Lower-limb isometric strength asymmetries do not impair running performance in recreational adult distance runners

This study aimed to: (1) examine differences in isometric strength asymmetry by sex and training level, (2) compare running performance between high- and low-asymmetry groups, and (3) explore associations between lower inter-limb strength asymmetry and running performance. Fifty-eight recreational endurance runners (34.2 ± 9.1 years, 50% male) completed six isometric strength tests assessing hip adduction and abduction, knee extension and flexion, ankle dorsiflexion and plantar flexion strength across one familiarisation and one testing session. The Cooper test was conducted on an outdoor athletics track and served as a proxy measure of running performance. Median-split analyses classified participants into high- and low-asymmetry groups per strength test. Between-group differences were evaluated using Kruskal–Wallis and Mann–Whitney U tests, and associations with running performance were examined using multiple linear regression. Asymmetry magnitudes ranged from 7.27% to 15.65%, with no significant differences by sex (p = 0.210-0.944) or training level (p = 0.278-0.629). Likewise, running performance did not differ significantly between asymmetry groups (p = 0.216-0.958). Positive associations were found between running performance and asymmetries in knee flexion (p = 0.032, B = 10.276) and ankle plantar flexion (p = 0.004, B = 20.934), whereas other strength measures showed no significant associations (p = 0.175-0.481). These findings indicate that lower inter-limb isometric strength asymmetries of up to ~16% are common among endurance runners and do not appear to impair running performance. Consequently, such asymmetry may reflect normal individual variation, supporting an approach to strengthen the weaker limb rather than considering it problematic.