Individuals with anterior cruciate ligament reconstruction (ACLR) have lasting impairments in movement patterns during walking (Kaur et al., 2016, Slater et al., 2017). For example, individuals with ACLR adopt walking patterns to offload demands for knee stability, which are characterized by more extended knee angles, lower knee flexion range of motion, and smaller knee extensor moments during stance (Davis-Wilson et al., 2019, Garcia et al., 2022, Kaur et al., 2016). Less is understood, however, about movement impairments above and below the knee after ACLR as many investigations are isolated to single-joint assessments focused mainly on knee-specific outcomes. While insightful, these assessments neglect the simultaneous motion occurring between adjacent joints (i.e., ankle or hip) throughout stance, limiting the ability to identify the secondary compensations that may be employed to accommodate impaired knee mechanics (Freedman Silvernail et al., 2018). In fact, some research has shown those with ACLR have different inter-limb coordination patterns between limbs and compared with controls (Armitano-Lago et al., 2024, Davis et al., 2019, Dennis et al., 2024, Gribbin et al., 2016, Samaan et al., 2022). Specifically, there is altered inter-joint coupling between the knee-hip in ACLR limbs in which inadequate knee extension may require additional hip motion during stance (Samaan et al., 2022). Importantly, altered hip-knee coupling was associated with worse patellofemoral cartilage composition (Samaan et al., 2022), suggesting impaired inter-joint coordination may contribute to future post-traumatic OA risk.

While above-mentioned evidence has unveiled altered coordination across the lower limb after ACLR studies are limited to mainly kinematic outcomes, which neglect adaptations that may occur in kinetic coupling of joints adjacent to the knee. It is probable the altered inter-joint coordination patterns displayed by persons with ACLR may be accompanied by redistribution of support and propulsion contributions from ankle, knee and hip joints, particularly given the profound muscle weakness persisting in this population. The total support moment (TSM) during walking offers a kinetic marker that complements joint coupling analyses (Winter, 1980) via the summed contributions of ankle, knee and hip moments in the sagittal plane. Lower TSM and alterations in joint-specific contributions to TSM have been observed in those with ACLR and in individuals with knee OA relative to controls (Harrison et al., 2023, Nawasreh et al., 2024, Zeni and Higginson, 2011). Thus, evaluation of TSM provides complimentary assessment to robustly evaluate how joint coordination influences functional characteristics of gait (e.g. force attenuation, propulsion). In neurological populations (i.e., Prader-Willi Syndrome) there are alterations in inter-joint coordination patterns that include a redistribution of joint demands across the limb both in support and propulsive phases (Pamukoff et al., 2022). However, no studies to our knowledge have evaluated if inter-joint coordination at the hip/knee or ankle/knee is redistributed to support demands away from the ACLR knee during gait.

The purpose of this study was to evaluate sagittal plane inter-joint coordination and lower-limb support across the ankle, knee and hip during walking in those 9–12 months after ACLR. Differences in joint coordination and support were evaluated between 1) ACLR and non-ACLR Limbs and 2) ACLR and control limbs. We hypothesized that 1) the ACLR limb would express joint coordination patterns at the ankle-knee and knee-hip motions consistent with less motion at the knee throughout stance compared to non-ACLR and control limbs and 2) the ACLR limb would exhibit smaller knee and greater ankle/hip contributions to support and reduce the TSM compared to non-ACLR and control limbs.