Belly Of Digastric Muscle Research Articles

Anatomical Study and Variation of the Anterior Belly of Digastric Muscle: Case Report

Anatomical Study and Variation of the Anterior Belly of Digastric Muscle: Case Report

A study of variations of anterior belly of digastric muscle

Abstract Aim : To study the anatomical variations of anterior belly of digastric muscle. Materials & Methods: In the present study, 30 human cadavers from the Department of Anatomy, Belagavi Institute of Medical Sciences, Belagavi were examined for the variations of anterior belly of digastric muscle during routine dissections of undergraduate and postgraduate students. The variations found were neatly dissected and photographs taken wherever necessary. Result: Out of 30 cadavers dissected3 specimens presented with variations. Discussion: First variation found had a unilateral accessory belly of digastric muscle on right side. Second variation was unilateral accessory belly of digastric muscle on left side and the third variation was bilateral accessory slips of digastric muscles. The details of these variations will be dealt in the article. Conclusion: As the variations of anterior belly of digastric muscle are common, the radiologists and the surgeons have to watch out for these while dealing with Sub mandibular region.

Clinical Implications of Accessory Anterior Belly of Digastric Muscle: A Case Study

Clinical Implications of Accessory Anterior Belly of Digastric Muscle: A Case Study

Craniovertebral junction 360°: A combined microscopic and endoscopic anatomical study.

Objectives:Craniovertebral junction (CVJ) can be approached from various corridors depending on the location and extent of disease. A three-dimensional understanding of anatomy of CVJ is paramount for safe surgery in this region. Aim of this cadaveric study is to elucidate combined microscopic and endoscopic anatomy of critical neurovascular structures in this area in relation to bony and muscular landmarks.Materials and Methods:Eight fresh-frozen cadaveric heads injected with color silicon were used for this study. A stepwise dissection was done from anterior, posterior, and lateral sides with reference to bony and muscular landmarks. Anterior approach was done endonasal endoscopically. Posterior and lateral approaches were done with a microscope. In two specimens, both anterior and posterior approaches were done to delineate the course of vertebral artery and lower cranial nerves from ventral and dorsal aspects.Results:CVJ can be accessed through three corridors, namely, anterior, posterior, and lateral. Access to clivus, foreman magnum, occipital cervical joint, odontoid, and atlantoaxial joint was studied anteriorly with an endoscope. Superior and inferior clival lines, supracondylar groove, hypoglossal canal, arch of atlas and body of axis, and occipitocervical joint act as useful bony landmarks whereas longus capitis and rectus capitis anterior are related muscles to this approach. In posterior approach, spinous process of axis, arch of atlas, C2 ganglion, and transverse process of atlas and axis are bony landmarks. Rectus capitis posterior major, superior oblique, inferior oblique, and rectus capitis lateralis (RCLa) are muscles related to this approach. Occipital condyles, transverse process of atlas, and jugular tubercle are main bony landmarks in lateral corridor whereas RCLa and posterior belly of digastric muscle are the main muscular landmarks.Conclusion:With advances in endoscopic and microscopic techniques, access to lesions and bony anomalies around CVJ is becoming easier and straightforward. A combination of microscopic and endoscopic techniques is more useful to understand this anatomy and may aid in the development of future combined approaches.

Botulinum Toxin Therapy versus Anterior Belly of Digastric Transfer in the Management of Marginal Mandibular Branch of the Facial Nerve Palsy: A Patient Satisfaction Survey.

BackgroundBotulinum toxin (BT) chemodenervation and anterior belly of digastric muscle (ABD) transfer are both treatment options in the management of an isolated marginal mandibular branch of the facial nerve (MMB) palsy. We compare the patient satisfaction following either BT injections or ABD transfer in the management of their isolated MMB palsy.MethodsPatients in the ABD-arm of the study were identified retrospectively from September 2007 to July 2014. The patients in the BT-arm of the study were identified prospectively from those attending the clinic. Both groups of patients completed a validated patient satisfaction survey. Statistical analysis was performed and a P-value <0.05 was considered statistically significant.ResultsSeven patients were in the ABD-arm and 11 patients in the BT-arm of the study. The patient satisfaction in both groups was high with 45% of ABD-arm patients and 40% of BT-arm patients rating their overall outcome as 'better' or 'much better', which was significantly more than the proportion rating their outcome as 'worse' or 'much worse' (P<0.001), although there was a significant trend towards those in the ABD-arm being more likely to be dissatisfied with their outcome (P=0.01).ConclusionsBT therapy is a good first-line intervention in the management of isolated MMB palsy. We have, however, shown that the overall satisfaction in both groups is high. Therefore, in patients who would prefer a more permanent solution to manage their facial asymmetry, ABD transfer remains a satisfactory treatment option with a good level of patient satisfaction.

