Misalignment between centre of pressure and musculoskeletal model after inverse kinematics affects lower limb joint kinetics and a method for adjustment.

INTRODUCTION: Musculoskeletal modelling uses external loads, ground reaction forces (GRF) and their centres of pressure (COP), in combination with kinematics to resolve net joint forces, torques, muscle forces and internal joint reaction loads. Global optimization procedures used to calculate kinematics from marker trajectory data can introduce spatial misalignment between the model and the external loads, which commonly remain largely unaltered following data collection.

METHODS: This study quantified the misalignment between the location of the external loads and the kinematics by calculating the position of the ankle joint centre with respect to the COP in the raw data and then following inverse kinematics. The effect of this misalignment was analysed on lower limb sagittal and coronal plane joint moments by comparing inverse dynamics results before and after adjustment of the COP based on the misalignment of the ankle joint centre position.

RESULTS: Compared to the raw data, the ankle joint centre had a root mean squared displacement of 19 mm in the transverse plane during stance phase following inverse kinematics. After correcting the COP using the ankle joint centre, significant increases in knee flexion impulse (32%, p = 0.023) as well as hip and knee adduction impulse (33%, p = 0.003 and 47%, p = 0.009) were found.

DISCUSSION: Inverse kinematics can introduce spatial misalignment between the external loads and the model. The present findings suggest that it may be necessary to adjust the COP to reduce this misalignment and thereby improve model estimations of joint kinetics.