SEX DIFFERENCES IN QUADRICEPS STRENGTH AMONG ADOLESCENTS WITHIN 1 YEAR OF ACL RECONSTRUCTION
Authors: Kuenze Christopher , Pietrosimone Brian , Lisee Caroline , Rutherford Margaret , Birchmeier Thomas , Lepley Adam , Hart Joseph M. 
Published: March 2019

Background Female patients with ACLR are 40% less likely to return to preinjury levels of sport after subsequent ACLR when compared to their male counterparts. Additionally, 24-30% of young female athletes who do return to sport will go on to experience a second ACL injury to the ipsilateral or contralateral knee within 2 years of ACLR. Despite these findings, sex-based differences in clinical outcomes following ACLR are not currently well understood within the adolescent population. Involved limb knee extension weakness, contralateral limb knee extension weakness, and between limb strength asymmetry persist months to years following ACLR among adolescent patients. Improvements in unilateral and symmetry-based measures of knee extension strength during and after the rehabilitation process have been linked to improved patient-reported knee related function, lower extremity functional performance, and decreased risk of subsequent ACL injury among individuals with recent ACLR. Currently, there is a lack of evidence describing the role that sex plays in persistent quadriceps weakness. Therefore, the purpose of this study was to investigate the effects of patient sex on unilateral and symmetry-based measures of knee extension strength, and patient-reported knee function among adolescent individuals with ACL reconstruction (ACLR) within the first year after surgery. Methods: Fifty-two male participants (age = 16.8±1.3 years, BMI = 25.6±5.6 kg/m2, time since surgery = 6.3±1.4 months) and 66 female participants (age = 16.7±1.4 years, BMI = 23.9±3.9kg/m2, time since surgery = 6.8±1.7 months) with unilateral ACLR, 4-12 months prior to enrollment were recruited for this multi-site cross-sectional study. Participants completed bilateral knee extension maximal voluntary isometric contraction (MVIC) torque assessments using a multimodal dynamometer with continuous verbal encouragement from the tester and visual feedback on a nearby monitor. Peak knee extension MVIC torque was identified for each of 3 trials and was normalized to body mass (Nm/kg). Limb symmetry indices were also calculated by dividing the MVIC torque obtained for the involved limb by the MVIC torque obtained for the contralateral limb. Participants also completed the International Knee Documentation Committee (IKDC) Subjective Knee Form to assess patient-reported knee function. Sex-based comparisons of quadriceps strength outcomes and IKDC score were made using separate ANCOVAs where time since surgery was utilized as a covariate. Cohen’s d effect sizes and 95% confidence intervals (CI95) were calculated to assess the magnitude of sex difference for all outcome measures. Odds ratios and CI95 were also calculated to assess the odds of an adolescent female achieving clinical threshold for acceptable involved limb quadriceps strength (MVIC = 3.0 Nm/kg), quadriceps limb symmetry (LSI = 90.0%), and patient-reported knee function (IKDC = 90.0). Results: The involved limb (male = 2.08±0.95 Nm/kg, female = 1.69±0.63 Nm/kg, p = 0.01, d = -0.50, CI95 = -0.13 to -0.87) and contralateral limbs (male = 2.75±0.75 Nm/kg, female = 2.31±0.64 Nm/kg, p = 0.001, d = -0.64, CI95 = -0.26 to -1.01) of male participants were stronger when compared to female participants but limb symmetry (male = 74.7±22.6%, female = 74.7±24.6%, p = 0.95, d = 0.00, CI95 = 0.36 to -0.36) and IKDC score (male = 84.4±16.1, female = 79.4±13.7, p = 0.08, d = -0.34, CI95 = 0.03 to -0.71) was not significantly different between the sexes. Male participants were 5.66 times (CI95 = 1.46 to 21.87) more likely to meet the clinical threshold for acceptable involved limb quadriceps strength and 3.52 times (CI95 = 1.54 to 8.05) more likely to report acceptable knee related function. The likelihood of achieving clinically acceptable limb symmetry was not significantly different between the sexes (OR = 1.17, CI95 = 0.48 to 2.83). Conclusion/Significance: Adolescent female patients may experience significantly worse involved limb and contralateral quadriceps weakness when compared to adolescent male patients despite displaying equally poor limb symmetry and patient-reported knee function during the first 12 month following surgery. Given the importance of involved limb quadriceps strength as a predictor of patient-reported function and movement quality, clinicians should incorporate patient education regarding the importance of quadriceps function and recommend progressive lower extremity loading when implementing patient-centered rehabilitation plans that promote safe return to healthy levels of physical activity following ACLR.