Changes in Quadriceps Force Control and Torque Quality Following Anterior Cruciate Ligament Injury and Reconstruction: Associations with Functional Performance—A Systematic Review and Meta-Analysis

BACKGROUND: Consistent force output is a critical indicator of the neuromuscular system's effectiveness. Although force signals inherently fluctuate, the ability of skeletal muscles to generate accurate and steady force offers insights into the system's adaptability and its ability to adjust motor control strategies to meet task demands. This systematic review and meta-analysis aimed to synthesize evidence on quadriceps force control in individuals with anterior cruciate ligament (ACL) injury and/or surgical reconstruction (ACLR). Additionally, it sought to explore the relationship between force control measures and physical function outcomes. METHODS: A literature search was conducted across several databases, including PubMed, EMBASE, Scopus, Web of Science, and SPORTDiscus. Risk of bias was assessed using the adapted Newcastle-Ottawa tool. The study included individuals with unilateral ACL injury and/or ACLR, with comparisons to uninjured controls or unaffected contralateral limbs. Primary outcomes included torque quality, force accuracy, and force/torque steadiness, while secondary outcomes included function-related clinical questionnaires and performance tests. Eligible studies consisted of observational studies and baseline data from interventional studies published in English. Standardized mean differences (SMDs) were calculated using a random effects meta-analysis. RESULTS: A total of 33 studies were included, comprising 20 individuals with ACLR, 12 with ACL injuries, and one with both. The meta-analysis showed significant effects of ACL injury (SMD = 0.84) and ACLR (SMD = 1.57) on quadriceps torque frequency content. Individuals with ACLR had a greater root mean squared error (RMSE) or absolute error (AE) in force output compared to healthy controls (SMD = 0.35) and exhibited a significant difference in the coefficient of variation (CoV) of the force signal (SMD = 0.22), indicating impaired force control in those with ACLR. CONCLUSIONS: ACL injury impairs quadriceps force control in the injured limb, as shown by torque frequency content analysis. While ACL reconstruction is the gold standard for joint stability, it may not fully restore neuromuscular function, potentially compromising physical functioning. However, the limited number of high-quality studies may weaken these conclusions. REGISTRATION: The review protocol was prospectively registered in the International Prospective Register of Systematic Reviews (PROSPERO; registration number: CRD42024571495).