Smartphone Instrumented 30-Second Chair Stand Recovers Body Composition and Strength Biomarkers

The 30-second chair stand test is widely used in geriatric assessment, yet clinical practice only records the repetition count, discarding the movement information within each cycle. This manuscript reveals that a smartphone held at shoulder level during the test can not only count stand-sit cycles, but also recover information related to body composition and strength. In a study of 78 adults, movement features extracted from the smartphone inertial measurement unit (IMU) signal during a chair stand test were used in simple regression models to predict clinical metrics from a dynamometer and dual-energy X-ray absorptiometry (DXA) scanner. Combined with height and weight to compute force and inverse kinematics, the smartphone-derived features predict dominant grip strength with Pearson R = 0.83, percent body fat with R = 0.62, and total lean mass with R = 0.92. Even without height and weight, the IMU-only features significantly exceed the repetition count alone performance for all three outcomes (p ≤ 0.02).