Study On The Electromyographic Activity Of Sternocleidomastoid Muscles During Chewing And Centric Maximal Voluntary Clenching

One of main function of sternocleidomastoid muscle(SCM)contributes to the movement of neck and head. Stomatognathic system muscular chain is made up of cervical and masticatory muscles in different layers. Whether there is a functional relationship between the sternocleidomastoid muscle and the mandibular functional movement such as chewing remains unclear.Whether there is a coordination of sternocleidomastoid muscle activity and that of the masseter muscles(MM) in the different level of bite force and chewing movement are also a question. Considering that occlusion is closely related to activity of MM,it is then reasonable to predict that occlusion may take a role in cervical spinal disorder(CSD) if there is a link between MM and SCM. So that currently, we firstly combined T-Scan III system and Bio EMG III system and used them to record the surface electromyographic(SEMG)activity of SCM and MM simultaneously for 25 healthy young volunteers. The recordings of normalized SEMG value,which is ratio of the SEMG value of a given bite force to that from the maximal voluntary clenching(MVC)bite force,was analyzed in the different level of bite force with the respect to MM. Analysis was carried out to exhibit changes to show relationship between SCM SEMG activity and level of bite force during MVC in ICP. We secondly synchronously recorded with the Bio EMG III system and Electrognathography(EGN) for the bilateral SCM and MM SEMG activity and traces of mandibular movements of 20 healthy young volunteers during their unilateral chewing. The time locations of SCM and MM SEMG activity peaks were analyzed and compared in different muscles.Results: 1, There is no difference between left side and right side SEMG activity of SCM and MM at any bite force level we detected during MVC in ICP. The ratios of the SEMG value of SCM to that of MM range from 9.13±4.77% to 10.23±6.47%. Normalized SEMG value of both SCM and MM increased from 0%-MVC to 75%-MVC,but declines from 75%-MVC to MVC-ICP(P < 0.05).2, During unilateral chewing, bilateral MM and SCM reached their SEMG peak values before reaching to terminal closing position(TCP).The balancing side SCM activity reached its peak level, although the energy levels were very low,6.95±3.48%~7.78±4.57% of the MM SEMG activity during chewing, earlier than the peak activity of working side SCM and the balancing and working sides of MM(P < 0.05). There was no significant time difference between the working side SCM peak activity and that of the balancing and working sides MM(P > 0.05).Conclusion: 1. SCM is engaged in centric maximal voluntary clenching and chewing movement, and shows a simultaneous collaborated activity with that of MM. However, asymmetric movement, such as chewing, SEMG activities of MM and SCM are not in same pace.2. Balancing side SCM, which is active earlier, may be functioned as a stabilizer in keeping normal neck posture to support stable structure which exerts bite force in unilateral chewing.