Development of Aerobic Exercise Equipment Using Universal Design: Treadmill and Arm Ergometer

Exercise products based on universal design, which reduce restrictions on the exercise environment and ensure convenience and safe use, are beneficial for people with a disability; however, the current universal design only considers the preferences of the general population, which is not suitable for the disabled population. This results in the exclusivity of the sports facilities and supplies for people with a disability. Consequently, we explored the components of universal design and product satisfaction by considering users with disabilities and proposed the direction for designing extended universal exercise equipment that is suitable for them. Specifically, this study focuses on developing exercise equipment for people with a disability. Based on the results from the evaluation of acceptance and satisfaction of universal sports products for people with a disability using design thinking, we suggest the following. First, it is necessary to consider safety devices for exercise products. Second, the user interface should be improved in terms of convenience. Third, the ergonomic instrument design should be improved. Finally, the instrument design should be centered on user convenience.


Introduction
Disability hinders the ability to perform daily life activities depending on the level or type of the disability. According to the World Bank report in 2022, 15% of the world's population has a disability, and Korea accounts for 5% of this total population [1]. The high proportion of people with a disability compared to the total population illuminates the various needs of the disabled population in society. Furthermore, the participation rate of people with a disability in daily sports is rapidly increasing; consequently, the demand for new welfare and sports equipment for this group is increasing [2].
The Ministry of Culture, Sports, and Tourism of Korea reported that the rate of participation in exercise by people with a disability tripled from 8.6% in 2010 to 24.9% in 2019 [3]. This indicates that the interest of people with a disability in sports activities has increased significantly. In addition, the more experienced people with a disability become, the higher the demand for support equipment during exercise; therefore, there is an increasing need to develop exercise equipment that all categories of people, including those with a disability, can use [3]. Compared to the normal population, people with a disability are at a disadvantage in terms of educational, socioeconomic, and occupational opportunities [4,5]. Moreover, the regular exclusion of people with a disability from design, service, and policy-making makes them less active and results in their isolation, depression, and loneliness [6]. Irregular physical activity among people with a disability may lead to adult and inactivity-related diseases [7]. According to the Center for Disease Control and Prevention (CDC), adults with disabilities are more likely to stop participating in physical activities in their leisure time than those with no disability. In addition, among teenagers, girls with disabilities in grades 9-12 reported at least 60 min of physical activity per day, for 3.1 days per week, whereas girls with no disabilities reported the same for 4.5 days per week [8]. The decrease in exercise participation among people with a disability reduces health-related benefits. In addition, the lack of regular physical activity can negatively affect their psychological and social health. This is because it can deepen their feeling of isolation, increase stigma, and worsen their social relations [9].
Therefore, gymnasiums and fitness centers for people with a disability are preferred places for improving their quality of life by participating in physical activities [10,11]. Fitness centers appeal to a wide range of disabled sports participants because they are relatively easily accessible, have flexible operating hours, and have no requirements for specific physical skills or fitness levels. However, the nature of current sports facilities still restricts people with disabilities from participating in sports due to their design, which does not consider the peculiarities of these groups. When efforts are required to use sports equipment, time and difficulty in procuring them can increase the stress of people with a disability [6]. For example, if these facilities lack user-friendly features for people with a disability, such as easy access to wheelchairs, which greatly undermines the emotional well-being of these participants [12]. Therefore, the design of sports facilities and equipment/devices should be user-friendly for people with a disability and nondisability in a diverse society. This means that more emphasis is needed on the application of universal design (UD).
UD was introduced in the 1970s by Ronald Mace, an American architect and product designer with a disability [13,14]. Ronald defined UD as a "product and environmental design that can be used by as many people as possible without adaptive or specialized design". Between 1994 and 1997, the National Institute on Disability and Rehabilitation Research (NIDRR), which is responsible for disability and rehabilitation among U.S. government agencies, made a major investment in the study of universal design. In the process, the Center for Universal Design introduced the following seven UD principles for architects, product designers, engineers, and environmental design researchers: (1) Equitable use, (2) flexible use, (3) simple and intuitive use, (4) perceptible information, (5) tolerance for error, (6) low physical effort, and (7) size and space for approach and use. Reportedly, various product designers have modified their products to accommodate these seven principles to increase usability by both people with a disability and those with no disability [15].
Exercise equipment facilitates educational and vocational social rehabilitation for people with a disability by mitigating physical and functional disabilities. Moreover, exercise equipment for people with a disability is an essential element that enables daily life and social activities and is a necessary means of improving the quality of life for the disabled and expanding opportunities for integration and independence. Equipment for convenience improvement and recreational activities for people with a disability has a relatively short history. For example, lightweight folding wheelchairs, which were first developed in the United States in 1933 [16], are still not very popular. Rimmer et al. [17] found significant opportunities to enable people with a disability to exercise and play in fitness centers and swimming pools that comply with the Accessibilities Disabilities Association (ADA) standard. In addition, Ludwa and Lieberman [18] studied the application of UD with the intention of enabling students with disabilities to participate in spikeball, which is a new sport.
Various studies in the field of sports have examined the applicability of UD to products for people with a disability. Gray, Zimmerman & Rimmer [19] conducted a study to measure the level at which the UD of human-made structures guarantees the physical activity of the disabled. Additionally, Kim & Chang [20] measured the user's perception of sports facilities by applying universal design. Currently, UD products are being actively researched around the world. In Korea, the exercise participation rate of people with a disability is reported to be low due to the lack of exercise infrastructure that is suitable for this group. The Ministry of Culture, Sports, and Tourism [3] surveyed sports facilities mainly used by the people with disability. The survey showed that 8.1% of public sports facilities, 3.9% of private sports facilities, and 3% of public facilities for people with a disability, and it was found that 81.5% of people with a disability did not use these sports facilities. Furthermore, 37.7% of people with a disability were found to have difficulty exercising alone because they did not use sports facilities. The Korean government has decided to build 150 new sports centers for people with a disability by 2025 to increase their participation in daily sports. The Sports Center for the Disabled, called the Bandabi Sports Center, will be operated as an integrated facility where people with a disability will have the right to use it first, while those with no disability will also use it. This implies that the need to develop exercise products with universal design, which enables people with a disability and those with no disability to participate in the exercise, is gradually increasing. Therefore, the purpose of this study is to develop an aerobic exercise equipment design, which employs a UD that can be used by people with a disability, those with no disability, and the elderly by applying the design thinking method.

