## 3 Results
### Muscle Strength Testing Results  
Patients completed the initial knee extension and flexion strength testing of the involved and uninvolved limb just before ACLR. ACLR was performed at a mean \(\pm\) SD of \(8.5\pm 9.03\) months after injury. The second testing was performed at approximately 6 months after ACLR (mean \(\pm\) SD 5.7 \(\pm\) 1.0 months). Thirteen (20.6%) patients achieved over 90% postoperative LSI for knee extension strength, 35 (55.6%) patients achieved over 90% postoperative LSI for knee flexion strength. Twenty-one patients (33%) achieved over 90% preoperative LSI for knee extension strength, 41 (65%) patients achieved over 90% preoperative LSI for knee flexion strength. Only 8 (12.7%) patients met over 90% EPIC knee extension strength levels (comparing the involved limb at approximately 6 months after ACLR to the uninvolved limb before ACLR) and none of them suffered a secondary ACL injury. Forty-one (65.1%) patients met over 90% EPIC knee flexion strength levels. Eight of the 13 patients who achieved postoperative 90% LSI for knee extension did not achieve 90% EPIC levels for knee extension strength 6 months after ACLR. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Factor**} & **All** & **No secondary Injury** & **Secondary Injury** & **Difference between** \\  & **(n = 63)** & **(n = 51)** & **(n = 12)** & **Groups** \\ \hline  & **Mean (SD)** & **Mean (SD)** & **Mean (SD)** & **\(p\) Value** \\ \hline Age (yrs) & 34.7 (12.3) & 36.2 (12.1) & 28.3 (11.7) & 0.045* \\ Body Mass (kg) & 76.7 (17.3) & 76.1 (18.8) & 79.0 (10.0) & 0.611 \\ Body Height (cm) & 174.9 (0.1) & 174.0 (8.7) & 178.6 (9.6) & 0.112 \\ Body mass index (kg/m\({}^{2}\)) & 24.9 (4.4) & 24.9 (4.8) & 24.7 (1.9) & 0.888 \\ Tegner Activity score & 6.7 (1.5) & 6.5 (1.4) & 7.2 (1.6) & 0.064 \\ Gender, male/female, n & 37/26 & 29/22 & 8/4 & 0.535 \\ Time from injury to surgery & & & & \\ (months) & 8.5 (9.03) & 11 (10.01) & 3.4 (2.3) & 0.001 * \\ \hline \hline \end{tabular}  
* indicates significant difference (\(p\leq 0.05\)) of mean values between secondary injury and no secondary injury groups (independent sample \(t\)-test). For gender differences between groups, chi-square test was used.  
\end{table} 
Table 1: Patient demographic data and preoperative measurements. Fifteen of the 21 patients who achieved preoperative 90% ISI for knee extension did not achieve 90% EPIC levels for knee extension strength 6 months after ACLR. In these patients, the mean knee extension strength deficit ranged from 89.9% to 37.1% (Table 2), and the mean knee flexion strength deficit ranged from 89.8% to 52.6% (Table 3). In the group of patients with secondary injury, three (2 ipsilateral ACL reinjuries, 1 contralateral ACL injury) of the 12 patients passed 90% of the preoperative LSI return-to-sport criteria in knee extension strength test 6 months after the initial ACLR, but none of these 3 achieved 90% EPIC levels (Table 4). In 10 of the 12 patients with a secondary ACL injury a significant knee extension muscle weakness of the uninvolved limb (EPIC-H) of a mean 17.5% \(\pm\) 5.9 (\(p\) = 0.001) between testing before and after ACLR occurred (Table 2). Twelve patients sustained a secondary ACL injury (median time from ACLR to secondary injury was 96 weeks; range 38-246 weeks) (Table 4). The group with a secondary ACL injury had a statistically significantly higher pre-operative extensor (\(p\) = 0.001) and flexor (\(p\) = 0.022) muscle strength of the uninvolved knee. Higher peak torque values of flexor and extensor muscles of the involved limb were observed in a group of patients with a secondary injury compared to the patents with no secondary injury (although this difference was not statistically significant in all variables) at preoperative and postoperative testing was observed (Tables 2 and 3). This might be associated with shorter time from injury to ACLR in patients with a secondary injury.