Conjugate momentum estimate using non-linear dynamic model of the sit-to-stand correlates well with accelerometric surface data

The purpose of this study was the development of a non-linear double inverted constrained pendulum model for the analysis of the movement of sit-to-stand (STS) transition. Ten able-bodied subjects perform five trials in their natural speed. Kinematics, kinetics as well as body worn accelerometer data were collected during the STS task using optoelectronic motion capture, force plate and inertial measurement unit, respectively. The conjugate momentum for the whole body which includes linear and angular motion correlates well with the accelerometric surface spanned by the accelerometer data. The partitioning of the conjugate momentum indicates a clear coordination between upper and lower limb after seat-off period. Moreover, the normalization procedure indicates a clear minimal and somehow invariant threshold value of the conjugate momentum to approximately 0.3 (body mass×body length) to perform the sit-to-stand for able-bodied subject. This threshold correlates well with the data obtained from accelerometeric index. The proposed accelerometric index is relevant to assess STS performance and to detect failed STS in clinics and outside a laboratory for patients with reduced mobility.