Procedure
This study is based on the principle of "design thinking". Self-thinking-based research is research that seeks a problem-solving process centered on people, and it is characterized by providing the process for the optimal combination of problem-solving, business, intuition and logic, etc. [21,22]. Therefore, to design universal exercise equipment, we conducted this study by applying the "design thinking" principle. Although the process of "Design Thinking" is diverse, the "Double-Diamond" process is presented among them, which consists of four steps-"Discover", "Define", "Develop", and "Deliver" [23]. Figure 1 explains the double diamond design process.
Healthcare 2022, 10, x FOR PEER REVIEW 3 of 27 of sports facilities by applying universal design. Currently, UD products are being actively researched around the world. In Korea, the exercise participation rate of people with a disability is reported to be low due to the lack of exercise infrastructure that is suitable for this group. The Ministry of Culture, Sports, and Tourism [3] surveyed sports facilities mainly used by the people with disability. The survey showed that 8.1% of public sports facilities, 3.9% of private sports facilities, and 3% of public facilities for people with a disability, and it was found that 81.5% of people with a disability did not use these sports facilities. Furthermore, 37.7% of people with a disability were found to have difficulty exercising alone because they did not use sports facilities. The Korean government has decided to build 150 new sports centers for people with a disability by 2025 to increase their participation in daily sports. The Sports Center for the Disabled, called the Bandabi Sports Center, will be operated as an integrated facility where people with a disability will have the right to use it first, while those with no disability will also use it. This implies that the need to develop exercise products with universal design, which enables people with a disability and those with no disability to participate in the exercise, is gradually increasing. Therefore, the purpose of this study is to develop an aerobic exercise equipment design, which employs a UD that can be used by people with a disability, those with no disability, and the elderly by applying the design thinking method.

