## 4 Discussion  
To the best of our knowledge, this is the first study to investigate the reliability and validity of GRF AI in community-dwelling adults at seat-off using a low-cost force plate. The results of this study indicate that the GRF AI has high intrasession and intersession intrarater reliability and poor validity for lower limb muscle strength. The GRF AI is a newly developed method of measuring functional lower limb muscle strength using STS and a low-cost force plate. In contrast to other tests using STS, the GRF AI could measure unilateral lower limb muscle strength and is less vulnerable to balance problems. Before our study, [PERSON] et al. [28] performed a study using a force plate during chair-rising in active older adults. They revealed that peak power and velocity during chair-rising were reliable and valid indicators of lower body power. Meanwhile, our study was significant in that it provided a novel measurement method and results by defining GRF AI. It is expected that the GRF AI enables the ready measurement of the lower extremity muscle strength for not only the general population but also the patients who have asymmetric lower extremity weakness. High reliability between test-retest is the most important factor in new test methods [29]. In this study, the GRF AI showed high reliability within the measured values of each test performed three times on the same session and between test session 1 and 2, with no significant difference in the results. Moreover, the single-time test-retest reliability was confirmed as high, indicating that even one test result is useful. This indicates that the test protocol is sufficiently familiarized for the test. Confirming the high reliability of this test demonstrates the possibility that it can be used as a diagnostic or patient evaluation method. To date, there has been no objective evaluation scale of lower extremity muscle strength that can be used in the clinical setting; therefore, muscle strength or functional status was indirectly predicted by grip strength [30]. STS performance is a basic motion required in daily life that is easy for the participant to perform, and it can be used to directly measure lower extremity muscle strength [12; 31]. The 30-s STS test (30 STS) records the maximum number of STS movements a participant can perform in 30 s [32]. It can be used to assess exercise capacity, and previous studies have revealed that 30 STS appears suitable to evaluate exercise tolerance. However, the participant can easily experience fatigue after 30 STS [33]. The five-repetition STS test (FRSTST) measures the time taken to complete five repetitions of STS movement and is used as an indicator of lower extremity strength [34]. FRSTST can be easily applied in various settings and provides an objective measurement of lower extremity muscle strength; however, age, body weight, and stature influence FRSTST and should be considered [35]. Furthermore, some participants face difficulty completing the evaluation, which is a limitation [36]. Previous studies on test-retest reliability reported that grip strength has an ICC of 0.91-0.95 [37], while that of 30 STS is ICC 0.87 [38], and FRSTS is ICC 0.64\(\sim\)0.96 [34]. Even when compared with these results, high reliability is confirmed by test-retest of the GRF AI. There are several advantages of the GRF AI compared with the conventional STS test. First, it is less susceptible to balance-related issues because of the inclusion of the wall touch. [PERSON] recommend that participants rest hands on the wall to reduce instability [10]. Second, it is less susceptible to endurance issues of participants because