Objectives: To explore arthrogenic muscle inhibition (AMI)’s impact on mobility after anterior/posterior cruciate ligament (ACL/PCL) reconstruction, identify early predictors of postoperative stiffness, and inform clinical rehabilitation. Methods: In this prospective cohort study, 146 ACL/PCL reconstruction patients were split into stiff (passive flexion 5° at 12 weeks [T2]) and non-stiff groups. Preoperative data, knee function (ROM, gait, muscle strength, proprioception) and AMI indicators (quadriceps VA, H/M ratio) were assessed at 4 weeks (T1) and T2. Boruta, multivariate logistic, and multiple linear regression analyzed predictors. Results: The stiff group had longer injury-to-surgery intervals, higher patellar tendon use, worse ROM/gait/strength/AMI metrics at T1-T2. The four most important predictors for T2 stiffness were T1 passive flexion, extension deficit, H/M ratio, and VA; T1 H/M (OR=4.82) and extension deficit (OR=3.85) were major risks. T2 H/M correlated negatively with passive flexion (β=-0.73), T1 VA with extension deficit (β=-0.38). The logistic model demonstrated good predictive performance (optimism-corrected C-index = 0.890). Conclusions: AMI associates with post-ACL/PCL mobility issues. Four early indicators predict T2 stiffness, with H/M impacting flexion and VA affecting extension, guiding personalized rehabilitation.