3d motion capture system

We utilized two wooden cubes with lengths of 10 cm. A total of five reflective markers were randomly attached to three of the six faces of the cube. Each cube was mounted on a pole for better handling (Figure 1).
Three experiments were performed, in which the two cubes were moved relative to one another. This included random collisions and the rubbing of the marker-free faces from time to time. We ensured that entire faces were rubbed against each other. Nine cameras from Qualisys (Gothenburg, Sweden) 3D motion capture system were used to record 15 s of the marker positions at a sampling frequency of 100 Hz.

For the ROM measurements, a 3D-motion capture system (Qualisys, Göteborg, Sweden) was used. Eight cameras were used in fixed positions, and the system was calibrated with a standardized L-frame and wand at the beginning of each day. Reflective markers were added according the Qualisys Gait module (type: Cast) to the participant’s hip (with two extra markers on the lateral iliac crest to ensure a proper measurement in a supine position) and test leg. The participant was then asked to perform three modified Thomas tests of the test leg for 5 s each on a medical treatment bed. The participant lay supine, with the ischial tuberosity close to the edge of the bed (Younis Aslan et al. 2018 ). The participant was asked to hold the knees by hand, with the arm extended to ensure the same hip angle between measurements, and a flat lumbar spine. The legs were completely relaxed. While holding the contralateral leg in position, the test leg was lowered toward the floor and the participant was asked to remain as relaxed as possible in the end position. After processing the Qualisys data with Visual3D Professional (C-Motion, Inc., Germantown, USA), the angles of the joints were assessed. The attempt with the lowest hip extension angle was taken for further analysis.

An obese boy with valgus knee [age: 8 years; height: 1.41 m; weight: 47 kg; body mass index (BMI): 23.64 kg/m²] and a healthy boy (age: 7 years; height: 1.30 m; weight: 28.5 kg; BMI: 16.86 kg/m²) participated in this study. BMI > 20.4 is classified as obesity based on Group of China Obesity Task Force for children with age of 7~8 [17 ]. The femur-tibia angle of the left valgus knee from the obese child is 13°. A 64-layer screwing CT machine (Somatom Sensation Cardiac 64, Siemens Corporation, Germany) was used to obtain the CT scan images of the knee valgus child’s left knee. A 3.0T MR machine (United Imaging uMR770, Shanghai, China) was used to obtain the MR scanning images of the healthy child’s left knee. The kinematic data of the lower limb and the ground reaction forces (GRF) during walk were recorded simultaneously by a 3D motion capture system (Qualisys, Sweden) and a force plate (Kistler Corporation, Switzerland). Sample rates were 200 Hz for the motion capture system and 1000 Hz for the force plate. All the system was calibrated before test.