Full length articleReliability of joint kinematic calculations based on direct kinematic and inverse kinematic models in obese children
Introduction
Most conventional gait models use direct kinematics (DK) to calculate joint angles and their derivatives. DK calculate joint angles as Euler angles between adjacent segment reference frames. These are defined directly from the experimental markers, which are assumed to be rigidly attached to the bones [1]. DK gait models are limited to joint kinematic and kinetic analysis. In contrast to DK models, the more sophisticated musculoskeletal (MSK) models (e.g. OpenSim [2] or AnyBody [3]) use inverse kinematics (IK), also known as global optimization, to calculate joint angles. MSK models have the main advantage of enabling additional analyses such as muscle-tendon length and force estimation [4], induced acceleration analysis [5], and joint contact force calculations [6]. These analyses may help to identify causes for secondary pathologies in obese populations (e.g. joint degeneration) and increase our understanding of gait in people with obesity by adding information about gait strategies at the musculoskeletal level.
If clinicians or scientists use either kind of these models to aid decision-making in clinical practice, reliability is an essential issue. In order to serve as a valuable tool, e.g. to evaluate therapy progress, one has to know if an obtained result reflects a true difference or if this difference falls into the variability of the underlying measurement technique [7]. The reliability of DK models has been assessed extensively over the last years. Research certifies moderate to good reliability in lean adults, typically developing children, children with cerebral palsy, stroke patients as well as acceptable reliability in obese children [8,9]. The reliability of IK models has only been assessed in a limited number of studies including healthy adult participants, lean typically developing children, and children with cerebral palsy [[10], [11], [12]]. So far, no study has assessed the reliability of IK models to compute joint kinematics in obese children, even though MSK-IK models have already been used in this population to calculate muscle forces and joint-contact forces [[13], [14], [15]].
During the entire workflow of three-dimensional (3D) gait analysis, regardless of using IK or DK approaches, the same pitfalls exist in identifying anatomical landmarks, in placing markers accurately and reliably, as well as problems associated with soft-tissue displacement [16]. In an IK approach, an additional processing step is necessary, where the pose and marker positions of a generic model is adjusted to attain the best match with the experimental markers [17,18]. The quality of the results during this process is highly dependent on the experience of the examiner. This process gets even more challenging in an obese population due to large soft-tissue offsets. As a consequence, this step may add additional variability to the results. Therefore, it is of utmost importance to assess the reliability of IK models in estimating joint kinematics in obese children to build up confidence in the MSK simulation results for overweight and obese populations.
Accordingly, the primary aim of this study was to assess the reliability of two different IK models in obese children and to compare their reliability to the reliability of a DK model. A secondary aim was to identify if there are any differences in joint kinematics between the IK models and the DK approach. Based on the findings of previous IK and DK reliability studies in lean participants [8,[10], [11], [12]], we hypothesized that the reliability of gait kinematics in obese children will be similar between the IK and DK models. Based on earlier published data [19], we also hypothesized that there will be clinically relevant differences in joint kinematics between the IK and the DK models.
Section snippets
Participants
This study used the data from an earlier study [9]. The data comprised of a convenience sample of two females and eight males with an age-based body mass index (BMI) above the 97th percentile (mean ± SD, age: 14.6 ± 2.8 years, height: 169.3 ± 11.3 cm, body mass: 99.2 ± 21.7 kg; BMI: 34.2 ± 3.9 kg/m2). Exclusion criteria were the existence of any syndromes associated with obesity (e.g. Prader-Willi syndrome), chronic joint disease, neuro-motor disease, or any history of a lower extremity joint
Results
Reliability was similar between the DK and both IK models. The Friedman test did not indicate significant differences in the SEM (χ2 (2) = 1.742, p = 0.419) and RMSDp (χ2 (2) = 1.826, p = 0.401) parameters between the analyzed models. The SEM, averaged across the min, max and ROM, was below 3° (range: 1.6–3.0°) for all analyzed models in the sagittal and frontal plane and below 5° for the transversal plane (2.1–4.9°). Results for the RMSDp and RMSDw showed a similar pattern for all three
Discussion
The primary aim of this study was to evaluate the test-retest reliability of two different IK models and to compare their results to a DK approach in a sample of young obese children and adolescents.
All three models were found to display a similar and clinically acceptable level of reliability. In detail, the DK approach yielded overall SEM values of less than 3° in the sagittal and frontal planes, and approximately 4° in the transversal plane. The RMSDp and RMSDw showed comparable, but higher
Conclusion
Our findings showed no differences in the reliability in joint kinematics between our analyzed IK and DK models. Reliability values were clinically acceptable and therefore support the careful use of MSK IK models in overweight or obese populations. Future MSK studies can use our obtained reliability results to judge if an observed difference is caused by a real difference, e.g. due to an intervention, or if an observed difference falls within the error margins of a test-retest scenario.
Conflicts of interest
There is no conflict of interest to declare.
Acknowledgements
This work was supported by the NFB−Lower Austrian Research and Education Company and the Provincial Government of Lower Austria, Department of Science and Research (grant number LSC13-009). Dr Hans Kainz was funded by a H2020-MSCA individual fellowship. We would like to thank David Artner, MSc. for his support during data recording and Prof. Dr. Susanne Greber-Platzer as well as DDr. Alexandra Kreissl for their great assistance in participant screening and recruitment.
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