Design of Metaverse-Based Physical Fitness Service for the Enhancement of Exercise Capability for Youth

. Tis study explains the case study to develop a metaverse-based training system for the management of the physical ftness of youth. Te study targeted developing a smart mirror-based AR physical ftness system and a bone age-based physical ftness management service to enhance exercise capability in youth. Accordingly, the data on youth’s bone age, physical strength, and physique were collected. Based on the collected data, the stage of the human development in youth was categorized, and the physical ftness management system providing the exercise program customized for the phases of physical development was implemented. Furthermore, a solution was devised so that youth with less physical exercise experience could practice physical ftness training without a coach using the smart mirror-based AR physical ftness system. Tis system uses an RGB-D camera and a three-dimensional posture estimation algorithm to recognize a user’s movement, determine movement accuracy, and provide feedback for the improved exercise efect. Te XR Physical Fitness, using a smart mirror implemented in this article, features two features of metaverse’s augmented reality and lifelogging, enabling the youth to enjoy more immersive and enjoyable exercise experiences.


Introduction
Youth is a signifcant time in the process of physical growth and mental maturity [1].However, recently, the obesity problem among children has become a signifcant social concern.Obesity in childhood is more likely to continue and lead to adult obesity.Moreover, obese children experience social and mental stress due to abnormal body shape formation.Another problem with healthcare for youth is their physical strength which is decreasing.As the diet level improves, children's nutritional status also improves, and their physique increases, but their physical strength decreases.Obesity and physical decline, caused by increased mental stress and a lack of physical activity, are resulting in various diseases in children.Terefore, it is essential to analyze whether the body is appropriately growing according to age and managing it properly [2].Tese situations show the importance of research for youth's physical strength improvement and health care programs.
Especially, the COVID-19 pandemic has confused the world ever since 2020 and change our daily lives.Limited indoor activities, such as telecommuting and online classes, have rapidly reduced the number of our daily activities to cause a health crisis and well-being.A new word such as "quarantine-15" has been coined, and healthcare for children has also become more difcult.As COVID-19 forced most outdoor activities to stop, many have quenched their thirst for exercise with various applications, such as wearable devices, YouTube, and home workouts.Ring Fit Adventure, released by Nintendo, has become an excellent solution for these people."Exergames" like Ring Fit Adventure allowed men and women of all ages to exercise in a limited space.Even the parents who were usually worried that their children would be too fxated on gaming were willing to buy the game, enjoy exercising with their children, relieve depression, and frustration from a lack of in-person activities and keep their bodies' minds healthy.
Te paradigm changes in the convergence of the real world and virtual reality, referred to as metaverses, are already quite noticeable in Korea at various points of our society, such as culture, education, and entertainment.As of 2022, the world has begun to think about how to enjoy a healthy "third life" in metaverses during the post-corona era beyond the COVID-19 pandemic.Te "Tird Life" is a phenomenon in which virtual space is organically connected to the real world to activate human senses and maximize experiences [3].Te world of the metaverse has begun to become a reality in our everyday lives, where one can enjoy a variety of sports activities without meeting friends in person and watch realistic games without having to go to the stadium.In other words, we now have to adapt to a new "Tird Life" in which real and virtual spaces interact through a physical "frst life" and a fctitious "second life" in virtual space.
Tis study aimed at developing a metaverse-based physical ftness services and management systems for effective youth athletic ability development.One factor that we concentrated on the most was preparing a plan for the young children to perform exercise programs without the trainers managing them in a non-face-to-face environment.Our conclusions can be summarized as twofold: the need for augmented reality technology and metaverse technology.A theoretical background was established by collecting research on extended reality technology and metaverse technology in the XR ftness and sports felds.Based on this, a smart mirror-based AR physical ftness system was developed.

Designing the Youth Exercise
Enhancing System

Te Importance of Physical Fitness Management for Youth.
Adolescence is an important time when physical and psychological changes occur the most as a part of human development, so various studies are required for healthy and stable physical development.In particular, young children should be treated diferently in comparison to sports and welfare services for adults, as there are many individual diferences in development speed and pattern [4,5].Terefore, research and implementation for health care services and systems to efciently provide appropriate physical activity services to children are needed.
We planned an integrated management system for physical ftness in youth as shown in Figure 1.In particular, we have developed a strategy that focuses on bone age.Because young children in the developmental process show outstanding exercise performance based on their weight and height, which are superior to those of immature young children [6].Tese results show that youth health care should focus on bone age rather than chronological age [7].In other words, since there are individual diferences in bone age, which measures biological maturity and chronological age, the current evaluation of exercise performance based on chronological age is not appropriate for young people with slow developmental growth [8].
Tus, this research project established a new Korean development stage index by measuring bone age and young children's ability to perform exercises, and based on this, we carried out clustering to derive characteristics of each development stage in young children.In addition, a management system was developed to improve exercise performance ability tailored to the characteristics of each cluster.

