Hints and pitfalls for estimating muscle-tendon lengths based on joint angles from the conventional gait model

Muscle-tendon length obtained from musculoskeletal simulations can improve clinical decision-making in children with cerebral palsy [1]. The standard workflow to estimate muscle-tendon length includes scaling a generic model, calculating joint angles via inverse kinematics and finally extracting muscle-tendon lengths [2]. Scaling includes uncertainties and errors [3], which affect joint angles and all further simulation results [4]. Alternatively, the joint angles from the conventional gait model (CGM) can be used to drive a musculoskeletal model without the need of scaling the musculoskeletal model [5]. However, the alternative approach potentially introduces inconsistencies between joint angles and muscle-tendon lengths due to the difference in segment reference frames and joint degrees-of-freedom between the CGM and most musculoskeletal models [6].