Detailed knowledge of regional anatomy and anatomical variations is fundamental to achieve successful surgical procedure. Although primary objective of neurosurgery is to restore physiological vital functions, remove intracranial mass and prevent further

Detailed knowledge of regional anatomy and anatomical variations is fundamental to achieve successful surgical procedure. Although primary objective of neurosurgery is to restore physiological vital functions, remove intracranial mass and prevent further brain damage, while preserving tissue and organ integrity, the neurosurgeon takes the risk of impairing non-vital functions. Occasionally, as with the hypoglossal nerve, the impairment of anatomical structures found on surgical route is due to their still barely known anatomical relations and variations. In order to provide an anatomically and surgically oriented classification to guide neurosurgical procedures and to ensure the preservation of nerve integrity, the aim of the present study is to detail the course of the 12th cranial nerve (CN) and to establish anatomical landmarks for surgeons. A combination of anatomical dissection of the neck and oral floor and skull base far lateral approach of 6 cadaveric human heads (3 male, 3 female, age 62+4) was performed, on both sides, to explore and follow the entire course of the 12th CN, from its emergence in the preolivary sulcus to the tongue. Skeletal, muscular and vascular relationships were meticulously analyzed and documented during anatomic and surgical dissections. According to our observations, hypoglossal nerve can be divided into five segments. The first two are intracranial, cisternal and intracanalar, and the other three, namely the descending, horizontal and ascending, are extracranial. Intriguingly, we found unreported relations of the nerve that, apart from their anatomical interest, have tremendous significance for surgeons operating on head and neck. Specifically, the intracanalar segment passes through a venous lacuna that, to the best of our knowledge, was never described before as such. This venous structure drains into the jugular bulb and acts as a sheath between the nerve and the osseous wall of the canal. The nerve in the venous sheath bends and it is elastically fixed to the osseous wall of the canal by fibrous bands. Therefore, the venous lacuna guarantees mobility to the nerve, and cushion the nerve from the bone. As for the descending segment, during its course it has very close relationship with the internal jugular vein, the internal carotid artery, the posterior belly of digastric muscle, and the styloid process and muscles inserting on it. The descending segment provides the ansa hypoglossi, branches to muscles inserting on the styloid process and to the sternocleidomastoid muscle. The horizontal segment has relationship with the intermediate digastric tendon, the stylohyoid, hyoglossus and mylohyoid muscles and the submandibular gland. The ascending segment might be very short and sometimes it is absent. The fifth and last segment becomes deeper at the anterior edge of hyoglossus muscle, and terminates into several branches supplying intrinsic and extrinsic musculature of tongue.

Facial nerve trunk variations with surgical implications: A cadaveric study

Purpose: Anterograde identification of facial nerve trunk (FNT) identification is paramount, because FNT injury causes substantial morbidity. This study reexamines controversial landmarks and considers the importance of gender and dentition in landmark choice. MethodsDistances from key landmarks to FNT were measured and compared by side, gender and dentition (92 specimens). Landmarks included posterior belly of digastric muscle (PBM), tragal pointer (TP), mastoid process (MP), external acoustic meatus (EAM) and transverse process of axis (TPA). ResultsTwo-sample T tests showing longer distances from: MP/TPA to FNT in men than in women (14.8 ± 2.2 mm vs. 13.5 ± 1.6 mm, P = 0.004; 37.6 ± 4.4 mm vs. 32.7 ± 4.2 mm, P = 0.001); EAM to FNT on occlusal sides than on the counterparts (14.2 ± 1.8 mm vs. 16.0 ± 3.8 mm, P = 0.020). One-sample T tests showing longer distances from: TP to FNT on right than on left side (21.4 ± 2.7 vs. 19.9 ± 2.9, P = 0.006); MP to FNT on the less dentulous maxillae than on the counterpart (14.4 ± 2.1 vs. 13.0 ± 1.6, P = 0.027); PBM/EAM to FNT on the less dentulous mandible than on the counterpart (9.8 ± 1.6 vs. 7.8 ± 2.5, P = 0.039; 16.4 ± 3.0 vs. 14.1 ± 1.5, P = 0.020). ConclusionSurgeons should be aware that distances of MP, PBM and EAM, to FNT, are lengthened in less dentulous patients, especially when maxilla and mandibles are non-occlusive. Overall, soft landmarks are less reliable than osseous landmarks.

