## 1 Introduction  
Innovative technologies contribute to the growth of the rehabilitation sector by providing effective and safe solutions [1; 2; 3; 4]. The technology involved in rehabilitation protocols is varied. Examples of applied technologies are assistive devices and robotics [5], optoelectronic systems [6], inertial measurement units [7], and virtual and augmented reality environments [8; 9; 10; 11]. Virtual reality (VR) and augmented reality (AR) have aroused particular interest in the creation of customized rehabilitation protocols in the study of body mechanics. Biomechanical outcomes, such as joint movement analysis, are effective not only in diagnosing but also in understanding the mechanism of symptom progression. Most experts' clinical assessments of these situations are based on observing a given movement performed by the subject or the manual measurement of angles on clinical images [12]. Several studies have proposed different innovative systems based on markerless motion tracking devices [13; 14; 15; 16; 17; 18; 19] and virtual or augmented reality systems [20; 21] to support diagnosis and biomechanical measurements. Furthermore, a significant aspect of rehabilitation in which these new cutting-edge technologies are applied is postural analysis, a fundamental factor regarding personal well-being. Various pathologies or bad habits can influence posture and lead to the deformation of the bone structure. Consequently, malfunctioning of the musculoskeletal, respiratory, and nervous systems might also occur. The study of posture is based on measurements of anatomical angles and alignment of bone and joint components [22; 23]. In many studies, practicing sports, such as martial arts, is recommended for correcting postural problems. In fact, in martial arts, posture and balance are crucial to the correct performance of exercises [24; 25]. For example, in karate, repetition of simple positions, such as the Juntzuki (lunge punch) or the Zenkutsu Dachi stance (forward leaning stance), is fundamental to learning the basics of karate. Positions are also subjected to evaluation in competition kata (i.e., a set of shots and stances combined to perform an imaginary fight), komite (i.e., fights with other athletes) [26], or to pass the exam to proceed to the next belt level. To be able to perform the positions correctly, control of coordination and total body balance, perception of the surrounding space, and knowledge of the anatomical angles to be achieved are necessary [22; 25; 26; 27]. In these disciplines, experienced instructors can guide their disciples on how to perform the movement and maintain the correct posture. Still, studies have highlighted the contribution that technological systems could make to support instructors and disciples who want to improve and correct their mistakes [28; 29].  
The present study proposes a project implemented to visualize in real-time bones and other anatomical components of a subject performing critical karate shots and stances. The system comprises an economic motion tracking device that recognizes the subject movements and the position of anatomical joints and an AR headset, with which the physicians can observe bones and anatomical information fidelity overlapped to the subject in real-time. This hybrid system has the potential to contribute to the monitoring of martial arts athletes after injuries to support the restoration of their movements and position to rejoin the official competitions by taking advantage of new innovative technologies.