The gait lab effect: symmetry restoration strategy after anterior cruciate ligament reconstruction is different in natural environments than the gait laboratory

Inertial measurement units (IMUs) offer a promising pathway to extend functional-recovery assessment after anterior cruciate ligament reconstruction (ACLR) beyond laboratory or in-clinic evaluations. Yet, it remains unclear how joint kinematics change in natural environments over time. In this study, 26 participants wore five sticker-like IMUs on the pelvis, thighs, and shanks for up to seven days, three and nine months after their surgery. They also participated in a laboratory assessment at each timepoint using standard marker- and inertial-based motion capture systems. Knee extension excursion (KEE) was estimated, and inter-limb KEE asymmetry was used as the primary outcome. Median KEE asymmetry decreased significantly from 7.7° at three months to 2.6° at nine months post-surgery, through a modest, 1.0° increase in the reconstructed KEE and a more salient reduction of 4.3° in the contralateral KEE. In the laboratory, however, only a 2.2° decrease in KEE asymmetry was observed, attributed to a 3.3° increase in reconstructed KEE and no significant change in the contralateral KEE. Sensitivity analyses revealed that findings were robust to walking-bout duration and number of days included in the analysis, and all reported group changes in KEE asymmetry exceeded disagreement between IMU- vs. marker-based tracking. Participant compliance to remote monitoring was high, with nearly 95% of the sessions completed. Together, these findings demonstrate that joint-level biomechanics can be captured longitudinally in natural environments after ACLR and that ecologically valid monitoring outside the laboratory can reveal aspects of functional recovery not observed in controlled settings.