## 4 Discussion  
The current study shows that knee extension strength EPIC levels and preoperative LSI for extension are more sensitive in predicting a secondary ACL injury compared to the postoperative LSI, which is commonly used for determining the time for RTS. All patients who suffered a secondary injury had statistically greater knee extensor muscle strength loss of the uninvolved limb compared to uninjured patients. This demonstrates that knee extension strength at 6 months after the initial ACLR does not guarantee that pre-injury strength levels of the uninvolved limb have been met and can inflate the LSI post-operatively. Thus 3 of the 12 secondary injuries might have been prevented if the RTS decision had been based on EPIC levels. This study supports evidence from previous observations by [PERSON] et al. (2016) who found that 6 of the 8 patients who sustained a secondary ACL injury after ACLR achieved postoperative LSI of 90% but did not achieve \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Patient**} & **Time from ACLR to Secondary** & **LSle** & **LSle** & **EpICEe** & **Side of Injury** \\  & **ACL Injury, wk** & **Preoperative** & **Postoperative** & **EPICEe** & **Side of Injury** \\ \hline
1 & 220 & 60 & 64 & 56 & ipsilateral \\
2 & 215 & 80 & 95 & 74 & ipsilateral \\
3 & 58 & 62 & 84 & 72 & ipsilateral \\
4 & 101 & 90 & 100 & 82 & ipsilateral \\
5 & 39 & 43 & 75 & 64 & ipsilateral \\
6 & 146 & 46 & 80 & 64 & ipsilateral \\
7 & 92 & 51 & 72 & 65 & ipsilateral \\
8 & 38 & 78 & 80 & 64 & ipsilateral \\
9 & 42 & 56 & 78 & 58 & contralateral \\
10 & 50 & 72 & 62 & 58 & ipsilateral \\
11 & 246 & 80 & 90 & 81 & contralateral \\
12 & 193 & 77 & 82 & 63 & ipsilateral \\ \hline \hline \end{tabular}  
LSle—limb symmetry index for extensor muscles; EPIC—estimated preinjury capacity for extensor muscles.  
\end{table} 
Table 4: Patients with secondary ACL injuries and LSIs and EPIC levels of knee extension strength. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **LSle Prep** & **LSf Freop** & **LSle Postop** & **LSIf Postop** & **EPICEe** & **EPICEf** \\ \hline
**Sensitivity** & **0.92** & **0.5** & **0.75** & **0.5** & **1** & **0.5** \\
95\% CI & 0.65–0.99 & 0.25–0.75 & 0.47–0.91 & 0.43–0.69 & 0.76–1 & 0.25–0.75 \\
**Specificity** & **0.41** & **0.68** & **0.21** & **0.57** & **0.15** & **0.68** \\
95\% CI & 0.29–0.55 & 0.55–0.80 & 0.12–0.35 & 0.61–2.22 & 0.08–0.28 & 0.55–0.80 \\
**Positive LR** & **1.55** & **1.95** & **0.956** & **1.15** & **1.18** & **1.59** \\
95\% CI & 1.17–2.07 & 0.79–3.20 & 0.67–1.37 & 0.48–1.63 & 1.05–1.34 & 0.79–3.20 \\
**Negative LR** & **0.20** & **0.72** & **0.159** & **0.87** & **0** & **0.72** \\
95\% CI & 0.03–1.36 & 0.40–1.32 & 0.38–3.52 & 0.37–4.65 & - & 0.40–1.32 \\
**PPV** & **0.26** & **0.27** & **0.18** & **0.21** & **0.23** & **0.27** \\
**NPV** & **0.95** & **0.85** & **0.87** & **0.82** & **1** & **0.85** \\ \hline \hline \end{tabular}  
LSle—limb symmetry index for extensor muscles; EPIC—estimated preinjury capacity for extensor muscles; LSIf—limb symmetry index for flexor muscles; EPIC—estimated preinjury capacity for flexor muscles; LR—likelihood ratio; PPV—positive predictive value; NPV—negative predictive value.  
\end{table} 
Table 5: Sensitivity and specificity of muscle strength indexes as tests for prediction of second ACL injury. 90% EPIC levels. Additionally, in patients who sustained secondary ACL injury after ACLR, greater differences in LSI of preoperative knee extension strength and EPIC levels were found. [PERSON] et al. 2020 [35], however, found no difference in the proportion of patients who recovered their postoperative knee extension and flexion absolute muscle strength compared to the patients who recovered their postoperative LSI at 8- and 12-months post ACLR. It should be stressed that the authors did not study the incidence of secondary ACL injuries. In our study, only 1 of the 5 patients achieved criterion of 90% LSI, and only 1 of the 8 patients achieved 90% of EPIC levels at 6 months after ACLR.