Single-Leg Squat Performance and Its Relationship to Extensor Mechanism Strength After Anterior Cruciate Ligament Reconstruction.

Abstract

Performance in strength and functional testing is important when considering return to sport after anterior cruciate ligament (ACL) reconstruction. Both knee extensor strength and the single-leg squat (SLS) have been used in this context. To evaluate the relationship between knee extensor strength and SLS performance after primary ACL reconstruction. Cohort study (Diagnosis); Level of evidence, 3. A prospective cohort of 100 patients was assessed 6 and 12 months after primary ACL reconstruction with a hamstring tendon autograft. Knee extensor peak torque was measured. Three sequential SLSs were performed, and the maximum flexion angle identified from frame-by-frame video analysis was used as the measure of squat performance. A limb symmetry index (LSI) was calculated and satisfactory performance defined as ≥90%. Extensor mechanism strength deficits were seen in 75% of patients at 6 months and 57% at 12 months postoperatively. Mean extensor mechanism strength showed a large improvement between 6 and 12 months (123.6 vs 147.8 N·m, respectively; <i>P</i> < .001; Cohen <i>d</i> = 1.10), and while there was also a statistically significant improvement in the mean maximum flexion angle, the change was small (66.1° vs 68.1°, respectively; <i>P</i> = .011; Cohen <i>d</i> = 0.26). There was a weak positive correlation between knee extensor strength and the SLS maximum flexion angle at 6 months (<i>r</i> = 0.342; <i>P</i> < .001) and 12 months (<i>r</i> = 0.245; <i>P</i> = .014). An SLS LSI <90% was 80% specific and 35% sensitive for extensor mechanism weakness at 6 months and 79% specific and 18% sensitive at 12 months. Extensor mechanism strength deficits are common after ACL reconstruction but reduce between 6 and 12 months. The SLS maximum flexion angle has a weak linear relationship to knee extensor strength. SLS performance has high specificity but low sensitivity in identifying extensor mechanism strength deficits. The SLS maximum flexion angle is therefore a suboptimal surrogate test to identify extensor mechanism strength deficits as diagnosed by isokinetic dynamometric testing. However, unsatisfactory SLS performance indicates a very high chance of underlying extensor mechanism weakness.