Quadriceps Strength and Volitional Activation After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis.

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). To assess differences of unilateral quadriceps isometric strength and activation between the involved limb and contralateral limb of individuals with ACLR and healthy controls. Web of Science, SportDISCUS, PubMed, CINAHL, and the Cochrane Database were all used during the search. 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). Meta-analysis. Level 2. 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). 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). 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.