Mandibular Jaw Movement and Masticatory Muscle Activity during Dynamic Trunk Exercise.

The examination of jaw movement during exercise is essential for an improved understanding of jaw function. Currently, there is no unified view of the mechanism by which the mandible is fixed during physical exercise. We hypothesized that during strong skeletal muscle force exertion in dynamic exercises, the mandible is displaced to a position other than the maximal intercuspal position and that mouth-opening and mouth-closing muscles simultaneously contract to fix the displaced mandible. Therefore, we simultaneously recorded mandibular jaw movements and masticatory muscle activities during dynamic trunk muscle force exertion (deadlift exercise) in 24 healthy adult males (age, 27.3 ± 2.58 years). The deadlift was divided into three steps: Ready (reference), Pull, and Down. During Pull, the mandibular incisal point moved significantly posteriorly (-0.24 mm, p = 0.023) and inferiorly (-0.55 mm, p = 0.019) from the maximal intercuspal position. Additionally, temporal, masseter, and digastric muscles were activated simultaneously and significantly during Pull (18.63 ± 17.13%, 21.21 ± 18.73%, 21.82 ± 19.97% of the maximum voluntary contraction, respectively), with maintained activities during Down (p < 0.001). Thus, during dynamic trunk muscle force exertion, the mandibular incisal point moved to a posteroinferior position without tooth-touch (an open-mouth position). Simultaneously, the activities of the mouth-opening digastric muscles and the mouth-closing temporal and masseter muscles led to mandibular fixation, which is a type of mandible fixing called bracing.

Rheological properties of elastomeric impression materials for selective pressure impression technique.

PURPOSE: The selective impression technique is recommended for removable prosthodontics. However, the viscoelastic rheological properties of impression materials, particularly the time-dependent viscoelastic changes in impression material immediately after tray seating, are unknown. The objective of this study is to clarify the viscoelastic rheological properties of impression materials by focusing on tray seating. METHODS: In this study, eight types of polyvinylsiloxane (PVS), two types of polyether, and two types of alginate impression materials were used. The storage modulus (G'; degree of stiffness) and loss tangent (tanδ; degree of hardening) were determined as functions of time from 0 to 360 s, commencing immediately after the completion of mixing, using a stress control-type rheometer. Thus, G' and tanδ at 0s and 20 s were compared. RESULTS: Stiffness was found to be widely distributed (4.49-0.26 × 10(4)Pa) among PVS-types, even immediately after mixing the impression material. There was also variation among polyethers (1.55-0.5 × 10(4)Pa) and among alginates (0.64-0.21 × 10(4)Pa). The hardening of all impression materials progresses beyond 20 s after the completion of mixing. CONCLUSIONS: The G' values varied with each impression material, even immediately after mixing, and the accurate impression-taking time was determined from the results of tanδ. These results provide unique insight into the selective impression technique.