## 2 Related Work
### The Usability and Acceptance of HMD-Based VR Exergames in Older Adults  
In a systematic review by [PERSON] et al. evaluating the effectiveness and feasibility of the use of VR/gaming systems by older adults at home to enable physical activity, the authors assessed the evidence as weak with a high risk of bias. In studies analyzing feasibility, strong retention (\(\geq\)70%) and adherence (\(\geq\)64%) were reported [18]. Nevertheless, this systematic review must be put into perspective since rapid technical development means that completely different systems are available in the current commercial market. For example, [PERSON] et al. did not consider HMD technology in 2014. [PERSON] et al. [19] analyzed the usability, user experience (UX), and feasibility of VR clinical systems with older adults in a more recent systematic review. Regardless of a number of technical and interactional issues, the included studies showed that the usability of clinical VR systems is good, well-accepted, appropriate, effective, and useful [19]. However, the findings indicated that non-HMD systems are considered better for older people. [PERSON] et al. [20] assessed the acceptance of VR HMDs with 76 older adults. The participants positively accepted and tolerated the VR HMD. The self-reported cybersickness was minimal and was not associated with VR exposure. Nevertheless, the digital divide between younger and older adults may inhibit older adults from achieving the benefits of immersive technologies [21]. Older people are confronted with individualized challenges when using VR or AR. They vary widely in terms of their use of and attitudes toward technology. Therefore, developers of VR/AR software and hardware should consider individual needs and skills [22; 23]. Physical and cognitive abilities can vary among older adults. Thus, mobility impairments, dementia and visual or hearing losses may occur. Especially in VR, these can lead to an increased risk of falls or limited use of the application. Therefore, it is important that these are considered in the development of VR/AR systems [23]. Initial analyses also showed a high acceptance and usability in relation to sports activities with a VR HMD system [24; 25; 26]. The Virtual Park, a VR-based system on a cycle-ergometer, aims at improving the performance of elderly patients during endurance exercise training. The training was found to be feasible and positively accepted by elderly people with chronic respiratory diseases. After a single session, the preliminary results for a group of eight patients demonstrated excellent usability and high acceptability [27]. A canoeing game was also tested on older people as a VR HMD. The results indicated that Canoe VR was received very well and can be utilized as an additional fitness tool [28]. Other types of training outside of the endurance category have been considered in only a few investigations involving older adults and HMD-based VR. [PERSON] et al. employed virtual reality-based high-intensity interval training for pulmonary rehabilitation in their feasibility and acceptability study [29]. The use of VR in their study did not lead to cardiovascular demands, higher dyspnea, tension or any serious adverse events or severe cybersickness. A few works have examined physical activity over a longer period of time in multiple training sessions. In a pilot study by [PERSON] et al. [30], 30 older adults tested nine VR applications that promote physical activities, among other tasks, for 15 min twice a week for six weeks. The older adults perceived VR as useful, easy to use, and an enjoyable experience. In an investigation by [PERSON] et al. [31], training for 30 min three times a week for six weeks in the VR was recommended for older individuals with disabilities. Most of the 135 participants accepted the VR training, and 65.2% provided positive comments. In some works, immersive HMD VR systems have been compared with traditional video exercises. [PERSON] et al.