A good friend of mine from Canada, Dr. Sahag Mahseredjian raised this question in an e-mail. I have edited and paraphrased the information below. Many professional vocalists impose very heavy demands on the mandible (lower jaw). When a singer performs vocal exercises for many hours, the jaw movements border upon the extremes of its physiologic range. Lower jaw motion is energized by the muscles that link the mandible above to the bones of the face and cranium, and below to the neck, including the hyoid bone, clavicle (collar bone) and sternum (breast bone).
Healthy mandibular movement for effective vocalization requires a neurophysiologic integration of all of the muscles and bones involved in carrying out this function. If the jaw is not in NM position, head & neck muscles are usually in a hypertonic (tight) state. This may affect their performance.
A common condition affecting professional vocalists is cranio-mandibular-cervical muscular dysfunction resulting from sprain of the muscles connecting the mandible to the head and neck. This dysfunction adversely affects the quality of the voice and also may cause head, facial and ear pains. The major muscles involved in voice production are strongly influenced by mandibular position. So, a craniomandibular-cervical muscular dysfunction can result in hoarseness and chronic voice irritation which interferes with the ability of the vocalist to produce sounds of good quality and high amplitude.
An explanation of the anatomy of this area is below:
The trachea is the main trunk of a system of tubes by which air passes to and from the lungs. The respiratory system which includes the lungs functions to inhale and exhale air through the trachea.
The larynx is the modified upper section of the trachea and contains the vocal cords. These include a pair of vocal folds that when drawn taut and subjected to a flow of breath, then vibrate to produce the sounds of the voice. In the context of the human voice, resonance refers to the quality imparted to sound vibrations originating in the larynx by resonator chambers formed by the oral and nasal cavities. The power or amplitude of the voice depends on respiration. Hence the breath control training for singers that emphasizes deep diaphragmatic breathing.
The larynx is composed of four principal cartilages: the thyroid, the cricoid, the arytenoid and the epiglottis. These cartilages are controlled by three primary groups of muscles. When contracted, the cricothyroid muscle brings the cricoid and thyroid cartilages together, thereby stretching the vocal cords to control the pitch of the vibrations produced thereby. The tension on the vocal cords is primarily controlled by the thyro-arytenoid muscle, while three sets of arytenoid muscles are involved with vocal cord adduction.
The proper balance of the muscles of the larynx is essential to effective vocalization. The laryngeal mechanism is a precision instrument requiring critical muscular tensions and cartilage positions to produce optimum sounds.
The larynx is suspended from the hyoid bone disposed in the throat between the thyroid cartilage and the roof of the tongue. The hyoid bone is linked by muscles to the mandible or lower jaw. The tension on these muscles is a function of the position of the mandible, and has a strong influence on the behavior of the larynx.
A Neuromuscular Orthotic exploits the relationship between mandible position and voice production to significantly improve the ability of a vocalist to produce sounds of fine quality and high amplitude.