Knee Extensor and Flexor Force Control after ACL Injury and Reconstruction: A Systematic Review and Meta-Analysis

Purpose: Reduced force control after anterior cruciate ligament (ACL) injury and reconstruction may contribute to poor function. Various metrics (linear and nonlinear) have been employed to quantify force control. The aims of this review were to synthesize evidence assessing knee extensor and flexor force control after ACL injury (ACLD) or reconstruction (ACLR) and to investigate the potential effects of injury management (e.g., surgery, graft type), mode of contraction (e.g., isometric, isokinetic), and speed and intensity of contraction.

Methods: We searched six databases for studies assessing knee extensor or flexor force control in individuals with ACLD or ACLR using a linear or nonlinear analysis method and comparing to the uninvolved limb or an uninjured control. Two independent reviewers assessed potential studies for inclusion and quality. Primary random effects meta-analyses were simultaneously separated by muscle, injury management, analysis method, and comparator.

Results: Twenty-seven studies were included. When assessed via linear methods, greater variability in knee extensor force (i.e., reduced force control) was observed only in comparison to uninjured controls (ACLD: P = 0.03, Hedges' g = 0.22, [95% confidence interval CI 0.02-0.42]; ACLR: P = 0.01, Hedges' g = 0.23, [95% CI = 0.05-0.41]). When assessed via nonlinear methods, lesser regularity of knee extensor force control (i.e., reduced force control) was observed in comparison to the uninvolved limb (ACLD: P = 0.008, Hedges' g = 0.44, [95% CI = 0.11-0.76]; ACLR: P < 0.00001, Hedges' g = 0.75, [95% CI = 0.61-0.88]) and uninjured controls (ACLD: P = 0.002, Hedges' g = 0.44, [95% CI = 0.16-0.72]; ACLR: P < 0.00001, Hedges' g = 0.69, [95% CI = 0.55-0.82]). Funnel plot data suggested a potential risk of bias.

Conclusions: Moderate-to-strong evidence indicates impaired knee extensor force control after ACLR in comparison to uninjured controls, regardless of analysis type. Nonlinear analyses detected more and greater force control differences, thus appearing to be more sensitive and highlighting a need for a standardized, clinically accessible methodology. These results may be partially explained by the neuromuscular mechanisms underlying motor control.