Bone Age-Based Physical Fitness Management System for
Youth.Te bone age-based physical ftness management system for youth was divided into four stages: (1) Data measurement and development stage classifcation, (2) cluster analysis and feature derivation, (3) ftness program development, and (4) management system development.

Measurement of Bone Age-Based Stages of Human
Development.In order to develop physical ftness standard indicators according to bone age, we conducted data measurements on a total of 2,931 people (1,469 males and 1,462 females) between October 2019 and February 2022.Te ages were distributed so that there were 361 people aged 6 to 7, 7, 529 people aged 8 to 9, 783 people aged 10 to 11, 931 people aged 12 to 13, and 327 people aged 14 to 16. Te measurement items were bone age, physical ftness, and physique data.As far as the bone age is concerned, we used the TW3 method to measure bone maturity after X-raying the left hand and wrist of the participant to be measured [9].For physique, we used inbody equipment to measure height and weight.To measure physical ftness, we measured a total of 7 physical ftness indicators: muscular strength (hand grip strength), balance (bass stick test), agility (plate tapping), power (standing long jump), fexibility (sit and reach), muscular endurance (sit-up), and cardiovascular endurance (suttle un).After that, individual stages of human development were classifed based on bone age based on the measurement data.Te development stages were categorized into fve stages (early childhood, middle childhood, late childhood, early adolescence, and middle adolescence) following the study by analyzing the theories suggested by existing experts, such as Piaget, Kephart, and Stratz [10][11][12].
Te analysis results of our target cluster group in the study (the 11-13-year-old) group show that there was a signifcant diference in men in the physical ftness category of muscular strength, power, muscular endurance, and cardiovascular endurance, and in women in muscular strength, balance, agility, power, fexibility, muscular endurance, and cardiovascular endurance [6].showing statistically signifcant diferences [6].It was confrmed that there was a clear diference between the evaluation of exercise ability based on chronological age and the evaluation of exercise ability based on bone age.

Cluster Analysis and Feature Derivation through Big
Data Analysis.Tis step shows that the critical factors for recommending personalized exercise were extracted using the data collected through the previous step's physique and physical ftness measurements (Table 2).K-means clustering was conducted to identify the data characteristics of the measurement targets and derive characteristics between groups.Although the number of clusters is planned to increase up to K � 5 in the future, clustering was conducted with K � 3 to derive meaningful features as sufcient data has not been accumulated yet.Finally, these data are again clustered into three groups through the three flters (gender, bone age-based stages of human development, and physical ftness measurement value).Te clustering results were represented as a graph (Figures 2 and 3) making it easy to grasp the characteristics of the physical ability of the same group and the heterogeneity between groups.After the cluster analysis, the data is classifed once again.Tis process checks the current bone maturity by comparing an individual's chronological age and bone age.Bone maturity is classifed into three stages (immature, normal, and mature).Te bone maturity classifcation is to know the growth and development of young children.Particularly, we were interested in an immature group of young children.Teir bone age is lower than their chronological age, and they are at risk of being guided to perform excessive exercises compared to their abilities since their chronological age is higher.Since excessive exercise   might adversely afect growth and development because of their immature bones, these young children should set their exercise performance goals more carefully as per their abilities.
As such, the big data analysis program recommends the type, intensity, time, and method of exercise programs appropriate for individuals based on the analysis of the characteristics and bone maturity of physical ftness by

4
Mobile Information Systems means of clustering as well as physique and physical ftness data.Ten, the director observes the individual physical ftness of the young children more closely in the feld and trains them with the recommended exercise program.

Development of Physical Fitness
Programs.We constructed a total of 20 youth-tailored exercise programs, 5 of which were newly developed exercise programs considering the main characteristics of each stage of human development (Table 3).Te exercise program analyzed the validity and reliability of the draft developed through preliminary research and invited external experts (professors, physical education teachers, and youth sports center instructors) to develop the fnal draft through expert meetings.Finally, the developed exercise program is applied to the big data analysis program described above.According to the data analysis result, an exercise schedule is generated that is suitable for the individual and recommended for the user.

