Implications of using hierarchical and six degree-of-freedom models for normal gait analyses

Hierarchical biomechanical models (conventional gait model, CGM) are attractive because of simple data collection demands, yet they are susceptible to errors that are theoretically better controlled using six degree-of-freedom models that track body segments independently (OPT1). We wished to compare gait variables obtained with these models. Twenty-five normal children walked while wearing a hybrid marker configuration, permitting identical strides to be analyzed using CGM and OPT1. Kinematics and ground reaction forces were obtained using a common motion capture system. CGM and OPT1 were implemented in Visual3D software, where inverse dynamics provided 20 clinically relevant gait variables (joint angles, moments and powers). These were compared between models using dependent t-tests (Bonferroni-adjusted alpha of 0.0025), and ensemble averages. We hypothesized that OPT1 would provide data similar to CGM in the sagittal plane, and different from CGM in coronal and transverse planes. Six variables were significantly different in the sagittal plane, suggesting that CGM produced a more extended lower extremity; this was explained by a posterior bias to the lateral knee marker during knee flexion, as a result of skin movement artifact. No significant differences were found in coronal plane variables. Four variables were significantly different in the transverse plane. Ensemble averages were comparable between models. For normal children, biomechanical interpretations based upon these tested variables are unlikely to change due to independent segment tracking alone (CGM vs. OPT1). Additional differences may appear due to pathology, and when segment reference frames are changed from those used in CGM to reflect individual anatomy.