Validation of smoothness evaluation during standing and sitting motions using an accelerometer.

[Purpose] To quantitatively evaluate smoothness during standing and sitting motion analysis using an accelerometer and to clarify the relationship between indices. [Participants and Methods] Seventeen healthy males participated in this study. We attached a 9-axis motion sensor to the spinous process of the third lumbar spine and measured the acceleration of standing and sitting motions under normal and unstable conditions. We estimated and compared the root mean square and entropy in the lateral, vertical, longitudinal, and triaxial composite directions. [Results] On comparing both conditions, the unstable condition indices were significantly high, except for the lateral direction of entropy. The root mean square was significantly negatively correlated with entropy under normal conditions. [Conclusion] The study results suggested that the acceleration index quantitatively evaluates motion smoothness. Since each index had different characteristics, the motion-specific index was observed to be significant.