Speeding Up or Slowing Down: The Effect of Decoupling Speed on Knee Biomechanics and Limb Loading During Split-Belt Treadmill Training in Persons With ACL Reconstruction

BACKGROUND: Surgical limb underloading is a common biomechanical adaptation after anterior cruciate ligament reconstruction (ACLR) and has been linked to early degenerative changes in knee cartilage, which are considered precursors to posttraumatic osteoarthritis. Split-belt treadmill training is an emerging rehabilitation approach that modifies load through asymmetric gait behavior, in which one limb walks faster than the other. While previous research has indicated that split-belt treadmill training can modify limb loading, its effects on post-ACLR biomechanics remain unexplored. PURPOSE/HYPOTHESIS: The purpose of this study was to examine the effects of decoupling speed on knee biomechanics and limb loading and model their relationship. It was hypothesized that at faster decoupling speeds, knee loads would increase, and at slower speeds, they would decrease. STUDY DESIGN: Controlled laboratory study. METHODS: Knee joint biomechanics were evaluated while 24 participants (15 females; mean age, 23.5 ± 6.5 years; mean height, 1.72 ± 0.08 m; mean mass, 75.61 ± 13.83 kg; mean postoperative time, 7.94 ± 1.74 months) with ACLR walked on an instrumented treadmill (2000 Hz) synced with a 12-camera motion capture system (200 Hz). Participants completed 5 minutes of baseline walking at 1.1 m/s on tied treadmill belts. Afterward, the authors manipulated the speed of the belt under the ACL-reconstructed leg with 8 randomized 5-minute decoupled speed configurations (ACL-reconstructed limb at 30%-170% of 1.1 m/s in 20% increments) with 2 minutes of tied-belt walking between each speed. Bilateral sagittal plane knee moments, angles, and vertical ground-reaction forces were calculated. Statistical parametric mapping was used to evaluate the effects of decoupling speed on the dependent variables during the stance phase of the gait. RESULTS: A clear dose-response relationship between decoupling speed and knee/limb loading was found. Specifically, faster decoupling speeds (130%-170% of 1.1m/s) generally increased peak loads and slower speeds (30% and 50% of 1.1 m/s) decreased peak loads during early stance compared with tied-belt walking for both limbs. In contrast, slower decoupling speeds (30%-90% of 1.1 m/s) led to higher bilateral knee/limb loads at midstance, while faster decoupling speeds (130%-170% of 1.1 m/s) resulted in less knee/limb loading during midstance. CONCLUSION: These findings suggest that split-belt treadmill training offers a promising method for modulating knee/limb loading post-ACLR. CLINICAL RELEVANCE: Split-belt treadmill training could be a viable intervention to target the loading asymmetry that is prevalent in persons post-ACLR.