Knee joint underloading does not evolve after a two-week reintroduction to running program after anterior cruciate ligament reconstruction

Objectives: Knee underloading patterns have been reported mid- and long-term after return to running post-ACLR, but changes in these patterns during the reintroduction to running are unknown. We evaluated knee biomechanics in individuals within 6 months of ACL-R at the start and completion of a reintroduction to running program. Design: Longitudinal laboratory study. Setting: Three-dimensional running biomechanics during instrumented treadmill running. Participants: 24 participants post-ACL-R with hamstring autograft and 24 healthy, matched controls. Main outcome measures: Tibiofemoral joint (TFJ) and patellofemoral joint (PFJ) contact forces, peak knee extension moment and peak knee flexion angle. Results: Significant LIMB∗GROUP interactions (all p < 0.05) but no TIME effects were found. PFJ and TFJ contact forces, peak knee flexion angle and peak knee extensor moment were lower (all p < 0.001) on the injured-limb compared to both contralateral-limb and CONTROL. PFJ and TFJ contact forces and peak knee flexion, knee extension moment were greater (all p < 0.01) on the contralateral-limb of ACL-R compared to CONTROL. There was no change in knee biomechanics after two weeks of the reintroduction to running. Conclusions: Clinicians should be aware that substantial and persistent knee underloading does not resolve upon reintroduction to running after ACL-R. Level of evidence: Longitudinal observational study, level III.