Centroid position estimating method for observational analysis

[Purpose] This study aimed to develop a clinical observation method to evaluate the position of the mass center. From the human visual capability, we considered it would be practical to divide the body into two parts: the upper and the lower body mass. If we could identify their optimal position, we could observe the middle point in between as the center of mass. [Participants and Methods] Twenty healthy males performed forward bending, backward bending, squatting, and walking. The three-dimensional coordinates were analyzed using a conventional model. In addition, five "virtual" markers were assigned as upper and lower mass, respectively. The midpoints of each five virtual marker combinations defined the mass centers, providing 25 coordinates. We calculated the difference in the coordinates between mass centers from virtual markers and mass centers using a conventional model. The combination with the slightest error was evaluated to determine the 95% confidence interval of the observed points and whether the value was clinically beneficial. [Results] The optimal combination of the upper and lower mass was Th8 and in the middle of both hip and knee centers. [Conclusion] The overall magnitude of error was about 30 mm and enough to evaluate the center of mass with macroscopy.