Procedure
This study is based on the principle of "design thinking." Self-thinking-based research is research that seeks a problem-solving process centered on people, and it is characterized by providing the process for the optimal combination of problem-solving, business, intuition and logic, etc. [21,22]. Therefore, to design universal exercise equipment, we conducted this study by applying the "design thinking" principle. Although the process of "Design Thinking" is diverse, the "Double-Diamond" process is presented among them, which consists of four steps-"Discover", "Define", "Develop", and "Deliver" [23].  The first step is the discovery step, where ideas are reviewed, reclassified, and selected. In other words, it refers to literature and field research processes such as market research and user research. The second stage is the define stage, where the direction is established. The third stage is the development process, where problem-solving ideas are proposed through various methodologies. The final stage is the delivery stage, which re- The first step is the discovery step, where ideas are reviewed, reclassified, and selected. In other words, it refers to literature and field research processes such as market research and user research. The second stage is the define stage, where the direction is established. The third stage is the development process, where problem-solving ideas are proposed through various methodologies. The final stage is the delivery stage, which refers to the process of deriving and testing the final concept through the process of converging the diffusion of various ideas and applying the design [23]. The overall procedure of the study is shown in Figure 2. fers to the process of deriving and testing the final concept through the process of converging the diffusion of various ideas and applying the design [23]. The overall procedure of the study is shown in Figure 2.

Discover
In this stage, a basic investigation of UD sports equipment was carried out. Market research on sports equipment was conducted, and a focus group interview (FGI) was conducted with trainers at 13 sports facilities used by both disabled and non-disabled people. Through the basic survey, the arrangement status of exercise equipment, problems, and demands for exercise equipment was investigated and analyzed. The characteristics of the participation of the FGI are shown in Table 1. For FGI, data were analyzed according to the FGI content analysis method proposed by Krueger and Casey [24]. First, to rule out study bias, the FGI contents were transcribed in the form of oral transcripts by researchers not subordinated to the study. Second, the transcribed contents were compared with the audio file again, and the observation results were added to the transcript based on the chapter memos developed during the interview process. Third, the data was transcribed into Microsoft ® Office Word 2019 (Microsoft Corporation, Redmond, WA, USA) and read iteratively to analyze the content of the raw data. Then, meaningful words were entered into Microsoft ® Office Excel 2019 (Microsoft Corporation, Redmond, WA, USA) to extract the theme. We grouped the common features of the subjects and then reclassified keyword. By repeating the classification process, we were able to identify and analyze common attributes in the responses in terms of frequency, specificity, emotion, and abundance [25]. The characteristics of the participation of the FGI are shown in Table 1.

Discover
In this stage, a basic investigation of UD sports equipment was carried out. Market research on sports equipment was conducted, and a focus group interview (FGI) was conducted with trainers at 13 sports facilities used by both disabled and non-disabled people. Through the basic survey, the arrangement status of exercise equipment, problems, and demands for exercise equipment was investigated and analyzed. The characteristics of the participation of the FGI are shown in Table 1. For FGI, data were analyzed according to the FGI content analysis method proposed by Krueger and Casey [24]. First, to rule out study bias, the FGI contents were transcribed in the form of oral transcripts by researchers not subordinated to the study. Second, the transcribed contents were compared with the audio file again, and the observation results were added to the transcript based on the chapter memos developed during the interview process. Third, the data was transcribed into Microsoft ® Office Word 2019 (Microsoft Corporation, Redmond, WA, USA) and read iteratively to analyze the content of the raw data. Then, meaningful words were entered into Microsoft ® Office Excel 2019 (Microsoft Corporation, Redmond, WA, USA) to extract the theme. We grouped the common features of the subjects and then reclassified keyword. By repeating the classification process, we were able to identify and analyze common attributes in the responses in terms of frequency, specificity, emotion, and abundance [25]. The characteristics of the participation of the FGI are shown in Table 1.

