New estimation model of the initial lower limb angle to improve angle estimation during the extension phase of standing-up movement

[Purpose] An estimation model of the knee and ankle joint angles during the extension phase was proposed in the previous study. However, it had limited use because of the fixed initial lower limb angle before standing up. This study aimed to propose a new estimation model of the initial lower limb angle to improve the angle estimation during extension phase. [Subjects and Methods] Seven healthy male volunteers were enrolled. The new estimation model approximated the initial lower limb angle using a force sensor plate that measured the plantar pressure of the subjects. The estimated angle and force were compared to those obtained by a motion capture system and force plate. [Results] The new estimation model of initial lower limb angle showed no significant difference compared with the true values obtained by motion capture, except for the subject who had a greater foot-pressure measurement error compared with the force plate measurement, with maximum errors of 5.98° and 6.31°, respectively. [Conclusion] The proposed model in this study can estimate the initial lower limb angle before standing and can be applied to the angle estimation model during the extension phase of the standing-up movement.