The muscle can be tender to touch and can refer to the upper teeth, over the eyebrow and on the side of the head and sometimes to the temporomandibular joint (TMJ).
It is responsible for closing the jaw (when both contract), moving the jaw from side to side (when one contracts) and a little bit of retraction (pulling the jaw backwards)
It is often affected by excessive gum chewing, jaw clenching, trauma to the muscle and head position.
In my practise I often use active release technique (ART) or acupuncture to specifically treat this muscle in addition to addressing the causes of this muscle being over worked
To perform your own self treatment of this muscle (self myofascial release) take your thumb or a ball (the smaller the ball the more focused the pressure can be). Pin the tender point down. You don’t need to crush the muscle to do this. Mild to moderate discomfort is fine. Since this muscle fans out from its insertion, once you have pinned the muscle out direct the ball/thumb in different direction.
To add a stretch to it simply open up your mouth. If you deviate your jaw to the other side you will add a little more stretch the the muscle.