# Knee Loading Asymmetries During Descent and Ascent Phases of Squatting After ACL Reconstruction
## 4 Discussion  
Our results indicate that at 6 months after ACLR, participants continue to exhibit deficits in knee loading symmetry during both the descent and ascent phases of squatting. Initially, greater asymmetry was observed during the ascent phase, as evidenced by vGRFi, KFMi and HKRi, compared to the descent phase. Specifically, the injured limb demonstrated approximately 30% lower external knee flexion moment impulse compared to the uninjured limb, along with nearly twice the hip flexion moment contribution relative to knee flexion moment contribution (HKRi). However, given that the ascent phase was performed at a faster speed than the descent (4% faster), we controlled for movement time to account for potential confounding. After adjusting for time, interlimb loading asymmetries persisted, but the previously observed difference in asymmetry between the ascent and descent phases was no longer statistically significant.  
The findings of our study align with and expand upon previous research. Sigward et al., Sanford et al. and Garrison et al. reported asymmetries when analyzing the squat as a single movement, while Strong et al. identified phase-specific asymmetries when comparing the limb of participants after ACLR to a healthy control limb [1,4,5,9]. In our study, asymmetries were evident in both phases; however, they were more pronounced during the ascent phase, which may be attributed to several factors. First, participants were instructed to perform squat sets without standardized control over time under tension and phase-specific velocity. Our findings indicate that differences in movement velocity across phases may influence an individual's control over the squatting movement, particularly during the ascent phase, thereby highlighting pre-existing asymmetrical movement patterns. Additionally, neuromuscular control of the lower limb and muscle activation are critical factors in squat execution during rehabilitation after ACLR. It is well established that neural control patterns differ across eccentric compared to concentric muscle contractions, with a lower rate of motor unit recruitment required under equal force conditions during eccentric versus concentric muscle contraction [21]. ACL injury and subsequent ACLR often result in arthrogenic muscle inhibition (AMI), a neural impairment that restricts proper quadriceps activation in the affected limb that is still present up to 2 years after ACLR [22,23]. Given that greater quadriceps activation is required during the ascent phase compared to the descent phase of a bodyweight squat, reduced quadriceps neuromuscular activation may contribute to the greater asymmetry observed in this phase [24]. Finally, the depth of the squats was selected at the discretion of the participants according to their comfort and ability. Deeper squats have been associated with increased muscle activation, particularly in the quadriceps and gluteus maximus [25]. Therefore, variations in squat depth, along with potential improper muscle activation, may contribute to greater asymmetries during the ascent phase compared to the descent phase.