Abstract
Joint kinematics can be calculated by Direct Kinematics (DK), which is used in most clinical gait laboratories, or Inverse Kinematics (IK), which is mainly used for musculoskeletal research. In both approaches, joint centre locations are required to compute joint angles. The hip joint centre (HJC) in DK models can be estimated using predictive or functional methods, while in IK models can be obtained by scaling generic models. The aim of the current study was to systematically investigate the impact of HJC location errors on lower limb joint kinematics of a clinical population using DK and IK approaches. Subject-specific kinematic models of eight children with cerebral palsy were built from magnetic resonance images and used as reference models. HJC was then perturbed in 6mm steps within a 60mm cubic grid, and kinematic waveforms were calculated for the reference and perturbed models. HJC perturbations affected only hip and knee joint kinematics in a DK framework, but all joint angles were affected when using IK. In the DK model, joint constraints increased the sensitivity of joint range-of-motion to HJC location errors. Mean joint angle offsets larger than 5° were observed for both approaches (DK and IK), which were larger than previously reported for healthy adults. In the absence of medical images to identify the HJC, predictive or functional methods with small errors in anterior-posterior and medial-lateral directions and scaling procedures minimizing HJC location errors in the anterior-posterior direction should be chosen to minimize the impact on joint kinematics.
Highlights
In children with cerebral palsy (CP), three-dimensional gait analysis is used for treatment planning and evaluating the outcome of an intervention [1]
The changes in joint ROM and mean joint angles caused by hip joint centre (HJC) location errors were slightly smaller in the 3-1-1-degrees of freedom (DoF)-Inverse Kinematics (IK) than in the 3-3-3-DoF-IK models (Fig. 3)
HJC perturbations in anterior-posterior and superior-inferior directions were responsible for significant differences between the ROM of hip flexion-extension and ab-adduction angles and all three rotations of the knee joint (Fig. 4). This is the first study to evaluate the impact of HJC location errors on lower limb joint kinematics in children with CP when using Direct Kinematics (DK) and IK approaches
Summary
In children with cerebral palsy (CP), three-dimensional gait analysis is used for treatment planning and evaluating the outcome of an intervention [1]. Joint kinematics are calculated by the conventional gait model as Cardan angles describing the relative pose of adjacent anatomical segment reference systems, which are defined from the experimental markers’ positions and a minimum set of anatomical measurements (Direct Kinematics, DK) [4,5]. Musculoskeletal software such as AnyBody [6] and OpenSim [7] use an Inverse. Musculoskeletal software may provide valuable additional information on the causes of gait abnormalities and improve clinical-decision making because it allows additional analyses such as musculotendon length estimation [10,11], joint contact force calculations [12,13] and induced acceleration analysis [14]
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