# Test-Retest Reliability of a 6 DoF Marker Set for Gait Analysis in Cerebral Palsy Children
## 2 Materials and Methods
### Data Processing  
Gait cycles were extracted using Qualysis Track Manager (QTM) (v2020.3 build 6020, Qualisys AB, Gothenburg, Sweden). The subsequent analysis and processing were done using Visual 3D software (Professional Version v4.80.00, C-Motion, Inc., Rockville, MD, USA). The marker set (Figure 1) that was used followed the calibrated anatomical system protocol (CAST) [30; 35] and CODA pelvis [36]. It was used to reconstruct the pelvis and both lower limbs [34]. The 22 individual markers and four marker clusters of four embedded markers each, allowed the reconstruction of seven body segments: feet, shanks, thighs, and pelvis. Each segment is considered to be independent and to have six degrees of freedom [37]. Lower limb segment masses were determined according to Dempster [38] while the remaining inertial parameters were computed based on Hanavan [39]. The pelvic anatomical coordinate system was defined by surface markers placed on the right and left anterior superior iliac spines (ASIS) and on the right and left posterior superior iliac spines (PSIS) and can be described as the origin at the midpoint between the right ASIS and the left ASIS; the Z-axis points from the origin to the right ASIS; the X-axis lies in the plane defined by the right ASIS, left ASIS, and the midpoint of the right PSIS and left PSIS markers and points ventrally orthogonal to the Z-axis; and the Y-axis is orthogonal to the previous two [41]. The hip joint centers were computed using the pelvis markers, according to [PERSON]'s regression equations [36]. Anatomical reference frames of the lower limb segments were defined in accordance with the International Society of Biomechanics (ISB) recommendations to the standard description of joint kinematics [41]. The thigh anatomical coordinate system was defined by the hip joint centers previously computed using the pelvis markers and the lateral and medial femur condyles; the origin was the hip joint center; the Z-axis points from the midpoint between the lateral and medial femur condyles and the origin; the Y-axis is perpendicular to the Z-axis and the frontal plane of the thigh (defined by an axis between the lateral and medial femur condyles and the hip joint center); the X-axis is orthogonal to the previous two. The bank anatomical coordinate system was defined by the femur condyles and malleolli markers; the origin was the knee joint center defined as the midpoint of the medial and lateral femur condyles; the Z-axis points from the midpoint between the lateral and medial malleoli and the origin; the Y-axis is perpendicular to the frontal plane of the shank and Z-axis; X-axis is orthogonal to the previous two. The foot anatomical coordinate system was defined by the malleolli markers and the metatarsal markers; the origin was the ankle joint center defined by the midpoint between the lateral and medial malleoli markers; the Z-axis points from the midpoint between the 1 st and 5 th metatarsal heads and the origin; the Y-axis is perpendicular to the frontal plane of the foot and the Z-axis; X-axis is orthogonal to the previous two [42]. Lower limb and pelvis joint angles (calculated using a XYZ Cardan sequence) and moments (determined through inverse dynamics and normalized to subjects' body mass) were computed and expressed relative to the proximal segment. The XYZ Cardan sequence was used due to the ISB recommendations regarding its clinical and anatomical meaning [43], since the description of X, Y and Z are equal to flexion-extension, abduction-adduction and longitudinal internal-external rotation, respectively. A cubic spline smoothing routine was used to filter both kinematic and kinetic data. The segment length was defined as the distance between the proximal and distal ends of the segment. Kinematic and kinetic data were normalized to 100% of the gait cycle. Peak values for lower limb angles and moments, as well as time-distance parameters, were computed for each cycle and averaged for each subject [21].