Waveform-Level Validation of Continuous Shank-to-Vertical Angle Measurement Using a Wearable Posture Sensor with Independent Video-Based Gait Event Detection.

Accurate evaluation of the shank-to-vertical angle (SVA) is important for optimizing lower-limb alignment during gait, particularly during ankle-foot orthosis (AFO) tuning. This study investigated the validity of a wearable posture sensor for continuous SVA measurement during walking using an optical motion capture system as the reference standard. Nine healthy adults participated, and 88 gait cycles were analyzed. SVA was measured using a shank-mounted wearable sensor and a three-dimensional motion capture system. Gait events for the wearable sensor were identified independently using synchronized tablet-based video recordings rather than inertial signals. Agreement was evaluated at the gait-cycle and participant levels using waveform correlation, mean bias, root mean square error (RMSE), and Bland-Altman analysis. Across the full gait cycle, the trial-level waveform correlation was 0.935 ± 0.064 and the RMSE was 9.74 ± 3.71°. During the individually identified stance phase (mean toe-off, 63.67 ± 1.78% of the gait cycle), waveform correspondence increased to 0.990 ± 0.010 and the RMSE decreased to 6.78 ± 3.51°. Participant-level estimates were similar. For SVA range, participant-level Bland-Altman analysis yielded a bias of -5.58° with 95% limits of agreement from -14.01° to 2.86°, and repeated-measures analysis produced similar estimates. Temporal error analysis showed smaller and more stable deviations during the majority of the stance phase than during swing, with larger deviations in late swing. These findings support the potential use of the wearable posture sensor as a targeted tool for stance-phase SVA assessment, although further validation of pathological gait and actual orthotic tuning is required.