Cleared But Not Ready? The Impact of Fatigue on Return-To-Sport Testing Post Anterior Cruciate Ligament Injury

Abstract

Anterior cruciate ligament (ACL) injury is associated with prolonged rehabilitation, substantial healthcare costs, reduced sports participation, and an elevated risk of reinjury after return to sport (RTS). Despite routine RTS testing to guide clinical decisions after ACL injury and reconstruction, reinjury rates remain high, suggesting current assessments may not adequately evaluate readiness for sport demands. Most RTS assessments are done in a rested state, although fatigue is an inherent feature of sport that alters neuromuscular control, movement quality, force production, and landing mechanics. These effects may be especially relevant in female athletes, who have higher rates of non-contact ACL injury and already exhibit higher-risk movement patterns that fatigue may further compromise. Consequently, athletes may meet RTS criteria when rested but show deficits under fatigue. This thesis investigated fatigue's influence on ACL RTS assessment and explored whether fatigue-informed testing identifies deficits missed during conventional rested evaluations.

To address this aim, two studies were conducted. The first, a scoping review, evaluated fatigue protocols used in ACL RTS research regarding clinical relevance, feasibility, and translational potential. Twelve studies were identified, with protocols classified as localised neuromuscular (n = 7), whole-body sport-specific (n = 4), or isolated joint (n = 1). Fatigue consistently reduced absolute hop and jump performance, movement quality, and biomechanical variables linked to ACL injury risk. However, substantial heterogeneity existed across fatigue definitions, induction methods, verification criteria, and RTS assessment batteries. Notably, limb symmetry indices often remained unchanged despite reductions in absolute performance, suggesting symmetry-based measures may mask bilateral functional declines and underestimate fatigue-related deficits.

The second study, a repeated-measures experiment, examined fatigue's acute effects on RTS outcomes in 15 recreationally active female athletes (23.1 ± 4.7 years). Healthy participants were recruited to isolate fatigue effects without the confounding influence of ACL injury or rehabilitation. Participants completed a standard RTS testing battery under rested and fatigued conditions. Fatigue was induced through repeated hopping, jumping, squatting, and step-down tasks until countermovement jump height dropped by at least 30%. After fatigue, quadriceps strength decreased by 9.8% (p < .001) and single-leg hop distance by 17.3% (p = .041). While most traditional RTS outcomes and limb symmetry measures remained unchanged, force-time-derived kinetic variables showed consistent reductions, including eccentric peak force, eccentric braking impulse, concentric impulse, and landing rate of force development.

Collectively, these findings indicate fatigue can reveal neuromuscular and biomechanical impairments not detected by conventional RTS assessment done in a rested state. Traditional RTS measures retain clinical value but may not fully reflect functional capacity under physiological stress. Fatigue-informed assessment, especially through kinetic measures, can identify residual neuromuscular deficits concealed during conventional RTS testing. Further research is needed to establish clinically feasible fatigue protocols in ACL-reconstructed populations and determine whether fatigue-related RTS outcomes improve prediction of successful return to sport and reinjury risk.