System Design and Prototype.
User service and management programs were developed as a system that considered individual bone age, physical strength, and physique measurement data.Tis system is confgured as shown in Figure 4. Users can select and use the application and the web service according to their goals.Youth users can check attendance, recommended exercise programs, and physical ftness measurement results through the application (Figures 5 and 6).Coaches use the application and the web service as per the situation.First, the coach checks youth attendance using the application in the exercise feld, checks each youth's customized recommended exercise program, and teaches them whether they are performing it correctly.Te web service was developed to easily manage all youth members, exercise management, and exercise recommendation management (Figures 7 and 8).Te application has a small screen, making it difcult to compare information between each member and to grasp much data at once.Moreover, the customized recommended exercise results can be checked in the program, determined, and modifed from the position of coaches.

Developing Metaverse-Based Youth
Exercise System

Te Need for Metaverse-Based Physical Fitness Services for
Youth.Before the COVID-19 pandemic, ftness programs and services with Extended Reality and XR technology such as smart mirrors, VR, and AR glasses were also sometimes released.However, the COVID-19 pandemic brought the need for exercise to become more prominent and began to emerge more competitively.Extended reality exercise programs may be recognized as replacing face-to-face exercise activities for a while only due to COVID-19 situations.However, in reality, they can collect and analyze a lot of valuable data for increasing exercise efects, such as customized exercise content or posture feedback.It also has the potential to provide an evolved exercise experience that cannot be obtained from existing exercise methods.
However, quality problems such as insufcient content, inaccurate feedback, and incorrect technology continue to be pointed out, and many of these programs are often ignored after briefy attracting attention from users in the early stages of release.Terefore, if the upcoming extended reality exercise program fails to solve the current problem, no matter how good the technology is applied, it will not satisfy users' desire to participate in exercise and exercise satisfaction.Augmented reality afordance can be a great way to solve quality problems that may arise in mixed reality exercise programs.Augmented reality afordance is a concept consisting of a technology that mixes virtual objects into real space to interact [13], and an afordance called behavioral induction is an essential attribute that induces an action in the user [14].In other words, this can be understood as a method of inducing behavior for a natural learning process in the user using augmented reality.If this augmented reality afordance is applied to exercise performance, it is considered an efective tool to supplement or replace existing learning methods [15].In addition, extended reality exercise programs and services have continuously been chiefy developed for adults, so young children were not the primary target.However, the modern youth generation lives in and out of the real world through various digital technologies.In the future society, the real world and the virtual world will be closely connected.Accordingly, it is becoming common for humans to interact with and infuence the virtual world and their experiences in the real world.For that reason, in the future educational environment, there is a need for educational content that develops future capabilities to understand new metadata and freely perform social and economic activities and various hands-on activities in the world.Teaching the ability to live independently while actively responding to digital technology and the metaverse for new extended realities will be an important mission for future education.Tis makes it necessary to research and implement future education actively.Exercise activities are mostly difcult to deal with in a metaverse environment.However, a metaverse-based training platform needs to expand the boundaries of limited experiences in the virtual world to afect real users' experiences directly.[16].However, various types of metaverses are already emerging in various felds.In addition to technology development for metadata such as VR, AR, and various IoT devices, various research on content and service planning, storytelling, production and design, immersion, and reality are being conducted.

