## 1 Introduction
Therefore, these systems are widely used for research purposes rather than for practical applications in the field. Existing pose-estimation technology is based on 2D or Kinect sensors; however, the results are inaccurate and difficult to use as objective indicators. The need for related research has emerged with the recent development of 3D sensing technology. Given that  
Figure 1: Global market for a digital medical therapy. pose-estimation technology is not actively used in the clinical field, most preceding studies are published in the field of general engineering, and research that can present the clinical meaning, value, and utilization of the data extracted by this technology is urgently needed. Recently, multi-view image-based motion-analysis systems that are capable of 3D implementation have significantly affected the research and diagnosis of the musculoskeletal system in sports players and even ordinary people. These systems have the ability to simultaneously observe and analyze various movements by using images obtained from various angles in 3D [4]. These 3D-based multi-view image sensor systems are of great help in identifying and preventing potential injury factors in sports players who may experience musculoskeletal accidents when performing high-intensity movements in competitions. The multi-dimensional data collection and processing capabilities of a multi-dimensional system allow athletes to analyze their movements accurately and quickly identify the causes and effects of injuries [5]. The validity of the angular root may indicate whether the measured angular data of the system correlates with the actual motion [6]. This validity is important for reliable motion analysis and the determination of the cause of injury in sports musculoskeletal accident research [7; 8]. The reliability of the 3D implementable multi-view image-based motion-analysis system introduced in this study indicates the consistency and stability of the results when repeatedly analyzing the same movement using this system. In situations involving sports musculoskeletal accidents, this system can facilitate a better understanding of accident-related factors and the development of safe exercise methods [8]. The meaning, advantages, and disadvantages of motion capture are well known. Marker-based motion capture technology is widely used for motion analysis; it involves the capture and analysis of data by using cameras and small markers to track a person's musculoskeletal movement and estimate the actual joint position [9]. Marker-based motion capture allows the precise tracking of musculoskeletal motion, thereby enabling accurate motion analysis [10]. Motion analysis using this motion capture device is well-established and is widely used in research and various industries [11]. Additionally, it is compatible with various motion-analysis systems, can be easily adapted for multiple purposes [11], and can provide instantaneous feedback during movement by allowing for real-time tracking [12]. However, marker-based systems have several disadvantages, and the special cameras and markers used in these systems are expensive. In addition, the marker may be obstructed from the field of view of the camera used, thus resulting in incorrect or missing motion-tracking data [13]. This pilot study aimed to develop a new multi-view image sensor-based system (namely, 4 DEYE) to compare and analyze the reliability and validity of the sensor developed via the simultaneous analysis of three joint angles with existing VICON sensors (VICON motion capture system). In addition, we present the possibility of the clinical application of a new motion-analysis system that has the advantages of convenience and economic feasibility.