Variations in morphological lower inter-limb asymmetry by sex and training level, and its link to performance in recreational runners: A cross-sectional study

Background Body composition plays a critical role in endurance running. While anthropometric characteristics have been incorporated into performance prediction models, potential inter-limb morphological asymmetries in lower-limb body composition remain largely unexplored. Therefore, this study aimed to (1) quantify lower-limb morphological asymmetry and examine differences across sex and training level, (2) compare running performance between low- and high-asymmetry groups using split analyses, and (3) investigate the link between lean mass (LM), bone mineral content (BMC) and fat mass (FM) asymmetry as well as leg length discrepancy (LLD), and running performance in healthy adult endurance runners with varying training backgrounds. Methods Sixty-eight healthy adult recreational runners (53% male) were stratified by sex and training level (novice, intermediate, trained). Segmental lower-limb body composition (LM, bone mineral density (BMD), BMC, and FM) was assessed using dual-energy X-ray absorptiometry (DXA). LLD was determined anthropometrically as the distance between the anterior superior iliac spine and medial malleolus. Running performance was evaluated using the Cooper 12-minute run test. Low and high asymmetry groups were created by categorizing participants according to the sample median asymmetry values for DXA-derived metrics and using a 2 cm threshold to classify LLD. Group differences were assessed using t-tests and ANOVAs, and multiple linear regression examined predictors of running performance. Results Mean asymmetry magnitudes ranged from 1.91% to 4.04% across metrics. LLD ranged from 0.5 to 0.9 cm. Female runners demonstrated greater LLD than males (p = 0.004), and trained runners showed greater BMD asymmetry than novices (p = 0.006). LM, BMC and FM asymmetry as well as LLD were no significant predictors of Cooper test performance (p = 0.316 to 0.686). Furthermore, no significant differences were found between the low and high asymmetry groups (p = 0.581 to 0.999). Conclusion DXA-derived inter-limb asymmetries at lower limb level and LLD in recreational endurance runners are small and largely independent of sex and training level. Importantly, these asymmetries are not linked to running performance, suggesting limited practical relevance for performance optimization. Trial registration: ClinicalTrials.gov (NCT06808399). Registered on 02 April 2025.