Define
Based on the content analyzed in the "Discover" stage, concept ideas were presented according to the improvement elements and requirements of the existing exercise equipment through an expert meeting, and the components of each product were identified to classify the ideas. Improvement plans for problems that may occur in the process of using the product concept ideas were derived through the first design concept by classifying ideas into items of usability, safety, economic feasibility, sustainability, and aesthetics according to the major UD elements for each derived idea.
Experts conducted semi-structured interviews with the participants. Interviews were conducted until data was saturated, coded and coded in Microsoft ® Office Excel 2019 (Microsoft Corporation, Redmond, WA, USA), and analyzed thematically through repeated perusal. The validity of the study was determined using the method proposed by Guba and Lincoln [26]. The final results were shared in writing or emailed to 10 experts for review and feedback on what needs to be corrected or deleted. The results confirmed the accuracy of the final study results. The participants of the expert meeting are shown in Table 2. The acceptance of the aerobic exercise equipment design concept idea to which the first UD derived from the 'Define' stage was applied was evaluated. A total of 396 users (131 seniors, 133 disabled people, and 132 non-disabled people) from 13 sports facilities used by people with a disability and those with no disability were selected as the participants. To determine the acceptability, frequency analysis was performed using IBM SPSS Statistics 23 (IBM, Armonk, NY, USA). The characteristics of the first design concept idea acceptance evaluation participants are shown in Table 3. From the evaluation of acceptance of the first design concept idea, two types of 3D models of the treadmill and arm ergometer, which are aerobic exercise equipment based on universal design, were developed. Furthermore, these models were reviewed by experts. Afterward, the design satisfaction was evaluated using 60 users (including both people with a disability and those with no disability) from sports facilities. The satisfaction evaluation was determined by using IBM SPSS Statistics 23 (IBM, Armonk, NY, USA) to conduct frequency analysis. The characteristics of 3D design satisfaction evaluation participants are shown in Table 4.

Deliver
A secondary design concept was prepared based on the design satisfaction evaluation and 3D modeling design concept idea acceptance evaluation results derived through the 'Develop' stage, and the final design was created by modifying and supplementing the design concept through the expert meeting. The analysis method of the results of the expert meeting was the same as the analysis method performed in the "Define" step.

Exercise Equipment Market Research Results
Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Table 5. Aerobic exercise equipment used by both people with a disability and those with no disability.

Exercise Equipment Market Research Results
Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. Market research was conducted on exercise equipment that can be used by both people with a disability and those with no disability. The market research results are shown in Table 5. A focus group interview was conducted on the current status of trainers and aerobic exercise equipment at 13 public sports facilities used by people with a disability and those with no disability together, and the related problems and requirements were also investigated. Table 6 shows the results of the FGI. In the 'Discover' stage, the components of each product of the existing aerobic exercise equipment researched in the market were identified, the ideas were classified, and the FGI results were reflected to derive the concept ideas according to the improvement factors and the user's needs.
To derive the conceptual idea of treadmill, first, the main characteristics and operation methods of treadmill investigated in the existing market were analyzed. Second, through FGI, the concept idea of a treadmill applied with a universal design was developed by reflecting the problems of the existing treadmill and the needs of users. The concept idea of the derived treadmill is shown in Figure 3 and Table 7.      To derive the conceptual idea of an arm ergometer, the main characteristics and operation methods of the arm ergometer investigated in the existing market were primarily analyzed. Second, through FGI, the concept idea of an arm ergometer applied with a universal design was developed by reflecting the problems of the existing arm ergometer and the needs of users. The concept idea of the derived arm ergometer is shown in Figure 4 and Table 8. Table 8. Problems of existing arm ergometer and concept ideas applying universal design.

