Effect of Acute Neuromuscular Fatigue on Dynamic Inter-Limb Asymmetry in Clinical and Athletic Populations: A Systematic Review

Abstract Background: Neuromuscular fatigue may reveal inter-limb differences that stay hidden in rested testing, yet return-to-sport batteries are usually completed unfatigued. Evidence on how acute fatigue changes dynamic inter-limb asymmetry has not been systematically synthesised. Objective: To synthesise peer-reviewed evidence on the effect of acute neuromuscular fatigue on dynamic inter-limb asymmetry (primary outcome) and loading rate (secondary outcome) in clinical and athletic populations. Methods: PubMed/MEDLINE, Scopus, Web of Science and SPORTDiscus were searched. Eligible studies were peer-reviewed, English-language pre–post or controlled studies measuring dynamic kinetic, kinematic or performance-based asymmetry before and after an induced fatigue exposure. Risk of bias was appraised with an adapted Joanna Briggs Institute checklist and certainty of evidence with GRADE-informed judgements. Results were synthesised narratively following SWiM guidance. Results: Seven studies (152 participants; samples from one research group may overlap) were included: post-ACL reconstruction (ACLR; n = 2 studies), healthy adults (n = 1), resistance-trained adults (n = 1) and amateur runners (n = 3). Risk of bias was moderate in six studies and low–moderate in one. After fatigue, the hop-test limb symmetry index (LSI) in ACLR patients narrowed because the non-operative limb declined more, and landing quality deteriorated. In healthy adults and lifters, group-level asymmetry did not change meaningfully. In runners, changes were joint- and plane-specific. No included study examined knee osteoarthritis or vertical loading rate. Certainty of evidence was low to very low. Conclusions: Current evidence does not show that fatigue consistently increases dynamic asymmetry. Its effect depends on population, task, metric and on which limb deteriorates, and the LSI can improve for the wrong reason. Clinicians should report absolute values for both limbs alongside ratio indices. Adequately powered, prospectively registered studies with standardised fatigue protocols are needed. Keywords: neuromuscular fatigue; limb asymmetry; limb symmetry index; ground reaction force; anterior cruciate ligament reconstruction; return to sport; running.