Context:: Quadriceps function is a significant contributor to knee joint health that is influenced by central and peripheral factors, especially after anterior cruciate ligament reconstruction (ACLR).
Objective:: To assess differences of unilateral quadriceps isometric strength and activation between the involved limb and contralateral limb of individuals with ACLR and healthy controls.
Data sources:: Web of Science, SportDISCUS, PubMed, CINAHL, and the Cochrane Database were all used during the search.
Study selection:: A total of 2024 studies were reviewed. Twenty-eight studies including individuals with a unilateral history of ACLR, isometric knee extension strength normalized to body mass, and quadriceps activation measured by central activation ratios (CARs) through a superimposed burst technique were identified for meta-analysis. The methodological quality of relevant articles was assessed using a modified Downs and Black scale. Results of methodological quality assessment ranged from low to high quality (low, n = 10; moderate, n = 8; high, n = 10).
Study design:: Meta-analysis.
Level of evidence:: Level 2.
Data extraction:: Means, standard deviations, and sample sizes were extracted from articles, and magnitude of between-limb and between-group differences were evaluated using a random-effects model meta-analysis approach to calculate combined pooled effect sizes (ESs) and 95% CIs. ESs were classified as weak ( d < 0.19), small ( d = 0.20-0.49), moderate ( d = 0.50-0.79), or large ( d > 0.80).
Results:: The involved limb of individuals with ACLR displayed lower knee extension strength compared with the contralateral limb (ES, -0.78; lower bound [LB], -0.99; upper bound [UB], -0.58) and healthy controls (ES, -0.76; LB, -0.98; UB, -0.53). The involved limb displayed a lower CAR compared with healthy controls (ES, -0.84; LB, -1.18; UB, -0.50) but not compared with the contralateral limb (ES, -0.15; LB, -0.37; UB, 0.07). The ACLR contralateral limb displayed a lower CAR (ES, -0.73; LB, -1.39; UB, -0.07) compared with healthy control limbs but similar knee extension strength (ES, -0.24; LB, -0.68; UB, -0.19).
Conclusion:: Individuals with ACLR have bilateral CAR deficits and involved limb strength deficits that persist years after surgery. Deficits in quadriceps function may have meaningful implications for patient-reported and objective outcomes, risk of reinjury, and long-term joint health after ACLR.
Keywords: ACLR; central activation ratio; isometric knee extension strength; quadriceps function.