Problems and needs Access
• Difficulty in recognizing the manual for the visually impaired and the elderly • Difficulty entering the proper location for wheelchair users when using a wheelchair • When a wheelchair user uses a wheelchair, it is difficult to set an appropriate distance between the handle and the wheelchair. • Inconvenient for wheelchair users to move chairs when using a wheelchair • Inconvenient for wheelchair users to move into a chair when using a chair

Deriving the First Design Concept Idea
The design concept idea for aerobic exercise equipment was developed from the first design concept idea by classifying ideas into usability, stability, economic feasibility, sustainability, and aesthetics items according to UD elements. The design concept ideas were derived from six types of treadmills with UD and six types of arm ergometers with universal design. The first design concept idea is shown in Figures 5 and 6.

Deriving the First Design Concept Idea
The design concept idea for aerobic exercise equipment was developed from the first design concept idea by classifying ideas into usability, stability, economic feasibility, sustainability, and aesthetics items according to UD elements. The design concept ideas were derived from six types of treadmills with UD and six types of arm ergometers with universal design. The first design concept idea is shown in Figures 5 and 6.

Concept Idea Acceptance Evaluation in Progress
The acceptance of the first design concept idea was evaluated for 396 users from sports facilities used by both disabled and non-disabled people. The concept ideas were

Concept Idea Acceptance Evaluation in Progress
The acceptance of the first design concept idea was evaluated for 396 users from sports facilities used by both disabled and non-disabled people. The concept ideas were designed for six treadmills and six-arms ergometers, which were based on universal design, and the acceptance of each concept was evaluated. The evaluation results for each concept idea applying UD are shown in Figures 7 and 8.

Design Development of Aerobic Exercise Equipment Based on Universal Design
Among the aerobic exercise products, which were applied with universal design, six treadmills and six arm ergometers were found to have accepted the concept ideas. The acceptance results were reviewed by experts and the ideas were classified into usability,

Design Development of Aerobic Exercise Equipment Based on Universal Design
Among the aerobic exercise products, which were applied with universal design, six treadmills and six arm ergometers were found to have accepted the concept ideas. The acceptance results were reviewed by experts and the ideas were classified into usability, stability, economic feasibility, sustainability, and aesthetics items according to UD elements through a meeting. Based on this content, 3D m dels of two treadmills and two-arm ergometers were created and reviewed by experts. The first 3D modeling is shown in Figures 9 and 10.
Healthcare 2022, 10, x FOR PEER REVIEW 17 of 27 stability, economic feasibility, sustainability, and aesthetics items according to UD elements through a meeting. Based on this content, 3D models of two treadmills and twoarm ergometers were created and reviewed by experts. The first 3D modeling is shown in Figures 9 and 10. Figure 9. First 3D modeling of the treadmill design applying UD. Figure 9. First 3D modeling of the treadmill design applying UD.
Healthcare 2022, 10, x FOR PEER REVIEW 17 of 27 stability, economic feasibility, sustainability, and aesthetics items according to UD elements through a meeting. Based on this content, 3D models of two treadmills and twoarm ergometers were created and reviewed by experts. The first 3D modeling is shown in Figures 9 and 10.

Product Design Satisfaction Evaluation
Product Design Satisfaction Evaluation is based on 3D modeling based on two types of treadmill and ergometers, which are aerobic instruments that apply universal designs to 60 people who participate in an exercise in the gym used by the disabled and nondisabled people. A design satisfaction evaluation was conducted. Satisfaction evaluation evaluated the attribute evaluation and preference type of the product.

