Relationship between head position and the muscle hardness of the masseter and trapezius muscles: a pilot study.

The purpose of this study was to examine the hardness of the masseter and trapezius muscles at various head positions and to explore the relationship of these two muscles to each other in terms of their respective levels of muscle hardness at different head positions. Twenty-two asymptomatic male subjects participated in this study. Using a hand-held hardness meter, muscle hardness was first measured in a relaxed position as a baseline. The subjects were then asked to assume five deviated head positions, and the muscle hardness was measured again. The data obtained at each deviated head position were compared to those at baseline. In addition, the subjects were asked to maintain a five minute sustained anterior flexion of the head, and muscle hardness was also measured and compared to the baseline. As a result, there was a significant increase in muscle hardness at the point of the whole trapezius with 30-degree anterior flexion, while there was a significant decrease at the point of the right masseter. A significant increase in hardness was seen in the upper trapezius muscle in conjunction with right side bending and in the right upper trapezius muscle on left side bending. Also, a significant decrease in hardness was observed in the right masseter with right side bending. With reference to the axial rotation, there was a significant increase in hardness in the upper right trapezius muscle upon right axial rotation. Moreover, there was a significant increase in the muscle hardness in the whole trapezius after the five minute anteriorly sustained head task. This study provides evidence that deviated head positions lead to an increase in hardness of the trapezius muscle. The data also revealed the simultaneous occurrence of the elevation of muscle hardness in the upper trapezius muscle and the decrease in muscle hardness in the masseter muscle associated with right side bending and anterior flexion.