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The wide variations in patient demographics and concomitant injuries make the prediction of which patients will regain strength quickly following anterior cruciate ligament (ACL) reconstruction challenging. This study aimed to identify the dominant factor affecting quadriceps strength recovery after ACL reconstruction with a hamstring tendon autograft and to construct a predictive model for quadriceps strength recovery using decision tree analysis.Three hundred and eighty-six patients who underwent ACL reconstruction with a hamstring tendon autograft were included in this study. The isokinetic quadriceps strength at 60°/s was measured preoperatively and at 6 months after surgery. The quadriceps strength index (QSI) was calculated by normalising the peak torque of the involved leg with the uninvolved leg and multiplying it by 100. A stepwise multiple linear regression and a decision tree analysis were performed to assess whether or not the following parameters affect quadriceps strength recovery at 6 months: socio-demographic data and maximum isokinetic quadriceps strength.The preoperative QSI, age, and pre-injury Tegner activity scale were independently correlated with quadriceps strength recovery at 6 month after surgery. The decision tree analysis demonstrated that patients were expected to have a QSI ≥85% at 6 months after surgery if they met one of the following conditions: (1) age23 years with a preoperative QSI ≥78.8%, (2) age ≥29 years with a preoperative QSI ≥98.0%, or (3) age16 years with a preoperative QSI58.5% and pre-injury Tegner activity scale ≥9. By contrast, patients ≥29 years with a preoperative QSI98.0% were likely to achieve a quadriceps strength index70% at 6 months after surgery.Based on the results of the decision tree analysis, younger patients could achieve good quadriceps recovery even if they have a lower preoperative QSI, whereas patients ≥29 years need a higher preoperative QSI to obtain good muscle recovery.

Patients typically return to sports activities 6–12 months after anterior cruciate ligament (ACL) reconstruction, and quadriceps strength has been used as one of the criteria to return to sports activities. The purpose of this study is to identify the factors that affect quadriceps strength recovery after ACL reconstruction with a hamstring tendon autograft 6 months after surgery. Isokinetic quadriceps strength at 60°/s was measured preoperatively and 6 months after surgery in 193 athletes who underwent ACL reconstruction with a hamstring tendon autograft. The quadriceps strength index was calculated by normalizing the peak torque of the involved leg with the uninvolved leg. The subjects were divided into two groups according to the quadriceps strength index 6 months after surgery: at least 85 % (n = 84) or less than 85 % (n = 109). Multivariate logistic regression analysis and a receiver operating curve analysis were performed. The preoperative quadriceps strength index [odds ratio (OR) 1.02; 95 % confidence interval (CI) 1.01–1.03], age (OR 0.92; 95 % CI 0.86–0.98), sex (OR 2.45; 95 % CI 1.19–5.18), and knee pain (OR 0.17; 95 % CI 0.04–0.52) were independently associated with quadriceps strength recovery. The cut-off value of the preoperative quadriceps strength index was 70.2 % to obtain at least 85 % quadriceps strength index 6 months after surgery. To achieve sufficient quadriceps strength recovery at the timing of return to sports activities following ACL reconstruction using a hamstring tendon autograft, preoperative quadriceps strength, age, sex, and knee pain should be considered. III.