Association between inter-limb asymmetry and determinants of middle- and long-distance running performance in healthy populations: a systematic review

Backgroundː The presence of inter-limb asymmetry in the human body has traditionally been perceived to be detrimental for athletic performance. However, a systematic review addressing and comprehensively assessing the association of asymmetry between the lower limbs and middle- and long-distance running performance-related metrics is currently lacking.
Objective: The main purpose of this systematic review was to examine the relationship between lower inter-limb asymmetry and determinants of running performance in healthy middle- and long-distance runners. The secondary objective was to identify possible avenues for further research in this area.
Methodsː Pubmed, Web of Science and SPORTDiscus were systematically searched for studies investigating the relationship between lower inter-limb asymmetry and (determinants of) running performance in healthy and injury-free middle- and long-distance runners. The quality of studies eligible for inclusion was assessed using the Downs and Black Quality Index Tool.
Resultsː Out of 4817 articles screened, 8 studies were included in this review which assessed the association between functional, morphological, kinematic and kinetic asymmetry and running performance-related metrics. The quality score of the included research varied between 5/10 and 9/10. Our results revealed mixed findings, showing both significant negative (n = 16) and positive (n = 1) associations as well as no significant associations (n = 30) between inter-limb asymmetry and running performance-related metrics.
Conclusionsː A high heterogeneity across study methods and outcomes was apparent, making it difficult to draw a straightforward conclusion. Our results indicate that the majority of metrics of functional, morphological, kinematic and kinetic inter-limb asymmetry are negatively or not associated with running performance (and/or its determinants). Thus, a more extensive high-quality body of research using standardised asymmetry magnitude metrics is essential to determine whether, and to what extent asymmetry between the lower limbs could affect middle- and long-distance running performance. Future studies should establish potential trade-off values to help practitioners in developing evidence-based training programs.

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