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INTRODUCTION: Kinetic loading asymmetries during bilateral squats have been reported following anterior cruciate ligament reconstruction (ACLR). Evidence is however limited to discrete value data extracted at specific knee angles from cross-sectional studies where side-toside strategies are presented only at group level. It is therefore unclear whether loading asymmetries occur throughout the entire squat, whether they change during rehabilitation and how they are distributed between sides.

METHODS: Bilateral bodyweight squats were performed by 24 individuals (13 females) post-ACLR on three occasions: 1) Early rehab - 2.9(1.1) months; 2) Mid-rehab - 8.8 (3.1) months; 3) Return to sport (RTS) - 13.1 (3.6) months; and 29 asymptomatic controls (22 females) on one occasion. Motion capture and two force plates were used to calculate time-normalized curves of vertical ground reaction forces andhip, knee, and ankle moments. Outcomes were compared between sides and groups, and over time, using functional t-tests with p-values adjusted by the interval-wise testing procedure. Individual knee loading strategies, i.e., under-/overloading of the ACLR side, were classified when asymmetry in favour of the respective side exceeded the 95% pointwise confidence interval of controls during at least 50% of the squat.

RESULTS: At Early rehab, ACLR had significantly greater (adjusted P < .05) asymmetry in knee flexion moment than controls during thetime-normalized interval of 15-100% of the eccentric phase and the entire concentric phase, as well as ankle flexion moment during 56-65% of the concentric phase. At Mid-rehab, ACLR had significantly greater asymmetry than controls for knee flexion moment during 41-72% of the eccentric phase and for ankle flexion moment during 56-69% of the concentric phase. No significant between-group differences were found at RTS. At RTS compared with Early rehab, ACLR significantly reduced asymmetry for hip (21-46%, eccentric phase), knee (27-58%, concentric phase), and ankle flexion moment (21-57%, eccentric phase). Individual asymmetry strategies for knee flexion moment atEarly rehab were mainly due to individuals underloading the ACLR side (46%) rather than overloading (4%). At RTS, however, more individuals overloaded (25%) than underloaded (17%) the ACLR side.

CONCLUSION: Curve analyses revealed significant kinetic loading asymmetries throughout bilateral bodyweight squats for our ACLR group compared with controls at Early rehab and Mid-rehab, but not at RTS. Significant reductions in hip, knee and ankle flexion moment asymmetry from Early rehab to RTS showed modifications during rehabilitation. An expected underloading strategy was evident for almost half of the ACLR participants at Early rehab, but an overloading strategy was the main reason for knee flexion moment asymmetry at RTS.Loading asymmetries during bilateral tasks should thus not be assumed due to underloading of the ACLR side, but may depend on anoverloading strategy, particularly late in rehabilitation.