## 1 Introduction  
Regular exercise is a key strategy to improve overall health, decrease the risk of musculoskeletal, metabolic, cardiovascular, and neurological conditions, and reduce all-cause mortality [1, 2]. Despite these clear and well-known benefits of exercise, one-third of adults do not meet exercise guidelines [3], and low enjoyment during exercise, among other reasons, can be a powerful factor leading to exercise avoidance [4, 5, 6]. Common components of exercise include stretching [7], and flexibility is generally considered a necessary component of sports and fitness [8]. Lower extremity flexibility, especially, is important for successful performance of sports movements [9] and activities of daily living [10, 11], and is essential for reducing the risk of injury [12, 13]. Hamstring muscle shortening is a major risk factor for knee joint stiffness due to increased patella and femur compression, muscle dysfunction, and cartilage decline [14]. Gastrocnemius and soleus muscles play a major role in weight-bearing activities. Therefore, lower extremity stretching helps decrease leg pressure and cramping, increases range of motion (ROM) in the ankle joint, and improves dynamic balance through an improved ability to lower the center of mass, as well as changes in length and stiffness of muscle tendon units [15; 16; 17; 18]. Additionally, long-term stretching programs can be an alternative to general strength training or rehabilitation exercises in cases where active force production is not possible due to limitations in joint movement [19]. Digital therapeutics (DTx) are evidence-based behavioral treatments delivered online that can increase accessibility and effectiveness of healthcare [20]. They are defined as evidence-based, clinically evaluated software to treat, manage, and prevent a broad spectrum of diseases and disorders [21]. DTx help to prevent, manage, and treat diseases using high-quality software programs. These evidence-based behavioral treatments are delivered online to increase healthcare access and effectiveness [20]. The advent of social media platforms, wearable devices, and cloud-based data platforms has garnered digital health worldwide attention. Health monitoring is gradually expanding from hospitals and treatment centers to the digital world [22]. Programs delivered online through DTx are recognized as a \"secure arena\" by users, which increases self-disclosure and honesty [23], and can improve physical functionality [24; 25; 26]. DTx can also enhance user satisfaction by enhancing usability, providing enjoyment and satisfaction, and improving quality of life through enhanced functionality [24; 25; 26]. Previous studies have shown that patient participation improves the quality of care, but programs that increase patient participation in exercise programs are lacking [27]. However, DTx allow providers to monitor patient adherence and enable patients to independently engage in treatments, providing quality, non-invasive care for a large population [28]. Home-based programs utilizing DTx can improve exercise adherence [26], especially through a push notification [29]. Recently, a digital therapeutic with a calf muscle and hamstring stretching training protocol was developed. There are many studies confirming the effectiveness of enhancing functionality through DTx; however, despite the impact of exercise satisfaction and adherence on exercise effectiveness, factors such as satisfaction with the exercise program, enjoyment, immersion in exercise, motivation for exercise, and exercise adherence lack sufficient research confirmation [27; 30; 31; 32]. Low enjoyment during exercise contributes to physical inactivity and exercise avoidance in healthy adults [4; 5; 6], and despite the fact that DTx can provide exercise enjoyment [25], few studies have identified a correlation between DTx use and satisfaction in healthy adults. Additionally, few studies have identified pure adherence without researcher intervention [26].