Understanding the Metaverse. When Marc Zuckerberg renamed the company Meta in 2021, many people began exploring what the Metaverse was
Te American Foundation for Future Acceleration Studies (ASF) defned "metaverse" as "a complex concept, a fusion of virtually enhanced physical reality and physically permanent virtual space [17]."Terefore, the metaverse is not just a 3D space but a space where virtual space and reality actively interact and can be understood.In order to embody the concept of the metaverse, the metaverse roadmap is Mobile Information Systems divided into two axes and provides four key elements (Figure 9).Te longitudinal axis is the spectrum of "augmentation and simulation."Augmentation means the ability to add new realities to existing systems, and simulation means the reality of providing new environments such as virtual space.Te horizontal axis is the spectrum of "intimate and external."External information is about the surrounding world, while intimate focuses on individuals and individuals and focuses on users [17].
Metaverses can be classifed into augmented reality, lifelogging, virtual worlds, and mirror worlds depending on the implementation space and type of information.Augmented reality is a technology that enhances virtual information in a real environment and combines virtual threedimensional images into reality to increase a sense of reality further [18].Pokemon Go, which was released in 2016, has become a craze among the public and is a representative example of being used as an augmented reality game.Lifelogging refers to the overall activity of recording, storing, and sometimes sharing things and people's daily experiences and information for their purposes [19].Examples include Facebook, Instagram, and Twitter, such as SNS and social media.Mirror Worlds imitate the real world as it is, as if it were a mirror, and express the real world in digital form in a metaverse [20].Unlike augmented reality, Mirror World focuses on "realistically reproducing" information related to the location of the actual area.Google Earth, Airbnb, and Earth 2 are examples of Mirror World.Google Earth is building a database of satellite images, maps, topography, and 3D building information around the world using satellite image maps.Virtual Worlds have already been used for a long time in computer games.It is built into a virtual world created based on 2D and 3D engines that are inherent and simulated [21].Te virtual world can be divided into three types (game-type virtual world, community-type virtual world, and work-type virtual world).Te game-type virtual world aims to achieve common or individual goals of players while cooperating or competing with each other with specifc rules.In the community-type virtual world, activities such as setting avatars to explore the virtual world and communicate with other users are carried out.Te working virtual world enables remote work by reproducing the working environment of companies or public institutions as it is.
Tese metaverse platforms enable diferentiated experience values with new convergence technologies, and the 4I, immersion, interaction, imagination, and intelligence are achieved, enabling new experience designs beyond time and space [22].In other words, metaverses provide modern people with diferentiated experience value, which allows modern people to create the same appearance as themselves in the real world, or a completely new created self, as a metahuman (Avatar).Not only can it be designed in the virtual world by cloning the physical space of the real world into an entirely new imaginary space, but it can also implement or recreate the real past in the virtual space in terms of time and provide new experience value to explore the predictive future.

Understanding the XR Physical Fitness.
Te non-face-toface ftness environment has several limitations.Te critical problem is that it is impossible to meet with a trainer.In ftness, coaching by trainers plays a signifcant role.First of all, one needs to fgure out what kind of exercise one needs based on their current physical condition.However, one needs a trainer because it is pretty challenging to do it yourself without professional knowledge [23].In addition, maintaining correct posture while exercising can prevent injuries and increase exercise efects.However, most people are not used to controlling their movements and posture accurately and do not know how to exercise, so they have to learn from their trainers.Finally, leadership is necessary to  6 Mobile Information Systems provide continuous motivation.Trainers play an important role in presenting exercise goals and steadily providing motivation to not give up continuously.However, due to the absence of a trainer, these three points-physical conditioning, exercise guidance, and motivation-are bound to weaken in a non-face-to-face ftness environment.
For that reason, XR physical ftness has recently attracted attention.XR physical ftness provides ftness services with a coaching system that sufciently analyzes the condition of athletes, monitors their movements through XR technology using cameras and sensors, explains the how and efects of exercise, and provides exercise content to report and follow.Many YouTube channels and ftness applications provide various exercise content, but they have limited available streaming content.In addition, some ftness applications check the user's exercise time and running distance, but they cannot correct and motivate posture or determine the current athlete's physical condition.As a result, various XR physical ftness systems and services are being released to compensate for these shortcomings.
MIRROR is a digital healthcare service using smart mirrors.A smart mirror is a device with an interactive display that looks like a mirror and a built-in camera and speaker.MIRROR helps one do ftness exercises easily with accurate movements at home.Te professional coaching system of MIRROR optimizes the exercise course to obtain the maximum exercise efect according to exercise goals and preferences by tracking and managing the user's profle and   8 Mobile Information Systems biometric data.More than 50 exercise programs can be trained in real-time by professional instructors or exercise with friends [24].Peloton provides connected ftness equipment (bike, treadmill, and various exercise tools) and subscription-type online ftness streaming services.Users can use online trainer lessons and training programs in real-time using connected ftness devices.A large tablet installed on a bike or treadmill allows one to view training programs and enjoy exercise with other users in real-time.Data measured by sensors mounted on ftness equipment are transmitted to coaches conducting real-time classes.Feedback can be received from an expert trainer through the measured data.And one can compare his or her training with that of other users or be recommended a training program that fts your body data [25].
One of the most popular XR physical ftness services is Zwift in recent years.Zwift is a simulation program that connects with Smart Bike Roller for door cycling and allows virtual riding through a computer or mobile.Various training programs are provided to help improve one's athletic ability.As an event, one can ride together with several people, such as in group riding or a race, to compare their skills through competition with others.Group riding has a diferent difculty level so that one can participate according to his or her skills.If one achieves a specifc mission while riding, one can receive various jerseys to decorate virtual characters.Virtual characters can change not only clothes but also bicycles, helmets, goggles, gloves, and socks.Zwift also provides training programs for running.When a device called Runpod is worn, the speed and cadence (SPM, number of steps per minute) are measured.Tese two data allow virtual characters to run at the same pace as real users run on the treadmill [26].
XR Fitness is attempting to provide the same exercise environment and efectiveness as ofine in combination with various high-tech felds such as wearable devices, motion sensors, and AI technology [27].In particular, for this purpose, smart mirrors are actively used in XR Fitness as an intelligent health management system [28].Smart mirrors have the most important feature of being able to easily understand and follow expert movements compared to other XR Fitness methods because users can see their exercise movements through the mirror while exercising [29].Te currently released exercise coaching system using smart mirrors is currently divided into three types.First, it is a form of measuring the number of exercises and the accuracy of the posture by recognizing the user's movement through the camera.Second, it is a form in which exercise equipment is combined with a smart mirror.Tis type can also perform lifting exercises.Tird, it is a form of measuring exercise in conjunction with wearable devices such as smartwatches and heart rate sensors.Tese three forms may be confgured in combination for each service or may be confgured alone.Te MIRROR introduced above measures the user's motion through such a camera and also measures the amount of exercise with a wearable device.TONAL and TEMPO combine with exercise equipment to provide content for various muscle exercises.Users must select smart mirrorbased XR Fitness devices and services according to their desired exercise purpose.However, no matter how smart a mirror-based XR Fitness is, there are common essential requirements to provide a satisfactory exercise experience for users.First, it is the easy accessibility and composition of exercise programs that can produce sufcient exercise effects.Te following is a stimulus to the belief that you can receive accurate exercise coaching even in a non-face-to-face environment and have a steady desire for exercise.Te last is having a pleasant exercise experience and obtaining efects through it.Terefore, a feature analysis that can accommodate these requirements should precede the development of the XR Fitness system.