Identification of Facial Nerve During Parotidectomy: A Combined Anatomical &amp; Surgical Study

To find out the most easily identifiable and anatomically consistent landmark for identification of facial nerve during parotid surgery. Ten cadaveric dissections and ten live parotid surgeries for different types of parotid tumours were done. Cadaveric dissection was performed in the Department of Anatomy and the surgeries were done in the Department of ENT and Head and Neck surgery of R. G. Kar Medical College of Kolkata. The distance of the facial nerve trunk from three most commonly used landmarks (viz., tympanomastoid suture, tragal pointer and posterior belly of digastric muscle) was measured in both cadaver and live patients. The ease of identification of the nerve trunk using each of the landmarks, particularly during live surgery was also assessed. The mean distance of the tympanomastoid suture from the facial nerve trunk was 3.5mm (cadaver) and 3.87mm (live surgery), the tragal pointer was found to be at a mean distance of 16.61mm (cadaver) and 16.36mm (live surgery) and in case of the posterior belly of digastric muscle it was 7.41mm (cadaver) and 8.03mm (live surgery). During live surgery the posterior belly of digastric was found to be the most easily identifiable landmark with a consistent anatomical relationship with the nerve trunk. The posterior belly of digastric muscle is the most easily identifiable and a very consistent landmark for facial nerve dissection during parotidectomy. When supplemented with the tragal pointer, accuracy in identifying the facial nerve trunk is very high, thereby avoiding inadvertent injury to the nerve trunk.

Changes in activity and structure of jaw muscles in Parkinson's disease model rats

Parkinson's disease (PD), a major neurological disease, is characterised by a marked loss of dopaminergic neurons in the substantia nigra. Patients with PD frequently show chewing and swallowing dysfunctions, but little is known about the characteristics of their stomatognathic functions. The purpose of this study was to evaluate the influence of PD on jaw muscle fibre and functions. PD model rats were made by means of the injection of 6-hydroxydopamine (6-OHDA) into the striatum of 8-week-old Sprague-Dawley male rats. Five weeks after the injection, a radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter and anterior belly of digastric muscles. Muscle activity was recorded for 3days and was evaluated by the total duration of muscle activity per day (duty time). After recording the muscle activities, jaw muscles were isolated for immunohistochemical and PCR analyses. In PD model rats, the following findings of the digastrics muscles verify that compared to the control group: (i) the higher duty time exceeding 5% of the peak activity level, (ii) the higher expression of the mRNA of myosin heavy chain type I, and (iii) the tendency for fast to slow fibre-type transition. With respect to the masseter muscle, there were no significant differences in all analyses. In conclusion, PD leads to the changes in the jaw behaviours, resulting in a PD-specific chewing and swallowing dysfunctions.

Variant Facial Artery in the Submandibular Region

Facial artery has been considered to be the most important vascular pedicle in facial rejuvenation procedures and submandibular gland (SMG) resection. It usually arises from the external carotid artery and passes from the carotid to digastric triangle, deep to the posterior belly of digastric muscle, and lodges in a groove at the posterior end of the SMG. It then passes between SMG and the mandible to reach the face after winding around the base of the mandible. During a routine dissection, in a 62-year-old female cadaver, in Kasturba Medical College Mangalore, an unusual pattern in the cervical course of facial artery was revealed. The right facial artery was found to pierce the whole substance of the SMG before winding around the lower border of the mandible to enter the facial region. Awareness of existence of such a variant and its comparison to the normal anatomy will be useful to oral and maxillofacial surgeons.

