Posted on 05 Jan 2011 22:03
By Ground Up Strength, Eric Troy
The digastric is a double muscle of the throat which is located under the chin, behind and below the corner of the jaw, immediately in front of the top of the sternocleidomastoid, one for each side of the jaw and neck. It gets its name from the Greek word for "two bellies". The Greek word dia means double and gaster means belly hence digastric meaning "two-bellied".1 The digastric is made up of an anterior and posterior belly. The anterior belly extends from the digastric fossa of the mandible and the posterior belly extends from the mastoid notch of the temporal bone. Both bellies then insert to the body of the hyoid bone via a fibrous loop over a common intermediate tendon between the two bellies.
The digastric assists the lateral pterygoid in depressing the mandible (opening the jaw), primarily during maximum depression or very quick forceful opening of the mouth. Together with the stylohyoid, geniohyoid, and mylohyoid muscles it is known as a suprahyoid muscle and these other hyoid muscles are also its synergist in assisting in mandible depression though the lateral pterygoid is the more important muscle in this action. The suprahyoid muscles, in general, elevate the hyoid bone and are important for control of the esophagus and pharynx during swallowing and speaking.
Overwork occurs in the digastric usually due to the pressure of an overactive masseter accompanied by habitual open mouth breathing due to sinus problems or secondary to bruxism.
Digastric Origin, Insertion and Action and Synergists
Origin: Digastric Fossa of Mandible, close to symphysis of mandible.
Insertion: Both bellies united by a common intermediate tendon, attaching to hyoid bone through a fibrous loop.
Action: Elevates hyoid bone, depresses mandible to open mouth, assists retruding mandible.
Synergists: For mandible depression: Lateral pterygoid (for maximum jaw opening) and infrayoid muscles; For mandible retrusion: Posterior temporalis fibers and masseter.
Antagonists: To jaw opening: the mandible elevators including the masseter, temporalis, medial pterygoid and superior lateral pterygoid.
Origin: Mastoid Notch of Temporal Bone under attachments of longissimus capitis, splenius capitus and sternocleidomastoid.
Insertion: Same as anterior belly.
Action: Same as anterior belly.
Synergists: Same as anterior.
Antagonists: Same as anterior.
Digastric Trigger Points Causes and Symptoms
Trigger points in the digastric commonly occur secondary to trigger points in the antagonistic masseter muscle, which is the workhorse jaw muscle responsible for most of the closing force of the jaw and chewing. Problems occur in the digastic because of habitual mouth breathing, which often occurs from chronic sinus problems, nasal blockage such as from nasal polyps, or a deviated septum. This puts added stress on the digastric and the pain they cause is likely to be compounded by referred pain from the lateral pterygoid, its synergist in opening the jaw. Both the masseter and temporalis are far more powerful than the digastric and lateral pterygoid so when these muscles are overactive the mandible depressors easily become strained.
Bruxism is a major cause of problems in the masseter as well as the other jaw muscles, including the digastric.
Retrusion of the mandible can also give rise to trigger points in the digastric. Retrusion means that the lower jaw is placed in a position posterior to its normal one. This is often hereditary, due to a Class II malloclusion1. This may also occur when woodwind instruments are played as the mandible is often brought back to form the embouchure. However, it should be noted that playing woodwind and brass includes muscles of the mouth, face, jaw, and tongue and the actions of these muscles are very complex. The actions used to form the mouth into the embouchure are not natural and musicians often report a host of problems. TMJ is often blamed for these problems but may be a misdiagnoses. Some common symptoms that woodwind and brass musicians have are called “embouchure dystonia”, dystonia being a movement disorder that causes the muscles to contract and spasm involuntarily. 5, 6
Each belly of the digastric has its own referred pain patterns. The most widespread and common pain is referred from the posterior belly and this causes pain in the upper part of the sternocleidomastoid, under the chin, and upward to the occipital portion of the occipitofrontalis muscle. This pain may become apparent only after TrP's in the sternocleidomastoid are resolved. Pain may then be left in the upper part of the SCM muscle. This SCM pain that is caused by TrP's in the posterior belly of the digastric is sometimes called 'pseudo-sternocleidomastoid pain'. The superior SCM may still be tender to touch but will itself be free of trigger points or twitch responses.
The stylohyoid muscle has also been associated with head and neck pain but the digastric and stylohyoid are difficult to differentiate. Trigger points in the anterior belly sometimes refer pain the lower front teeth and together with TrP's in the mylohyoid muscle can cause throat pain and difficulty when swallowing. Associated trigger points in the masseter and temporalis are likely to occur with digastric TrP's.
Anterior Digastric Trigger Points
As stated above, trigger points in the anterior belly of the digastric refer pain to the lower front teeth. The trigger points will be located underneath the point of the chin just lateral to the midline. Massage this area freely with long even strokes.
Anterior Digastric Belly Trigger Point and Referred Pain
Posterior Digastric Trigger Points
Trigger points of the posterior belly are located under the corner of the jaw. Start with your fingers in the soft part of the flesh underneath the jaw and in front of its corner, just anterior to the upper part of the sternocleidomastoid. From here run your fingers along the front border of the SCM up toward your earlobe, feeling for the very tender spots. Sustained pressure may reproduce the referred pain symptoms.
Posterior Digastric Belly Trigger Point and Referred Pain
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This page created 05 Jan 2011 22:03
Last updated 23 Feb 2015 20:02