###### Abstract
Most of them use LSI for bilateral limb comparison or additional comparison to normative data of healthy controls. Some of the most common assessments are muscle strength measurements, hop tests, agility tests, Y-balance test, tuck jump assessments, or landing error scoring systems [21]. High incidence of secondary ACL injuries has been the matter of debate in the literature regarding the appropriate use of postoperative LSI in currently recommended RTS muscle strength testing criteria [22; 23]. Several studies have shown that athletes passing the objective RTS muscle strength assessment are at a lower risk of any knee injury, contralateral ACL injury, and/or ACL graft reinjury [15; 23; 24]. Persistent knee extension (quadriceps) muscle weakness is the most commonly reported impairment 6 months after ACLR; longitudinal studies have shown, however, that they can persist up to 3 years and longer [9; 25; 26; 27]. [PERSON] et al. [15] have shown that the degree of muscle strength deficit in knee extensor is more significant than hop testing in preventing a secondary ACL injury. Being unable to produce sufficient knee extensor force is also associated with changed knee biomechanics during hopping and subjective hop testing assessment is not sufficiently specific to pick up the deficits [27; 28; 29]. Isokinetic dynamometry represents the gold standard in the evaluation of muscle strength and is the most commonly used tool in the studies evaluating the strength deficits in RTS following ACLR [15; 25]. Muscular weakness may be defined as LSI--the difference between peak torque of the ACLR limb and the uninvolved limb. The commonly used criterion of 90% symmetry between the limbs has been proposed as the minimum standard for the involved limb before the patient returns to unrestricted sports after the injury [15; 24; 30]. But the use of 90% LSI has been under scrutiny due to the reports of concurrent loss of knee extensor muscle strength of the uninvolved limb after ACLR, which can overestimate muscle strength recovery of the ACLR knee [31; 32]. This raises concern of using the LSI to confirm symmetrical muscle function, which could be based on bilateral weakness and could lead to subsequent premature RTS decision. Therefore, the question is whether the comparison of the involved to the uninvolved side (LSI) after ACLR is a good enough measure to evaluate the adequate postoperative rehabilitation status for safe unrestricted RTS [33]. To date, several studies have investigated an interrelationship of isokinetic strength performance before and 6 months after ACLR [28; 34; 35]. [PERSON] et al. [34] have found that preoperative knee extension and flexion strength are moderately correlated to postoperative strength performance after ACLR, but with high intraindividual variability. A preliminary study of [PERSON] et al. [36] has demonstrated a potential benefit of using the estimated preinjury capacity (EPIC) index instead of postoperative LSI. EPIC compares the knee extension strength of the ACLR operated limb with the strength of the uninvolved limb prior to ACLR to determine the readiness for RTS. The same study suggests that inability to restore the knee function exhibited before ACLR (EPIC) may increase the risk of secondary ACL injuries and is a better predictor of secondary injury than postoperative LSI. However, the authors have not reported preoperative LSI. Thus, the objective of our study was to investigate the predictive value of preoperative and postoperative LSI and EPIC index for secondary ACL injury. To our knowledge, there have not been studies investigating these three parameters as predictive factors for secondary ACL injury.