Smartphone-Based Ankle Stability Assessment and Real-Time Biofeedback Training Using an Open-Source Blockly-Based Programming Environment (PhyphoxTM): Design and Preliminary Metrological Validation

While ankle proprioception training on wobble boards is widely used in chronic ankle instability rehabilitation, objective and real-time instrumented assessment and biofeedback are still largely restricted to expensive laboratory-grade platforms or custom hardware. The aim of this study was twofold: (1) to present the design of a free, open-source smartphone-based system for ankle stability assessment and real-time biofeedback training; (2) to assess its accuracy in tracking the dynamic tilt of a wobble board during a single-leg stability test. The system was implemented through the Blockly-based visual programming editor of phyphox and deployed via the phyphox app. The smartphone’s quaternion-based attitude sensor was used to measure wobble board tilt in the mediolateral (roll) and anteroposterior (pitch) directions at 100 Hz, computing stability metrics and providing real-time audio biofeedback triggered by a user-defined isotropic threshold. Validation against the Biodex Balance System SD was performed in eleven healthy participants performing a single-leg stance protocol with both legs. Biofeedback was disabled to isolate tilt measurement accuracy. Roll and Pitch waveform similarity and accuracy were quantified using the linear fit method and root mean square difference (RMSD), respectively. The linear fit method yielded mean R2 of 0.929 ± 0.043 and 0.958 ± 0.038, amplitude ratios (a1) of 0.943 ± 0.038 and 0.953 ± 0.039, and offsets (a0) of −0.000 ± 0.029° and −0.004 ± 0.046° for Roll and Pitch respectively. RMSD was 0.275 ± 0.195° and 0.246 ± 0.210°, indicating high validity in tracking wobble board tilt. The experiment is freely available as a QR code for direct use through the phyphox app and as an editable source file.