Product Design Satisfaction Evaluation
Product Design Satisfaction Evaluation is based on 3D modeling based on two types of treadmill and ergometers, which are aerobic instruments that apply universal designs to 60 people who participate in an exercise in the gym used by the disabled and non-disabled people. A design satisfaction evaluation was conducted. Satisfaction evaluation evaluated the attribute evaluation and preference type of the product.
As a result of the evaluation of treadmill properties, the evaluation of 'type B' was high in most of the properties. In addition, it was found that the participants with disabilities had a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of treadmill product design is shown in Figure 11.
Healthcare 2022, 10, x FOR PEER REVIEW 19 of 27 As a result of the evaluation of treadmill properties, the evaluation of 'type B' was high in most of the properties. In addition, it was found that the participants with disabilities had a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of treadmill product design is shown in Figure 11. The preferred treadmill design was type B (63.3%). The reason for the preference for type B was 'safety of use' (71.1%), followed by 'convenience of use' (57.9%) and durability/strength. (7.9%). The evaluation result of the preferred type of treadmill product design is shown in Figure 12. As for the properties of the arm ergometer, the evaluation of type B was high. Participants with disabilities were found to have a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of the arm ergometer product design is shown in Figure 13. The preferred treadmill design was type B (63.3%). The reason for the preference for type B was 'safety of use' (71.1%), followed by 'convenience of use' (57.9%) and durability/strength (7.9%). The evaluation result of the preferred type of treadmill product design is shown in Figure 12.
Healthcare 2022, 10, x FOR PEER REVIEW 19 of 27 As a result of the evaluation of treadmill properties, the evaluation of 'type B' was high in most of the properties. In addition, it was found that the participants with disabilities had a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of treadmill product design is shown in Figure 11. The preferred treadmill design was type B (63.3%). The reason for the preference for type B was 'safety of use' (71.1%), followed by 'convenience of use' (57.9%) and durability/strength. (7.9%). The evaluation result of the preferred type of treadmill product design is shown in Figure 12. As for the properties of the arm ergometer, the evaluation of type B was high. Participants with disabilities were found to have a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of the arm ergometer product design is shown in Figure 13. As for the properties of the arm ergometer, the evaluation of type B was high. Participants with disabilities were found to have a particularly high evaluation of the 'safety of use' attribute. The result of the attribute evaluation of the arm ergometer product design is shown in Figure 13. As for the preferred arm ergometer design, type B (78.3%) was overwhelmingly high. As the reason for the preference for type B, 'convenience of use' was found to be the highest, followed by 'visual beauty' (30.8%) and 'durability/strength' (19.1%). The evaluation result of the preferred type of arm ergometer product design is shown in Figure 14.

Design Acceptability Verification
Based on the acceptance and satisfaction evaluation results of the first aerobic exercise equipment applying UD, which was first developed in the develop stage, the design concept was prepared by modifying the product design. As for the preferred arm ergometer design, type B (78.3%) was overwhelmingly high. As the reason for the preference for type B, 'convenience of use' was found to be the highest, followed by 'visual beauty' (30.8%) and 'durability/strength' (19.1%). The evaluation result of the preferred type of arm ergometer product design is shown in Figure 14. As for the preferred arm ergometer design, type B (78.3%) was overwhelmingly high. As the reason for the preference for type B, 'convenience of use' was found to be the highest, followed by 'visual beauty' (30.8%) and 'durability/strength' (19.1%). The evaluation result of the preferred type of arm ergometer product design is shown in Figure 14.

Design Acceptability Verification
Based on the acceptance and satisfaction evaluation results of the first aerobic exercise equipment applying UD, which was first developed in the develop stage, the design concept was prepared by modifying the product design.

