###### Abstract  
Long-term muscle weakness may increase the risk of knee reinjury after anterior cruciate ligament reconstruction (ACLR) and of osteoarthritis. The incidence of secondary injuries after ACLR and the predictive value of preoperative and postoperative limb symmetry index (LSI) and estimated preinjury capacity (EPIC) index were studied for predicting the risk of reinjury in a retrospective study. Sixty-three recreational and professional athletes after ACLR with hamstring autograft were followed for secondary injury in the period from 2012 to 2014, 5 years after ACLR. Peak torque values of knee extensor and flexor muscle strength of the involved and uninvolved limb were measured with an isokinetic dynamometer at 60 degrees per second before ACLR and 6 months after ACLR and were used to calculate LSI and EPIC index. The results suggest that the preoperative LSI and EPIC indexes predict a secondary ACL injury better than the postoperative LSI for extensor muscles which is often used as a criterion to determine the time for returning to normal sports activities. Individuals with secondary ACL injuries suffer greater loss of knee extensor muscle strength of the uninvolved limb between preoperative and postoperative ACLR testing compared to the individuals without secondary injury. a 2021 12 April 20  
The risk of a secondary injury is a multifactorial phenomenon, the prediction of which should involve a broader approach to understand the complex relationships between risk factors and the occurrence of injuries better [7]. The individual's risk is influenced by different physical, psychological, and other factors. Main physical factors are based on impairments found in patients after ACLR. They include poor neuromuscular control and poor biomechanics known to contribute to secondary ACL injuries, such as greater knee abduction moment, smaller knee flexion angle during landing with compensatory hip flexion, patterns of favoring the uninjured contralateral limb, decreased quadriceps limb symmetry index (LSI) below 90%, decreased rate of force development and hop test LSI below 90% [8; 9; 10; 11; 12]. All these variables are influenced by other factors such as sex, age, previous injury, the amount of time elapsed since ACLR, and increased exposure to sports precipitation, unanticipated environmental events, and higher training load [7; 8]. There is still a lack of adoption of the simplest RTS tests in clinical practice. The time from ACLR to RTS has been the most frequently reported criterion in the literature [13]. However, the recent consensus statement is that purely time-based RTS decision-making should be abandoned in clinical practice, and it must include objective physical examination data (e.g., clinical tests and measurements) [14]. A general recommendation for unrestricted RTS used to be 6 months, but recent evidence indicates that RTS should be delayed for at least 9 months; some authors even suggest that it should be delayed up to 2 years after ACLR [15; 16]. There are many suggested prevention strategies with limited plausibility and dubious efficiency [17]. Currently, the best prevention programs include the restoration of all the reported neuromuscular impairments after primary ACLR; use of objective RTS assessment; and use of a prevention exercise program that includes pyometric, strength, and proprioception training [18; 19]. Return to sport assessments are administered following ACLR with the goal of evaluating modifiable risk factors associated with secondary injury, such as physical and mental readiness to safely return to unrestricted activity [20]. Physical readiness is assessed with the goal of capturing the overall impact of knee injury on knee function. Knee function can be assessed with different instruments, such as patient reported outcome measures, performance-based outcome measures, and clinical outcome measures. An important part of RTS assessments is the use of different performance-based outcome measures and screening tools that measure unique constructs.