Analysis of Features of the XR Physical Fitness.
Since XR Physical Fitness is diferent from existing ofine ftness programs, sustainable elements must be identifed and included in the program composition.Analyzing the XR Physical Fitness service discussed above, factors necessary for successful program development are derived.Tere are six of these elements, which can be divided into three groups.Te frst group is "usability" and "professionality."Professional ftness clubs already have an environment for exercising, and users can start exercising as soon as they go to the ftness club.However, it takes much time to prepare for XR Physical Fitness at home, and if a complex preparation process is required, it hinders the continuous participation of users.Terefore, it should be conveniently confgured to start an exercise program as soon as possible when you decide to exercise.In other words, it should be highly useable, which means convenience.Convenience should not be misunderstood as easy or simple.Since the XR Physical Fitness program requires the user to perform exercise alone, the provided exercise program must have expertise.As pointed out earlier, if sufcient information about exercise programs that must be performed without a trainer is not Mobile Information Systems provided, it adversely afects the user's motivation to exercise.Terefore, professional exercise programs must be organized as much as possible.Information necessary for exercise must be provided to users promptly during the exercise process so that they can be immersed in exercise without being disturbed as much as possible [30].
Te second group is "feedback" and "motivation."Users can focus on exercise with confdence only when they receive feedback on whether they are continuously performing accurate movements.Although it is quite difcult for the system to accurately feedback in situations where user stamina and physique characteristics are diferent, it is a necessary factor in XR Physical Fitness.To this end, the XR Physical Fitness system is focusing its eforts on developing technologies to collect users' exercise performance data as much as possible using cameras and various sensors, analyze them in real-time, and give feedback.Accurate data analysis increases trust in exercise results, and this trust provides a "motivation" for the user to exercise continuously.Another efective way to "motivate" is to provide a variety of exercise goals and help monitor the user's progress.Achieving a goal gives users a sense of accomplishment.Tis sense of accomplishment is positive energy and greatly infuences continuing to do something.If one exercises continuously through motivation, one will be able to get the efect he or she wants [31].
Te third group is "joy" and "exercise efect."Most people start exercise programs for health-related reasons, but interest and social experience are essential in maintaining them continuously [32].It is pretty challenging to make exercise enjoyable.Tis is because exercise itself is a complicated activity.One of the ways to make exercise enjoyable is to compete with others.Working out alone makes one feel more lonely than happy.Terefore, many people try to enjoy exercising with others at ftness clubs.However, XR physical ftness is an environment where one exercises alone in an independent space.Terefore, many XR physical ftness services provide connectivity between users and induce them to enjoy exercising through competition with each other.If a user exercises happily like this, the exercise efect naturally increases.No matter how much time one exercises, the exercise efect decreases if one cannot concentrate on oneself and exercise.However, if one focuses on exercising through competition with others, the exercise efect will inevitably improve.Terefore, competition with others in exercise can be a cooperative process that increases the efectiveness of exercise.
Te six elements analyzed as such play a crucial role in the development of XR physical ftness, and each element is not independent but strongly interrelated.Tese elements work for the goal of increasing sustainability to secure the user's exercise efect in XR physical ftness.Te elements of these three groups relate to 3I of virtual reality.Burdea and Coifet describe 3I, immersion, imagination, and interaction as important components of virtual reality [33].Imagination relates to joy and the exercise efect.Tis element is necessary to feel the efects of exercise while exercising joyfully and competing with other remote users.Users with weak imaginations cannot interact sufciently with the XR Fitness system.Interaction relates to the second group, feedback, and motivation.Trough interaction, users explore the XR Fitness environment and select and manipulate objects.Users must receive accurate feedback on their actions to increase their reliability in the system.Tis belief and trust provide the user with an incentive to continuously perform XR Fitness.Finally, immersion relates to usability and professionalism.Users exercise through XR Fitness based on imagination and interaction and are fully immersed in the exercise without any interference.As such, 3I is a key element for providing users with a sufciently meaningful experience in a virtual environment, and the six elements of XR Fitness derived in this paper are consistently understood as important factors in enhancing users' experience.
Te concept and characteristics of the metaverse and the core elements of XR Physical Fitness have been explored to develop a metaverse-based training platform to enhance the physical ftness of youth.As a result, we decided to develop a smart mirror-based AR physical ftness system for youth (Figure 10).Te smart mirror was selected because the most optimal environment where the youth can engage in exercise without coaching another person can be created with the smart mirror.Furthermore, it was deemed that a user can safely perform an exercise by following the movements directed by a professionally created exercise program shown on display.An RGB-D camera such as the Kinect can be used to sense the movements in real-time and verify the accuracy of the movements before fnally giving precise feedback to the user.
Tis system features the application of lifelogging and augmented reality of the metaverse (Figure 11).A user can get real-time feedback on the accuracy of their movements.Te exercise data will be consistently recorded and used to analyze the exercise efect.Te technologies derived during the development process are expected to be signifcantly instrumental in applying the experiential elements into metaverse-based training platforms.