Design Acceptability Verification
Based on the acceptance and satisfaction evaluation results of the first aerobic exercise equipment applying UD, which was first developed in the develop stage, the design concept was prepared by modifying the product design.
The consideration for applying the UD element to the existing treadmill consisted of a running belt, auxiliary footrest, handle and auxiliary handrail, display, operation button, and treadmill installation method. The second concept design and guideline of the treadmill to which UD is applied are shown in Figure 15 and Table 9.
Healthcare 2022, 10, x FOR PEER REVIEW 21 of 27 The consideration for applying the UD element to the existing treadmill consisted of a running belt, auxiliary footrest, handle and auxiliary handrail, display, operation button, and treadmill installation method. The second concept design and guideline of the treadmill to which UD is applied are shown in Figure 15 and Table 9.    The handle and auxiliary handrail should be of a shape and size that is easy for anyone to hold • Information on the screen should be displayed easily and clearly • Use a readable size for the text on the screen • The text on the screen should use a color considering the clarity • Provide appropriate feedback so that information such as operation status and operation results can be easily recognized • Provide information in a variety of ways, such as visual + auditory, visual + tactile, etc.
Operation button

•
The operation buttons should be easy and intuitive to use • Display the name of the operation button in text and braille • The operation button should be easy to press with little force • The operation button must be of a size and shape that can be operated accurately by all users • The height and position of the operation button should be placed in consideration of the body dimensions of various users • The operation buttons should be placed in consideration of the frequency of use • The emergency safety button should be placed in an easily visible location • The emergency safety button should be marked with a pictogram and highlighted color Etc.
• Provide a sufficient range of safety space around the instrument To apply the UD element to the existing arm ergometer, the saddle, handle, case, display, information board, and installation environment must be considered. The second concept design and guidelines for the UD-applied arm ergometer are shown in Figure 16 and Table 10. To apply the UD element to the existing arm ergometer, the saddle, handle, case, display, information board, and installation environment must be considered. The second concept design and guidelines for the UD-applied arm ergometer are shown in Figure 16 and Table 10.         • Provide a sufficient range of safety space around the instrument • The floor around the device should be made of non-slip material to support the feet

Design Completion
The design concept was revised and supplemented through expert evaluation. Based on the main considerations in the design of each product, the final product design was made by referring to the detailed example images based on the guidelines.
The main features of the final 3D design of the treadmill with UD are shown in Figures 17 and 18. on the main considerations in the design of each product, the final product design was made by referring to the detailed example images based on the guidelines.
The main features of the final 3D design of the treadmill with UD are shown in Figures 17 and 18.
The main features of the final 3D design of the arm ergometer with UD are shown in Figure 17.

Discussion
Using the design thinking process, this study explored five stages of the universal design development process. First, this study examined the demands of sports equipment for people with a disability. Second, it extracted design elements. Third, it evaluated the acceptance and satisfaction with the design by people with a disability. Fourth, an expert evaluation was conducted. Bianco [27] stated that people with physical and cognitive dis- on the main considerations in the design of each product, the final product design w made by referring to the detailed example images based on the guidelines.
The main features of the final 3D design of the treadmill with UD are shown in F ures 17 and 18.
The main features of the final 3D design of the arm ergometer with UD are shown Figure 17.

Discussion
Using the design thinking process, this study explored five stages of the univer design development process. First, this study examined the demands of sports equipm for people with a disability. Second, it extracted design elements. Third, it evaluated acceptance and satisfaction with the design by people with a disability. Fourth, an exp evaluation was conducted. Bianco [27] stated that people with physical and cognitive d The main features of the final 3D design of the arm ergometer with UD are shown in Figure 17.

