## 4 Discussion
This finding is consistent with other studies reporting that 6 months after ACLR, most patients have not yet recovered their knee extension muscle strength deficits [15; 24]. Furthermore, the patients with postoperative LSI lower than 80% have been reported to exhibit deficits even two years after ACLR [43; 44]. Similarly, others have found that more symmetrical preoperative limb-to-limb knee extensor muscle strength above 80% has a positive influence on ACLR outcomes even two years after ACLR [37; 38]. Low rates of patients passing this criterion raise concerns regarding the adequacy of the rehabilitation protocols used. On the other hand, it has been proven that patients undergoing supervised rehabilitation/physiotherapy with routine measurements of knee strength at different time points during rehabilitation can restore (LSI \(\geq\) 90%) knee extension and flexion muscle strength 6 months after ACLR [45; 46]. The decreased knee extension muscle strength of the uninvolved limb (EPIC-H) observed in this study likely indicates that a subgroup of patients exists in whom reduced physical activity may result in larger compensatory adaptations. The mean preoperative knee extension muscle strength of the uninvolved limb was statistically higher (\(p\) = 0.001) in the group of patients with a secondary ACL injury compared to patients without secondary injury. This might explain why stronger individuals suffer a greater strength decrease of the uninvolved knee extensor muscle and have larger between-limb asymmetry and are therefore at a higher risk of a secondary injury. Persistent asymmetrical knee extension strength is associated with asymmetrical knee biomechanics and plays an important role in the incidence of any knee injury, contralateral and ipsilateral ACL graft reinjury [11; 23; 27; 28], which might be a case in our cohort study. In our study, LSI of preoperative knee extension and EPIC for extensor muscles had higher sensitivity compared to other tested indexes and could be considered as more reliable predictive factors for second injury compared to other investigated indexes. Although both preoperative knee extension LSI and EPIC have high sensitivity for predicting second ACL injury, they have low specificity, which means that the percentage of correctly identified cases that will not suffer second injury is very low due to a low proportion of patients that reach the cut-off 90% preoperative LSI and EPIC. Consequently, there is a high proportion of false positive results of these tests. However, it has to be noted that in screening predictive values are more relevant than sensitivity and specificity [41]. In all indexes, we have observed relatively low positive predictive values and high negative predictive values; in this view, the indexes are better predictors for no reinjury. This indicates the necessity for the surgeon to apply these indexes before primary ACLR to identify the patients at a higher risk for secondary injury. The risk of secondary injury, resulting from preoperative limb asymmetry, could be minimized by using an extended preoperative rehabilitation protocol. During waiting for the ACLR patients should be empowered to pursue a more intensive preoperative rehabilitation protocol and should undergo a follow up testing. Moreover, by adopting a preseason testing strategy in the population at a higher risk for ACL, injury could be a good criterion to provide patient-specific rehabilitation milestones after ACLR. Thus, the use of reference matched age, sex and sport specific values could also present a good strategy to patient-specific preinjury data, but literature is still sparse and evolving [21; 47]. While in our cohort significant differences in time form injury to surgery were detected between tested groups, we did not find a significant association between the time from the ACL injury to the initial uninvolved limb testing (before ACLR) and EPIC levels, which is in line with the results of [PERSON] et al.