in accordance with the
contraction intensity of the masseter muscle using ultrasound during deep cervical flexor
training. The result was that as the contraction intensity of the masticatory muscle
increased, the thickness of the longus colli significantly increased, but the thickness of
the sternocleidomastoid exhibited no significant difference. In addition, the thickness of
the longus colli grew significantly thicker during the concurrent contraction of the
masticatory muscle to 80% of the MVIC relative to during only deep cervical flexor training.
When the masticatory muscle contracted to 10% and 50% of the MVIC, there was a tendency for
the muscle to become thick in proportion to the amount of contraction, but the differences
were not significant.  Therefore, it is our considered opinion that during deep cervical flexor training, the
concurrent contraction of the masticatory muscle at a submaximal intensity selectively
increased the thickness of the longus colli without contraction of the sternocleidomastoid,
and the probable mechanism behind this result is coordination of the masticatory muscle and
the longus colli.  The mastication system connects chains of the front and back muscles of the body. In
particular, the mandible and tongue are directly connected to the chains of the front
muscles, and the hyoid bone is pressured with the role of a basic axis.  During clenching, contraction of the masticatory muscle, and input of sensation by
proprioceptors within the teeth, gums, and temporomandibular joints, deliver a sensation to
the mesencephalic nucleus area within the brain stem and the sensation of mastication in the
mesencephalic nucleus is delivered to the trigeminal nuclei for activation.  The trigeminal nuclei are arranged lengthwise along the efferent pathway, which provides
descending delivery of postural information where information for tonic system equilibrium
is combined.  Resultant, excitation of the trigeminal nuclei affects the equilibrium of the tonic muscles
during clenching, and the masseter muscle is neurally dominated by the trigeminal nuclei;
therefore, it probably stimulates the contraction of the longus colli, and cervical postural
maintenance muscles11). Lee et al.12) reported that the contraction intensity of
the neck, upper limbs, and core muscles increased during masticatory muscle contraction,
compared to no-contraction, due to improved postural stability of the upper-lower jaw bite.
Crispin et al.13) also noted that
utilization of mandibular orthopedic repositioning appliances moves the mandible to a normal
position, thereby affecting the temporomandibular joint and improving the muscle strength,
endurance, and motor ability of the whole body muscles, as well as those of the masticatory
muscle.  The reason there was no significant change in the deep cervical flexor thickness when the
masticatory muscle was concurrently contracted at intensities of 10% and 50% MVIC was
because the weak muscle contraction failed to reach the threshold triggering coordination
with the cervical muscles. Thus, when the masticatory muscle is co-contracted at a
submaximal intensity, it would be more effective at strengthening the deep cervical flexor.
A future study should investigate the effect in patients with chronic neck pain, because the
subjects of this study were normal adults.

Question: Compared to 10% maximal voluntary isometric contraction what was the result of 50% maximal voluntary isometric contraction on Sternocleidomastoid thickness ?
Evidence: However, there was no significant difference in the muscle thickness
of the sternocleidomastoid according to contraction intensity (p>0.05) 
Answer: no significant difference