## 7 Motion Capture Systems--MoCap
### Optical Motion Capture Systems  
Optical MoCap systems exploit cameras to track the motion of reflective markers attached to specific locations of the subject's body. Two different approaches are commercially available: active and passive systems. The former is based on landmarks placed on the joint points implemented with powered markers made of light-emitting LEDs and daylight cameras. On the contrary, the latter only employs passive sensors traceable by infrared (IR) cameras. Some of them, namely deep optical MoCap marker-based (DeepMoCap), use multiple space-time aligned infrared depth sensors (IR-D) and retroreflective straps and patches (reflectors like passive markers) combined with fully convolutional neural networks (Figure 10). Figure 10: **(a)** In the active optical MoCap, infrared cameras recognize attached retroreflective markers on different body points to reproduce in motion analysis software; **(b)** in the passive optical MoCap, proper cameras recognize attached LED markers on different body points to reproduce in motion analysis software. DeepMoCap is a MoCap system that utilizes depth images to locate and label reflectors automatically, enabling the reconstruction of 3D spaces. The proposed system, employing a model-based adaptation technique, efficiently captures only one person's movement from the extracted optical data. By placing reflectors and IR-D sensors around the subject, body movements are fully acquired, overcoming the limitations of a one-sided view, such as partial occlusions or damaged image data. The logic behind the use of reflectors is the exploitation of the intense reflections that cause the IR fluxes of the IR-D sensors, allowing them to be detected on depth images. Spatial mapping and the alignment of the retrieved 2D points to 3D Cartesian coordinates using intrinsic and extrinsic depth data and IR-D camera parameters enable the frame-based 3D optical data extraction. DeepMoCap constitutes an alternative, low-cost, and flexible marker-based optical MoCap method that results in high-quality and robust MoCap outcomes. This process, called 2D reflector set estimation, replaces the manual marker labeling and tracking tasks required in traditional optical MoCap [212]. Active and Passive Optical Motion Capture Systems  
Active systems embed IR cameras that capture the light reflected by markers attached to bodies or objects under investigation. The camera lens properly lightens these markers, and the acquired images are used to calculate the position of such points within a 3D space. As an active optical system, Vicon is a reference tool for the calibration procedures of inertial and video markerless systems. This system works with cameras that detect proper markers worn by the players on various critical points of their bodies [213; 214]. To assess the influence of different axes of rotation, namely the minimum inertia axis, shoulder-center of the mass axis, and shoulder-elbow axis, on the four distinct phases of the tennis serve (loading, cocking, acceleration, and follow through), a T40 Vicon eight-camera system was set up. This system enabled precise evaluation and analysis of the subject's movement during each serve phase. The global orthogonal reference system was chosen so that the x-axis pointed forward, the y-axis pointed vertically upward, and the z-axis pointed laterally to the right. Forty-three markers were applied on the joint points of each participant to the study, and five additional markers were placed on the racket. From the 3D position of the markers, the evolution of arm joint angles and joint center positions was achieved using custom MATLAB R2023b scripts [215]. Vicon cameras (Centennial, CO, USA) were also used in tennis to analyze the influence of racket kinematics on the ball topspin angular velocity and accuracy during the forehand groundstroke [216]. In [217], the analysis process of the soft-soleed footwear, the kinematic variables for barefoot and footwear conditions were collected via the Vicon system by twelve cameras with twenty-nine markers.