Real-World Functional Gait Recovery Trajectories After Total Hip and Total Knee Arthroplasty via Smartphone Sensors: Observational Longitudinal Study (Preprint)

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BACKGROUND Recovery after total hip (THA) and total knee arthroplasty (TKA) is typically assessed with infrequent clinic-based evaluations that may not reflect patients’ everyday mobility. Smartphone inertial sensors enable scalable, repeated gait measurement in real-world settings, but the extent to which prompted walking tests reflect daily life walking during recovery is not well defined. OBJECTIVE The objective of this study was to characterize week-by-week recovery trajectories after THA and TKA using high-frequency smartphone sensor data and quantify the signal divergence between prompted in-app walking tests and passive daily life background walking. METHODS Methods We analyzed longitudinal smartphone-based gait measurements from 743 patients (THA, n=355; TKA, n=388) who underwent unilateral surgery between July 2022 and October 2025. Nine spatiotemporal gait parameters were monitored over the first 15 postoperative weeks across two conditions: prompted "in-app" walks and passively captured "background" walks. Recovery trajectories were evaluated using Kaplan-Meier time-to-baseline analyses, linear mixed-effects models, and two-sample t-tests comparing late-stage plateau levels (weeks 11-15). RESULTS Across all parameters, THA patients returned toward preoperative levels faster than TKA patients, with mean trajectories approaching baseline by week 4 for THA versus weeks 6-8 for TKA. Linear mixed-effects models showed that surgical procedure type significantly influenced recovery for all parameters except single support of the unaffected leg. Several metrics, including walk speed, cadence, and stride length, exhibited persistent differences through the late-recovery plateau (weeks 11-15). We also compared prompted "in-app" walks with passively captured background walking. Prompted and passive walking showed systematic divergence in both THA and TKA; prompted assessments generally indicated faster or more complete recovery than passive daily life walking. Finally, preoperative gait level in all conditions was strongly associated with postoperative trajectories, with baseline-stratified analyses showing distinct patterns of recovery between groups. CONCLUSIONS High-frequency smartphone gait assessment quantifies procedure-specific recovery dynamics and demonstrates that prompted tests and passive daily life measurements capture non-interchangeable functional signals. These findings suggest that declaring "full recovery" based solely on prompted functional capacity may overlook lingering habitual performance compensations.

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