Background/Objectives: Lower-limb muscle function is traditionally assessed with tests like the five times sit-to-stand test (5STS) and the 30-s chair stand test (30CS). The ADR Encoder, a linear transducer, measures power and velocity during the sit-to-stand test. This study aimed to validate its use as functional indicators of muscle function in older adults. Methods: A cross-sectional study with 114 participants (mean age: 72.92 ± 4.97 years) was conducted. The 5STS, 30CS, power, and speed were measured using the ADR Encoder. Participants were categorized into frail, pre-frail, and robust groups according to the FRAIL Scale. Criterion, intraobserver, and interobserver validity were assessed. Results: Strong correlations (r > 0.7; p < 0.001) were found between power, speed, and both 5STS and 30CS. The interobserver intraclass correlation coefficient (ICC) for speed was 0.90 and the intraobserver ICC was 0.85. For power, the interobserver ICC was 0.88 and the intraobserver ICC was 0.79, all statistically significant (p < 0.001). Significant differences in power (7.78 W vs. 11.26 W; p < 0.001) and speed (0.40 m/s vs. 0.58 m/s; p < 0.001) were observed between frail and non-frail participants. Conclusions: The ADR Encoder is a valid, reliable tool for measuring power and speed in older adults. Its strong correlations with the 5STS and 30CS demonstrate its clinical relevance, with high reproducibility across observers and time points. Its application in diverse clinical and community settings could broaden its utility in geriatric functional assessment.
Keywords: aged; cross-sectional studies; frailty; geriatric assessment; muscle strength; physical fitness; reproducibility of results; validation study.