Discussion
Using the design thinking process, this study explored five stages of the universal design development process. First, this study examined the demands of sports equipment for people with a disability. Second, it extracted design elements. Third, it evaluated the acceptance and satisfaction with the design by people with a disability. Fourth, an expert evaluation was conducted. Bianco [27] stated that people with physical and cognitive disabilities need accessible, usable, and safe designs. Currently, commercialized exercise devices provide insufficient control over safety risks for people with disabilities. When people with a disability use exercise equipment, operational support is needed to control various safety risks, such as the risk of falling while moving or exercising, impact from the use of equipment, and injuries caused by inexperience in operation [28]. Specifically, when a user with a disability performs upper and lower body exercises using an exercise device, the user's weight or driving torque should be considered. In particular, in the case of people with partial paralysis, the left and right muscle strength are different; therefore, it should be designed not to tilt to one side and fall when using the instrument. In addition, the design of exercise equipment should be made in consideration of the body size of the disabled user. In other words, the user should be able to exercise in an optimal posture according to the size of the body, such as the height of the disabled patient and the length of the limbs. This is because if the posture of the disabled user is unstable, it may cause a fracture risk depending on the user. Lezzoni et al. [29] reported that patients fear injury from movement without ancillary equipment. A safety-conscious UD can reduce their fear of injury and enhance their intention to participate in the exercise. Accordingly, many researchers have researched the safety of equipment for the disabled. Agaronnik et al. [30] studied the design of diagnostic equipment for the safety of disabled patients, while Hollis et al. [31] reported increased physical activity in accessible parking spaces with paved sidewalks and curb slopes. Calder et al. [32] also reported that the fitness center's ramp, the height of the device, and the location of the shower room device are designs that threaten the safety of the disabled. Accordingly, activating injury-preventing emotional engineering designs such as active injury-preventing systems, fall-preventing auxiliary handles, and shock-relieving materials is necessary.
Second, the user interface should be improved in terms of convenience. Specifically, for disabled people, it is difficult to check exercise information while using the exercise equipment. Accordingly, voice and visual feedback for confirming exercise information should be provided. Considering the visual and tactile interfaces attached to the exercise equipment is essential. Specifically, if the user is disabled, the user's hands and feet should be able to be fixed to the exercise equipment. In the case of people with hemiplegia disabilities, the limbs of the uncomfortable body part are not strong enough to hold on to the handle of the exercise equipment and to keep the lower body in a fixed position, so additional devices such as straps are needed. In addition, the accessibility of wheelchairs for wheelchair users should be considered. Many previous studies have been studied on the interface of universal design. Accordingly, Arbour-Nicitopoulos and Ginis [33] argued that exercise equipment should be able to secure a wheelchair space and that a system should be equipped to monitor the physical environment. Roger-Shaw et al. [34] stated that UD should provide various means for users to acquire information and knowledge. Baida and Ivanova [35] suggested information provision according to the importance and degree of disability as a guideline. Based on various previous studies, UD for the disabled should be a design that considers auxiliary equipment and accessibility for stable use of equipment, and intuitive information delivery is an essential factor. Convenience may be improved by applying more noticeable colors to areas that require attention during exercise or by marking Braille. It also increases the user's interest in the exercise and lacks providing exercise scenarios for each disability characteristic. The fun factor of exercise devices is a significant factor that affects exercise commitment and continuity. Therefore, building an interface that provides visual and auditory feedback is necessary.
Third, the ergonomic instrument design should be improved. In the case of wheelchair users as well as general users, equipment design should be made so that exercise can be performed at an appropriate location during its usage. Additionally, there is a need for a user-friendly design so that the operation can be performed easily and quickly when using the device. Finally, the instrument design should be centered on user convenience. In the case of handles or controls of exercise equipment, the degree of perception of discomfort varies depending on the type of disability or age, so it is necessary to consider a design that can meet their needs accordingly.
Moreover, these versatile designs are very cost effective and can benefit people with a disability and the general public. Social-ecological design for fitness facilities improves access to fitness facilities and enables proper investment recovery [36]. The total research and development cost of this study was about USD 315,000, and USD 52,000 was spent per unit. This shows a relatively high development cost compared to the development cost of general fitness equipment. However, the optimum design of a product, which considers both people with a disability and those with no disability, can ultimately maximize performance and minimize costs [37]. Applying universal design increases the accessibility of sports facilities by people with a disability, and the resistance of those with no disability to unfamiliar machines is low. Accordingly, UD will be able to fill the gap between sports facilities for people with a disability and general facilities.