Developing a Smart Mirror-Based AR Fitness System for
Youth.Te smart mirror, RGB-D camera, and server operate in connection with the smart mirror-based AR physical ftness system for youth.When a user stands in front of the smart mirror, the RGB-D camera uses the threedimensional body posture estimation algorithm to extract the skeleton data of the user.Te camera with a depth sensor fnds information on a body part corresponding to a user's skeleton.Ten, based on the image data from the camera, the positions of each joint of the user are estimated through the open-source library of OpenPose.OpenPose is the frst realtime multi-person human pose detection library [34].Figure 12 is the result of detecting the user's skeleton using OpenPose.Te user's motion is determined by measuring the angle of bending of the corresponding body part.For example, the movement of the left arm uses joint pointers on the left shoulder, elbow, and wrist.First, the system calculates the angle of the elbow by calculating the distance between each pointer.And we measure the arm's movement through the angle of the elbow.Te skeleton data collected 10 Mobile Information Systems accordingly are used to determine the accuracy of the movement during the exercise.Te process of examining the movement accuracy begins with pre-processing the joint position data.Ten, the angle of the skeleton of a user is compared against that of the skeleton of the professional in the video to evaluate the similarity between the two.Te movement data of the professional is created with the skeleton extracted every two frames out of the video of the professional and the angles between a total of 16 skeletons recorded.When a user performs the exercise, the angles will be compared against the prerecorded movement data of the professional.Suppose the skeleton of the user exceeds the threshold set in the movement data of the professional.In that case, it is determined that the user makes an inaccurate movement or posture, and the visual feedback will be provided accordingly.If there is a diference between the expert motion data and the angle range of the user's skeleton by 15 to 20 degrees or more, the system recognizes it as an incorrect posture (Figure 13(a)).Te danger range between 10 and 15 degrees (Figure 13(b)), and if it matches within 10 degrees, it is judged as the correct action (Figure 13(c)).Feedback on the user's motion accuracy is displayed in a visual graphic.Visual graphics help users intuitively recognize their motion accuracy.In order to reduce the range of determining operation accuracy, a plan is being developed to increase the accuracy of user motion measurement in the system by using machine learning.
Yoga, stretching, step box, tae bo, and dance were selected for exercise content appropriate for the environment where a smart mirror is used based on the consultation with experts.Te exercise programs were planned and designed to conduct the selected sports within the range in front of a smart mirror.Te exercise program video was flmed in 4K resolution in a Chromakey environment to create expert data and exercise content (Figure 14).Te difculty in producing exercise videos was to organize a program so that all movements could be made in the limited space of a smart mirror.Terefore, the movement to the left and right was reduced as much as possible, and the motion was carried out in the front-rear or vertical direction.Exercise may change the static while reducing left and right movement, but it is confgured to provide the same exercise efect even in this situation.For example, the movement had to move left and right, allowing the user to turn sideways and move.However, at this time, since the user cannot check the    14 Mobile Information Systems information through the smart mirror, it was instructed to see the front again quickly after a short operation.Confguring an exercise program so that dynamic movements can be performed in such a limited space is expected to be a continuing challenge in the future.Te level of difculty was categorized depending on the capabilities of the user.For example, the step box has three levels: beginner, intermediate, and advanced, while the K-pop dance has levels of lecture per part, checking per part, and full version.
Te program fow is shown in Figure 15.First, a user selects the desired exercise content.If the dance is selected, the user selects the music and the difculty level (Step 2).As below, the themes of the lecture per part, checking per part, and the full version can be selected (Step 3).When the difculty level is selected, the content will be played.Next, the user performs the exercise according to the video on the smart mirror (Step 4).Te program recognizes the user's movement in real-time.At the same time, the exercise is  Mobile Information Systems being performed, which compares the movement against the data of a professional, calculates the accuracy of the movement, and then provides feedback to the user.Te user can see the feedback information to recognize the accuracy of one's movement and understand one's exercise capability.In addition to the feedback, a user can check and adjust one's movement intuitively through the smart mirror.Once the content is fnished, the result window will appear, displaying information such as exercise time, calorie consumption, and heart rate (Step 5).
A user interface designed for the interaction optimized for the smart mirror was also developed (Figure 16).When the content is played, unnecessary icons are disabled to improve the user's engagement with the content.Furthermore, the transparency rate and font color were selected in consideration of the half-mirror transmissivity so that a user could obtain the information quickly while practicing the exercise.It required an intuitive and attractive design of UI elements such as visual icons to increase the system's usability.Terefore, icons such as home, shut down, and skip adopted a form commonly used in digital devices (Figure 16(c)).In addition, menus and UI elements were consistently confgured so that users would not be confused when using the program.Te distance between the device and the user is an important consideration for the UI design of the program.When selecting an exercise program, the user operates directly in front of the device.All components are concentrated in the center so that all components can enter the user's feld of view.Te size of the letters and icons is designed to be relatively small.However, when the program is all over, the letters are enlarged, and it is designed to check the exercise results through various efects (Figure 16(d)).In the future, further research on UI design according to user age and physical conditions will be needed.Tis is because it has been confrmed that there is a big diference in usability during system operation depending on the user's vision and height.

