Background Proprioception plays an important role in knee movements.Since there are controversies surrounding the overall recovery time of proprioception following surgery,it is necessary to define the factors affecting proprioceptive recovery after anterior cruciate ligament (ACL) reconstruction and to investigate the relationship between proprioception and muscle strength.Methods A total of 36 patients who had their ACL reconstructed with a semitendinosus/gracilis graft (reconstructed group:6 months post-surgery) and 13 healthy adults without any knee injury (control group) were included in the study.Knee proprioception was evaluated with a passive reproduction test.Isokinetic strength was measured using the Biodex System.Statistical analysis was used to compare proprioception of the reconstructed group versus the control group,and to define causal factors,including sex,hamstring/quadriceps ratio,and the course of injury before reconstruction.We also investigated the correlation between the passive reproduction error and quadriceps index.Results There was a significant difference in proprioception between the reconstructed and control groups (P ＜0.05).When the course of injury before reconstruction was less than 4 months,there was a linear correlation with proprioception 6 months after the operation (r=0.713,P ＜0.05).There was a positive correlation between post-surgery proprioception and the quadriceps index at 6 months post-surgery.Conclusions Impaired knee proprioception is observed 6 months after ACL reconstruction.Within 4 months of injury,early undertaking of reconstruction is associated with better proprioception outcome.Patients with enhanced proprioception have a better quadriceps index.