Background: Adolescent female athletes have a high incidence of ACL injury and re-injury(1). Knee strength is commonly assessed using the limb symmetry index (LSI) as a part of return-to-activity (RTA) clearance after ACL reconstruction (ACLR)(2). However, bilateral strength deficits persist after ACLR(3), suggesting the LSI may be an inadequate criterion for RTA clearance. Therefore, the purpose of this study was to identify whether pre-operative quadriceps and hamstring knee strength LSI and torque values change following ACLR among female adolescents, and identify the agreement of LSI between muscle groups (quadriceps vs. hamstrings) and contraction types (isometric vs. isokinetic). Hypothesis: We hypothesized that knee strength LSI increases and the difference between legs decreases over time. Secondly, there is a moderate agreement between muscle groups but not between contraction types. Methods: This longitudinal observational study included 30 female adolescents who had undergone ACLR (Table 1). All participants performed pre- and post-operative (after RTA clearance) isometric and isokinetic knee strength testing for flexion and extension. Isometric testing was performed for 5 seconds on a dynamometer at 60° knee flexion. Isokinetic testing occurred for 1 set of 5 repetitions at 90° per second ranging from 10-100° knee flexion. Torque data was normalized to body weight. LSI was calculated as (injured limb torque/non-injured limb torque)*100. A MANOVA was performed to identify the effect of time (pre- vs. post-operative) on LSI values, strength measures and net change in pre- and post-operative torque. Kappa tests were used to determine the LSI agreement between muscle groups and contraction types. Results: Only isokinetic flexion strength LSI increased after ACLR (Figure 1A). Injured and non-injured limb isometric flexion and extension, and isokinetic extension strength increased over time (Figure 1C-F), but did not induce changes in LSI. Only the injured limb's isometric knee flexion strength did not differ over time (Figure 1E). The net change in pre- and post-operative torque did not differ between limbs for each contraction (Figure 2). There was none to fair agreement between pre- and post-operative changes in LSI (Kappa range = -0.089–0.394). Conclusion: Despite increased torque output, pre-operative strength deficits were not recovered, and limb asymmetries persisted following ACLR in female adolescent athletes. Additionally, the findings of this study suggest the LSI lacks consistency across assessments. These results reinforce concerns surrounding the LSI's reliability when assessing knee strength in ACL-injured patients at time of RTA. Knee strength LSI should not be used in isolation when assessing female adolescent athletes for RTA clearance.
Carson et al. (Mon,) studied this question.