Discussion
Te system currently has certain limitations.First, the movement details of a professional should be prepared and optimized to create new content, which consumes a considerable amount of time.If the optimization is not processed sufciently, the standard threshold value setting for analyzing the accuracy of the movement will not be accurate.Consequently, the body posture feedback information cannot be reliable.Moreover, when the feedback is inaccurate, a user cannot check whether they perform the movement accurately.Hence, the user will not trust the exercise result, and consequently, there will be an adverse efect on performing the exercise continuously.Difculty with preparing the movement data of professionals will also infuence quick content expansion.Inducing interest in continuous exercise will need a variety of sports programs.A user will become easily bored after repeating a couple of sports programs a few times.Te difcult process of preparing and optimizing the movement data of the professional will interfere with fast content updates.It is expected to take less time as the know-how is accumulated after the repetition of program development.
Another limitation is that the exercise cannot be conducted with other users.As mentioned in the characteristics of XR physical ftness, exercising with others is more enjoyable and enables more extended exercise than engaging in exercise alone.However, the current system enables only one user to exercise in front of a smart mirror.When it comes to the youth who usually would not concentrate for a long span may well exercise for a relatively short period.Terefore, the communications feature enabling a group exercise can bring many advantages.A user can fnd an exercise partner or learn from more experienced users with the communications feature.Even if the system provides the sports program, voluntary coaching and learning among the users should be one of the strong points of a metaverse-based training platform.
Te last limitation is regarding the exercise space.A smart mirror limits the area where the exercise can be performed.Te exercise should be performed within the specifc area, right in front of the smart mirror, so that feedback on the movement accuracy can be provided through the camera.A user can receive intuitive feedback by checking on one's image in the mirror.Te limitation in terms of the area serves as a strong constraint in terms of content design, because the movement that goes out of the area in front of the smart mirror cannot be recognized.Another problem due to the useage of a smart mirror is that it is complicated to secure the exercise space in a small area.Te RGB-D camera requires a minimum distance of 2.5 M to recognize a user's movement accurately.Te distance between the smart mirror and the user can cause space problems and reduce the level of user engagement with the content.Te fsheye lens and the image compensation processing technology were used to reduce the distance to less than 2 M, but the space problem limited by the smart mirror remains.
Nevertheless, the smart mirror-based AR physical ftness system demonstrates the possibility of creating new experiences and highlighting the importance of the heuristic elements on the metaverse-based training platform.

