Centroid position estimating method for observational analysis

Abstract

[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.

Keywords: Center of gravity; Center of mass; Motion analysis.

Fig. 1.

Virtual markers. A. U1 was defined as the midpoint between the C7 spinous process and the center of the clavicles. U5 was defined as the midpoint between the xiphoid process and the Th10 spinous process. U2, U3, and U4 were on the line connecting U1 and U5, at 25%, 50%, and 75% of the distance from U1, respectively. B. L1 was defined as the midpoint of both hip joint centers. L5 was defined as the midpoint between both knee joint centers. L2, L3, and L4 were on the line connecting L1 and L5, at 25%, 50%, and 75% of the distance from L1, respectively. C. The COMs derived from UM and LM were defined as the midpoints of combinations of each of 5 coordinates as 25 coordinates. (Green circle)[U1–L1, U1–L2, U1–L3 • • • • • • • • • U5–L5].

Fig. 2.

Observational points. A. Left: Frontal plane, Right: sagittal plane. The UM was located at the height of Th6 (Th4–Th8) and the lower mass was at the middle point of both hip and knee joint centers or a little below that. Total COM was considered in the center of the UM and the LM. B. UM, LM, COG, and COP estimation during forward bending. Anterior-posterior distance of the UM and the LM of the left is larger than that of the right figure, suggesting the posterior side of the lumbar, pelvis, and hip joint in B is stretched larger than A. C. Squat and moment pattern (exhibited as internal). Upper part; The UM located posterior relative to the LM, knee extension moment was larger than hip extension moment. Lower part; The UM located anterior relative to the LM, hip extension moment was larger than hip extension moment. Each picture is represented at the moment of the dotted longitudinal line on the graph. D. Comparison hip and knee joint moment at the initial phase of gait (exhibited as internal). These graphs were typically detected examples of hip and knee extension moments. Each picture is represented at the dotted longitudinal line on the graph. Upper part; The UM located anterior relative to the LM, knee extension moment was small. Lower part; The UM located posterior relative to LM, knee extension moment was larger than the upper part. The maximum hip extension moment of the upper part was relatively higher than the lower part, but the maximum knee extension moment of the upper part was lower than the lower part. UM: upper mass; LM: lower mass; COM: center of mass; COP; center of pressure.