The magnitude of lower limb functional asymmetry in healthy endurance runners: A systematic review

Introduction: The magnitude of inter- and intra-limb asymmetry has extensively been studied in unilateral sports. Previous research highlighted the possible influence of asymmetry on sport performance and/or injury risk. However, research on the magnitude of inter- and intra-limb functional asymmetry in endurance runners is very scarce. Therefore, this systematic review aims to provide a thorough overview on the research performed on the magnitude of both inter- and intralimb functional asymmetry at lower-limb level in healthy endurance runners.
Methods: A systematic search was conducted using Pubmed, Web of Science, and SPORTDiscus for studies investigating the magnitude of inter- and intra-limb functional asymmetry in healthy endurance runners. Studies with sprint athletes (running up to 400m distances) or injured runners were excluded. A modified version of the quality assessment tool for observational and cross-sectional studies was used to identify the quality of the included studies.
Results: From the 4897 articles screened, 7 articles were included in the review. The quality score of the articles ranged between 7/10 and 9/10. Inter-limb asymmetry ranged between 0.20% and 14.20% whereas intra-limb asymmetry ranged between 1.61% and 4.04% in the asymmetry metrics evaluated. More specifically, inter-limb asymmetry magnitudes in range of motion varied between 16.30% and 39.60%, whereas inter-limb asymmetry in balance ranged from 3.26% to 31.72%, while an asymmetry in jumping height of 5.50% was observed.
Conclusion: The heterogeneity in study methods and outcomes emphasizes the high individual, task, test and metric nature of inter- and intra-limb asymmetry. The magnitude of inter- and intra-limb functional asymmetry varied between 0.20% and 39.60%. This large range in asymmetry magnitudes between studies highlights the need for more consistent and reliable research on asymmetry. In addition, future researchers should investigate which specific asymmetry scores are most relevant for measuring asymmetry.