We authors have attentions to the relation between the stomatognathic system and the systemic condition. And then we have had a strong belief in that relation. As a part of these studies, we lately studied the Influence of different sized of the occlusion supportive area on head muscular strength at the time of head anteflexion and some muscles activities using the Head-Neck-Shoulder Muscles analyzing system and electromyography analyzing system on 5 healthy athletes. And the following results were obtained.
　1. The maximum muscular strength was recorded when using most wide supportive area splint (7+7) in all subject and tended to decrease with the lose of posterior supportive area.
　2. The maximum muscular activity of the masseter and anterior tempolarlis were also recorded when using most wide supportive area splint (7+7) in all subject and tended to decrease as the outcome of a lose of posterior supportive area. And sternomastoid and tripedius had the same tendency,but it was only a very small difference compared with masticatory muscles.
　These results were much the same with back strength (Ohiwa). And they were considered as follows ; At first the lose of posterior supportive area occur the occlusal disharmony,and then the direction of occlusal force was any longer united that of masseter main muscle of occlusal force. So as to keep out of the pain in temporomandibular joint and periodontal ligament. These pains came into existence while strong clenching without enough posterior supporting area. Then above mentioned causes and so on had an effect on occlusal force so called clenching force and decrease it's performance.
　And these decreasing occlusal force affected head neck shoulder muscular strength at the time of head anteflexion in some relations between Stomatognathic System and cervical region. Such as 1 ; While clenching, the canalizatioin in H−reflex which is an indiction of motor nerve loop, is remarkable, and that is proportionate to the strength of clenching. 2 ; When the excitation in the motor area of mastication is activated strongly by clenching, it expands into other motor areas and actives the cells which cbncern skeletal muscle of whole body in medullary reticular formation.
　3. The central incidence from mesencephalic tract of trigeminal nerve in which the neurons from masticatory muscles has the path which enters the anterior horns of first and second cervical vertebral as well as the path which enters motor nucleus of trigeminal nerve. 4 ; Existence of reflex system between trigeminal nerve and neck muscles as Trigemino−Neck reflex.
　Consequently, when using mouthprotector without enough posterior supportive area and oral condition is badly in need for dental treatment like Prosthodontics, no muscle can work certainly. So a point of view safety and performonce enough posterior supportive area would be required. Thus wearing proper mouth protector and right oral health condition are able to be used for head−neck harm ptotection. And we dentists are in the hope of using the mouthprotector more widely.

DESCENTE SPORTS SCIENCE Vol.18/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE