## 2 Materials and Methods
### Measurement Protocol  
Calibration of the optical infrared camera-based motion capturing systems at both laboratories were performed according to the manufacturer's guidelines. This calibration process consisted of two main steps: (_i_) a static calibration to calculate the origin of the capture volume and define the 3D workspace orientation (x, y, z directions) and (_ii_) a dynamic calibration to calculate the relative position and orientation of each camera within the capture volume. Calibration quality was checked according to the manufacturer's guidelines.  
The PlugInGait marker setup for the lower extremity (kinematic model V 2.3) was used in both laboratories, based on the work of [PERSON] et al. (2019). Markers were attached to each subject by the same experienced staff member (Observer 1) at both gait analysis laboratories, according to a standardized protocol for anthropometric measurements, landmark identification and marker mounting.  
Each of the three subjects performed 20 barefoot gait trials with a self-selected walking speed resulting in a total of 60 CGA trials per gait laboratory. Individual gait speed was controlled and standardized between data collection sessions. The same two staff members (Observer 1 and Observer 2) performed data processing and analysis at both laboratories, using a standardized protocol for data processing, labeling and gait event detection. For each reliability analysis variation, the observer started with the original raw data in their data processing routine. These raw data were labeled, gait events were detected and possible gaps of reconstructed marker trajectories were filled. The kinematic parameters were extracted using the same template and again used a standardized workflow for both observers. Kinematic data used for reliability analysis, which consisted of specific movement parameters in the sagittal and frontal plane for the hip, knee and ankle. Parameters were selected according to the specifications of [PERSON] et al. (2019) (Figure 2).  
Data from the right leg of each subject were used for the analyses of reliability. Three different measures of reliability were performed (Figure 1):  
* Interobserver Reliability: The reliability of the two observers, using the same data set, was assessed using separate analysis software (Observer 1: GL1 (Workstation); Observer 2: GL2 (Nexus)).
* Intraobserver Reliability: The reliability of the same data set among the same observer was tested. This assessment was performed with a time interval of 12 months between analyses using the same software (Workstation). The CGA data for this assessment were collected at the GL2 site.
* Between-Laboratory Intraobserver Reliability: To compare the effect of laboratory environment and instrumentation while excluding observer-dependent influences, CGA data collected at both laboratories were analyzed by a single observer using the same analysis software.  
The following reliability variables were assessed. ICC\({}_{\text{mean}}\) was the average of all parameters for hip, knee and ankle. ICC\({}_{\text{total}}\) (Equation (1)) was the average of hip ICC\({}_{\text{mean}}\), knee ICC\({}_{\text{mean}}\) and ankle ICC\({}_{\text{mean}}\) for all three types of reliability (intraobserver, interobserver, between-laboratory intraobserver).  
ICC\({}_{\text{total}}\) = hip ICC\({}_{\text{mean}}\) + knee ICC\({}_{\text{mean}}\) + ankle ICC\({}_{\text{mean}}\) / 3 (1)