Occlusion, Sternocleidomastoid Muscle Activity, and Body Sway: A Pilot Study in Male Astronauts

Abstract

The modifications induced by microgravity on the coordinated patterns of movement of the head, trunk, and limbs are reported on extensively. However, apparently there is little data on the masticatory muscles. In normal gravitational conditions, information from the neck and stomatognathic apparatus play a role in maintaining the body’s balance and equilibrium. The current pilot study used normal gravity conditions to investigate the hypothesis of a functional coupling between occlusion and neck muscles and body postural oscillations. The immediate effect of modified occlusal surfaces on the contraction pattern of the sternocleidomastoid muscles during maximum voluntary clenching and on the oscillation of the center of foot pressure was analyzed in 11 male astronauts (aged 31-54 yrs). All subjects were healthy and free from pathologies of the neck and stomatognathic apparatus. Occlusal splints were prepared using impressions of their dental arches. The splints were modeled on the mandibular arch, had only posterior contacts, and were modified to obtain a more symmetric, standardized contraction of the masseter and temporalis muscles during teeth clenching. Surface EMG activity of the sternocleidomastoid muscles was recorded during a maximal voluntary clench with and without the splint. Sternocleidomastoid potentials were standardized as percent of the mean potentials recorded during a maximum contralateral rotation of the head, and the symmetry of the EMG waves of left- and right-side muscles was measured. Body sway was assessed with and without the splint, either with eyes open or closed. The variations of the center of foot pressure were analyzed through bivariate analysis, and the area of the 90% standard ellipse was computed. Within each visual condition (eyes open or closed), the difference between the areas of oscillation measured with and without the splint was computed. Muscular activity was more symmetric with the splint. The area of oscillation of the center of foot pressure was larger without the splint than with the splint, both with eyes open and eyes closed. The modifications, induced by the occlusal splint in the sternocleidomastoid muscles’ symmetry, and center of foot pressure differential area with closed eyes, were significantly related (p<0.05): the larger the increment in muscular symmetry, the smaller the area of oscillation with the splint as compared to without the splint. A functionally more symmetric maxillo-mandibular position resulted in a more symmetric sternocleidomastoid muscle contraction pattern and less body sway. Modifications in the contraction of the masticatory muscles may therefore affect the whole body.

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Notes on contributors

Chiarella Sforza

Dr. Chiarella Sforza received her M.D. degree from the University of Milan School of Medicine, Italy in 1986, and her Ph.D. degree in sports medicine in 1989 from the same university. She is a professor of human anatomy at the School of Sport Sciences of Milan University, and director of the Laboratory of Functional Anatomy of the Locomotor Apparatus, Department of Human Morphology, University of Milan, Italy. Dr. Sforza is involved in research on the functional anatomy of the locomotor apparatus and on the relationship between the stomatognathic apparatus and other body areas.

Gianluca M. Tartaglia

Dr. Gianluca M. Tartaglia received his D.D.S. degree from the University of Milan School of Dentistry, Italy in 1991, and his Ph.D. in morphological sciences from the same university in 1996. He is currently a staff member of the Laboratory of Functional Anatomy of the Stomatognathic Apparatus.

Umberto Solimene

Dr. Umberto Solimene is director of the Center for Research in Bioclimatology Medical, Biotechnology and Natural Medicine, Department of Human Morphology, Faculty of Medicine, University of Milan, Italy. The Center is a Collaborative Center for Traditional Medicine of the World Health Organization (WHO), and it cooperates with the Y. Gagarin Center for Astronaut Training for the study of oxidative cell stress and astronaut rehabilitation after missions.

Valery Morgun

Dr. Valery Morgun is the director of the medical staff at the Russian Y. Gagarin Center for Astronaut Training, Star City, Moscow Region, Russia.

Rustem R. Kaspranskiy

Dr. Rustem R. Kaspranskiy is assistant director of the medical staff at the Russian Y. Gagarin Center for Astronaut Training, Star City, Moscow Region, Russia.

Virgilio F. Ferrario

Dr. Virgilio F. Ferrario received his Ph.D. degree in bioengineering from the Polictecnico, Milan, Italy in 1969. He received his M.D. degree from the University of Milan School of Medicine, Italy in 1980. He is a professor of human anatomy at the Dental School of Milan University, and director of the Laboratory of Functional Anatomy of the Stomatognathic Apparatus, Department of Human Morphology, Faculty of Medicine, University of Milan, Italy. Dr. Ferrario has been involved in research on the anatomical bases of stomatognathic function and malfunction.