Fine structure of myotendinous junction between the anterior belly of the digastric muscle and intermediate tendon in adults rats

This study analyzed the ultrastructural characteristics of the myotendinous junction (MTJ) between anterior belly of digastrics muscle and the intermediate tendon in adult rats. Six male Wistar rats were used and were anesthetized with an overdose of urethane and sacrificed by intracardiac perfusion with modified Karnovsky solution, postfixed in 1% osmium tetroxide, dehydrated in increasing series of alcohols and embedded in Spurr resin for transmission electron microscopic analysis. Ultrastructural analysis showed conical shape of the fiber extremity in MTJ region, highlighting the presence of numerous mitochondria arranged in groups in the subsarcolemmal and intermyofibrillary regions. Atypical MTJ characteristics were seen interspersed with bundles of collagen fibers. Classic characteristics such as finger-like processes by means of sarcoplasmic projections were observed among interdigitations. Terminals and periphericals bundles of myofibrils showed close relationship with the adjacent muscle fiber's endomysium through lateral junctions. In the distal portion, it was observed that the communication region of microtendons forming the intermediate tendon of digastric muscle, and it can highlight the columns disposition of tenocytes. In conclusion, the MTJ ultrastructure between the anterior belly of digastric muscle and intermediate tendon of adult rats showed classical morphologic descriptions and presented an atypical region revealed by the subspecialization between the myofibrils bundles and collagen fibers in the MTJ region.

Sonographic anatomy of the neck: The suprahyoid region

The suprahyoid region extends from the base of the skull to the hyoid bone and includes the pharyngeal, parapharyngeal, parotid, carotid, masticator, retropharyngeal, and perivertebral spaces, as well as the oral cavity. The areas that can be explored by ultrasound include the parotid, carotid, and masticator spaces; the oral cavity; the submandibular and sublingual spaces; the floor of the mouth; and the root of the tongue. The parotid space contains the parotid gland and the excretory duct of Steno, the facial nerve, the external carotid artery, the retromandibular vein, and the intraparotid lymph nodes. The carotid space in the suprahyoid region of the neck contains the internal carotid artery, the internal jugular vein, cranial nerves IX to XII, and the sympathetic plexus. Only some parts of the masticator space can be explored sonographically: these include the masseter muscle, the zygomatic arch and the outer cortex of the ramus of the mandible, and the suprazygomatic portion of the temporalis muscle. The submandibular space houses the submandibular gland, the submental and submandibular lymph nodes, and the anterior belly of digastric muscle. The facial artery and vein and the lower loop of the hypoglossal nerve all pass through the submandibular space. The sublingual space includes the sublingual gland, the deep portion of the submandibular gland and its main excretory duct, the hypoglossal nerve (cranial nerve XII), the lingual nerve (branch of the mandibular branch of trigeminal), and the glossopharyngeal nerve (IX cranial nerve), and the lingual artery and vein. The mylohyoid muscle forms the floor of the mouth. The deepest portion of the oral tongue, the root, consists of the genioglossus and geniohyoid muscles and includes the septum of the tongue. In this article we present the ultrasound features of the structures located in the suprahyoid region of the neck.

Changes in EMG frequency and oxygen dynamics in anterior belly of digastric muscles by anterior traction of mandible

Oral appliances (OAs) are widely used for anterior positioning of the mandible, and many studies have investigated change in activity in the jaw-closing muscles, including the masseter muscle, caused by such appliances. Few studies, however, have investigated their effect on the jaw-opening muscles. In the present study, we chose the masseter and jaw-opening muscles because they allow non-invasive measurement by means of surface electromyography. We investigated physiological changes in the digastric and masticatory muscles brought about by OAs using electromyography and an oximeter. Subjects comprised 7 healthy males. Fabricated OAs were placed in the mouth to obtain either 70% maximal protrusion (advanced mandibular position: AMP) or a centric relation (CR). Subjects were then instructed to open their mouth 2 mm vertically for 180 s, during which time electromyograms were obtained and blood flow volume determined. No significant difference was observed between AMP and CR in the frequency analysis of the anterior belly of the digastric muscle (Dig). No significant difference was noted in muscle activity between CR and AMP. No stable tendency was observed in the masseter or temporal muscles. Oxygen content recovery time in Dig was significantly longer at 1, 5 and 7 h in AMP than in CR. Seven-hour retention in AMP caused physiological change in Dig and a delay in recovery time from muscle fatigue. However, recovery from muscle fatigue occurred shortly after OA removal.

