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

Am J Sports Med. 2019 Dec;47(14):3423-3428. doi: 10.1177/0363546519878432. Epub 2019 Oct 21.


Background: 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.

Purpose: To evaluate the relationship between knee extensor strength and SLS performance after primary ACL reconstruction.

Study design: Cohort study (Diagnosis); Level of evidence, 3.

Methods: 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%.

Results: 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; P < .001; Cohen d = 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; P = .011; Cohen d = 0.26). There was a weak positive correlation between knee extensor strength and the SLS maximum flexion angle at 6 months (r = 0.342; P < .001) and 12 months (r = 0.245; P = .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.

Conclusion: 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.

Keywords: ACL; extensor strength; rehabilitation; single-leg squat.

MeSH terms

  • Adult
  • Anterior Cruciate Ligament Injuries / surgery*
  • Anterior Cruciate Ligament Reconstruction / rehabilitation*
  • Cohort Studies
  • Exercise / physiology
  • Female
  • Hamstring Tendons / transplantation
  • Humans
  • Knee Injuries / rehabilitation*
  • Knee Injuries / surgery
  • Knee Joint / physiology*
  • Knee Joint / surgery
  • Male
  • Muscle Strength / physiology*
  • Prospective Studies
  • Range of Motion, Articular
  • Torque
  • Transplantation, Autologous