Conclusions
In this study, a bone age-based physical ftness management service was proposed to improve the exercise capability of youth.Developing the smart mirror-based AR physical ftness system was demonstrated as the metaverse-based XR physical ftness ofering physical ftness programs.Tis study was initiated to provide a physical ftness service for stable physical development after realizing that the youth shows individual diferences in terms of speed and pattern of physical development.Trough the physical ftness data analysis of youth, it has been confrmed that the categorization of growth development based on bone age rather than chronological age is the most accurate and reliable for estimating the growth status of youth.Continuous data accumulation is underway to establish the Korean standard indexes for growth development based on bone age.When the indexes are established, it is expected that the characteristics related to the physical strength of youth can be analyzed in more detail.Furthermore, the smart mirror-based AR physical ftness system provides an exciting exercise experience for youth.Te system collects the movement data and provides posture accuracy feedback using the RGB-D camera and body posture estimation algorithm.Accordingly, it ofers sufcient exercise in an environment without a coach or instructor.are still issues concerning the content, communications features, and exercise space.However, improvement plans are already prepared, and technology development is in progress.Collecting a user's movement data is essential for the research of interactive metaverse services.To this end, continuous eforts will be made to research the methods of collecting actual human data, processing, analyzing, and utilizing the collected data, and fnally substantiating the methods in the metaverse content.

Figure 1 :
Figure 1: Overview of bone age-based physical ftness management services for youth.

Figure 4 :
Figure 4: A diagram showing the structure of the physical ftness management system for youth.

Figure 5 :
Figure 5: Menu tree of the application.

Figure 8 :
Figure 8: Screenshots of the web service: (a) detail information of user, (b) member search, and (c) exercise plan management.

Figure 10 :
Figure 10: A smart mirror-based AR physical ftness for youth.

Figure 11 :
Figure 11: Position of a smart mirror-based AR physical ftness system.

Figure 13 :Figure 14 :
Figure 13: Process of posture accuracy evaluation: (a) perfect is matching all motions, (b) good is matching partial motions, and (c) bad is mismatching most of the motions.

Figure 14 :
Figure 14: Expert video for content production and content-applied screen: (a) step box, (b) dance, and (c) tae bo.

Figure 15 :
Figure 15: Progress step of the content.

Figure 16 :
Figure 16: UI design of the system: (a) introduction to exercise, (b) choose a practice type, (c) content progress and icons, and (d) completed exercising.

Table 1
compares the physical ftness measurements of bone age of 11 years old and the chronological age of 11 years old.Here, as far as a man was concerned, values were higher in bone age than chronological age on the fve categories (muscular strength, agility, power, muscular endurance, and cardiovascular endurance), showing statistically signifcant differences.As far as women were concerned, the values were higher in bone age than chronological age in the six

Table 1 :
[6]diference in physical ftness according to bone age and chronological age of males and females aged 11[6].

Table 2 :
Average of physical strength measurements of late childhood.

Table 3 :
Major exercise programs and expected efects on the stages of human development.