Bilateral, asymmetric anomalies of the anterior bellies of digastric muscles

Bilateral, asymmetric anomalies of the anterior bellies of digastric muscles were observed during dissection of the submental region. Specifically, four extra muscle bundles were found between the anterior bellies of the digastric muscle. Although anomalies of the anterior bellies of digastric muscles are often observed, this complicated pattern of digastric anomalies has not been previously reported. Our findings and previous observations illustrate the morphogenetic complexity of the anterior belly of the digastric muscle derived from the first pharyngeal arch, which gives rise to jaw musculature such as the mylohyoid muscle.

Adaptation of rat jaw muscle fibers in postnatal development with a different food consistency: an immunohistochemical and electromyographic study

The development of the craniofacial system occurs, among other reasons, as a response to functional needs. In particular, the deficiency of the proper masticatory stimulus affects the growth. The purpose of this study was to relate alterations of muscle activity during postnatal development to adaptational changes in the muscle fibers. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a solid (hard-diet group) or a powder (soft-diet group) diet for 63 days. A radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time), the total burst number and their average length exceeding specified levels of the peak activity (5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of fibers by means of immunohistochemical staining and their cross-sectional area was measured. All muscle fibers were identified as slow type I and fast type IIA, IIX or IIB (respectively, with increasing twitch contraction speed and fatigability). At lower activity levels (exceeding 5% of the peak activity), the duty time of the anterior belly of the digastric muscle was significantly higher in the soft-diet group than in the hard-diet group (P < 0.05). At higher activity levels (exceeding 20 and 50% of the peak activity), the duty time of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.05). There was no difference in the duty time of the anterior temporalis muscle at any muscle activity level. The percentage of type IIA fibers of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.01) and the opposite was true with regard to type IIB fibers (P < 0.05). The cross-sectional area of type IIX and type IIB fibers of the superficial masseter muscle was significantly smaller in the soft-diet group than in the hard-diet group (P < 0.05). There was no difference in the muscle fiber composition and the cross-sectional area of the anterior belly of the digastric and anterior temporalis muscles. In conclusion, for the jaw muscles of male rats reared on a soft diet, the slow-to-fast transition of muscle fiber was shown in only the superficial masseter muscle. Therefore, the reduction in the amount of powerful muscle contractions could be important for the slow-to-fast transition of the myosin heavy chain isoform in muscle fibers.

Anatomic Variation of the Anterior Belly of Digastric Muscle and Positional Relationship between the Posterior Belly of Digastric and Stylohyoid Muscle

Anatomic Variation of the Anterior Belly of Digastric Muscle and Positional Relationship between the Posterior Belly of Digastric and Stylohyoid Muscle

Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition

Skeletal muscles have a heterogeneous fiber type composition, which reflects their functional demand. The daily muscle use and the percentage of slow-type fibers have been shown to be positively correlated in skeletal muscles of larger animals but for smaller animals there is no information. The examination of this relationship in adult rats was the purpose of this study. We hypothesized a positive relationship between the percentage of fatigue-resistant fibers in each muscle and its total duration of use per day. Fourteen Wistar strain male rats (410-450 g) were used. A radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter, deep masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time) exceeding specified levels of the peak activity (2, 5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of the fibers by means of immunohistochemical staining. At lower activity levels (exceeding 2 and 5% of the peak activity), the duty time of the anterior belly of digastric muscle was significantly (P < 0.01) longer than those of the other muscles. The anterior belly of digastric muscle also contained the highest percentage of slow-type fibers (type I fiber and hybrid fiber co-expressing myosin heavy chain I + IIA) (ca. 11%; P < 0.05). By regression analysis for all four muscles, an inter-muscular comparison showed a positive relationship between the duty time (exceeding 50% of the peak activity) and the percentage of type IIX fibers (P < 0.05), which demonstrate intermediate physiological properties relative to type IIA and IIB fibers. For the jaw muscles of adult male rats, the variations of fiber type composition and muscle use suggest that the muscle containing the largest amounts of slow-type fibers (the anterior belly of digastric muscle) is mainly involved in low-amplitude activities and that the amount of type IIX fibers is positively related to the generation of large muscle forces, validating our hypothesis.

