# Knee Loading Asymmetries During Descent and Ascent Phases of Squatting After ACL Reconstruction
## 1 Introduction  
Squats are a commonly performed exercise during rehabilitation after anterior cruciate ligament (ACL) reconstruction (ACLR) [1, 2]. During early- and mid-rehabilitation stages after ACLR, intralimb compensations during squats that shift loading toward the hip and ankle joints of the injured limb, as well as shifted loading to the non-injured limb,occur [3,4]. These compensatory movement patterns often persist after patients have completed rehabilitation, as seen up to 7 years after ACLR [5]. Restoring symmetric movement patterns is important because altered knee joint loading is associated with negative outcomes such as a second ACL injury and the development of knee osteoarthritis (OA) [4,6, 7, 8].  
Previous research has focused on loading asymmetries during the entire length of squats (descent combined with ascent phase) in individuals after ACLR. A study performed by Sigward et al. compared the injured vs. uninjured limb of individuals after ACLR during the entire length of squats, finding that individuals exhibited lower peak knee flexion angles, knee extension moments and peak vertical ground reaction forces while squatting at 3 and 5 months after surgery [1]. Through investigations of squat biomechanics by Sanford et al. and Garrison et al., the injured knee of individuals after ACLR was compared to healthy controls during the entire length of squats, with increases in anterior femoral translation relative to the tibia, asymmetric ground reaction forces, increased hip energy absorption contributions and decreased knee energy absorption contributions present [5,9]. While previous studies have demonstrated squat-related asymmetries, the distinct phases of the squat [descent and ascent] impose different demands on the musculoskeletal and neuromuscular systems (e.g., knee extensors elongate during descent and shorten during ascent), warranting phase-specific investigation [10,11]. Only the work performed by Strong et al. described squat asymmetries during the descent and ascent phases of squats [4]. However, their data compared the injured knee after ACLR to a single limb of healthy controls without measures of interlimb asymmetries, reporting greater differences in knee flexion and ankle flexion moments during both the descent and ascent phases at 2.9 months and the descent phase alone at 8.8 months post-ACLR (\(p<0.05\)) [4]. Nonetheless, the comparison of interlimb squat biomechanics during the descent compared to the ascent phase of squats remains absent from the current literature.  
The purpose of this study was to determine if loading symmetries differ between the descent and ascent phases of bilateral squats at 6 months after ACLR and if the length of time during each phase influences squat loading patterns. We hypothesized that interlimb loading asymmetries would be present and similar during the descent and ascent phases of the bodyweight squats. Findings from this study may inform more phase-specific assessment and rehabilitation strategies aimed at reducing persistent movement asymmetries and optimizing knee joint loading after ACLR. Ultimately, reducing asymmetries during functional movements may help decrease the risk of early osteoarthritis and reinjury following primary ACL reconstruction.