The precision of four commonly used surgical landmarks for locating the facial nerve in anterograde parotidectomy in humans

In addition to using intra-operative facial nerve monitoring in helping to locate the position of the facial nerve in anterograde parotidectomy, numerous soft tissue and bony landmarks have been proposed to assist the surgeon in the early identification of this nerve. There is still dispute within the literature as to the most effective method, if any, of locating the nerve. The purpose of this study was to measure the distance (in twenty-six embalmed cadavers) from four of the most commonly used surgical landmarks to the main trunk of the facial nerve—the posterior belly of digastric muscle (PBDM), the tragal pointer (TP), the junction between the bony and cartilaginous ear canal (EAM) and the tympanomastoid suture (TMS). The main trunk of the facial nerve was found 5.5±2.1 mm from the PBDM, 6.9±1.8 mm from the TP, 10.9±1.7 mm from the EAM and 2.5±0.4 mm from the TMS. From this, the TMS can be used as a reliable indicator for locating the main trunk of the facial nerve. In addition, this study also demonstrated a statistically significant difference between the sexes in relation to the two bony landmarks used here, the EAM and the TMS, with the facial nerve found further away from those landmarks in females compared to males. With the advent of 3D construction and reformatting of images, these values may come to the forefront in pre-operative planning for locating the facial nerve in anterograde parotidectomy.

A complex variation of the digastric muscle: a case report

The anterior belly of the digastric muscle varies greatly in its shape and size. It is important to know the variations of the digastric muscle, as it is used as a landmark in certain surgical procedures. A complex variation of the digastric muscles was detected during the routine dissection of a 55-year-old male cadaver. The anterior belly of the digastric muscle on the left side had an accessory bundle, which arose from the anterior part of the intermediate tendon and divided into two parts after a 2-cm course. The superior part coursed toward the mylohyoid raphe and fused with the mylohyoid muscles of both sides. The inferior part travelled parallel to the hyoid bone and fused with the right mylohyoid muscle. Another accessory bundle was found between the origin of the anterior belly of the right digastric muscle and the bifurcation site of the aforemen- tioned bundle. This latter accessory bundle had no attachment to the mylohyoid muscle. Knowledge of the variations of the digastric muscles may prevent complications when surgery is performed in the suprahyoid region or when imaging is inter- preted for this region.

Study on Motor Learning of Sternocleidomastoid Muscles during Ballistic Voluntary Opening

Neck pain is one of the main symptoms of temporomandibular disorder. Muscle activity of the sternocleidomastoid muscle during occlusion has been clarified in recent years. We reported that when healthy individuals were instructed to chew rapidly, the activity of the sternocleidomastoid muscle responded to activity of the masseter muscle, however, during voluntary jaw opening, activity of the sternocleidomastoid muscle did not respond, but worked actively due to motor programming. The objective of the present study was to investigate the learning effects of repetitive training, that is, changes in activity mode of the neuromuscular system. The sternocleidomastoid and the anterior belly of digastric muscles in 8 healthy male adults were analyzed. In response to acoustic stimulation, each subject was instructed to open their mouth as quickly and widely as possible a total of 30 times with a break between measurements. EMG-reaction times (RT) of the sternocleidomastoid and anterior belly of digastric muscles were measured, and the length of time from the start of EMG activity of agonist to the start of actual movement was measured. In all subjects, at first measurement, EMGRT of the sternocleidomastoid muscle did not precede that of the anterior belly of digastric muscle. With each measurement, the difference in EMG-RT between the sternocleidomastoid and the anterior belly of digastric muscles decreased, and in 6 of the 8 subjects, EMG-RT of the sternocleidomastoid muscle preceded that of the anterior belly of digastric muscle. Repetitive task movement alters the start times of muscular activities, and from the perspective of EMG kinesiology, motor learning effects were confirmed with maximum ballistic voluntary jaw opening.