Limb asymmetry in swimmers and its correlation with sprint performance

Background: Inter-limb strength asymmetry is well-established as an injury risk factor, yet its relationship with athletic performance remains controversial and context-dependent. In swimming-a bilateral cyclical sport theoretically requiring symmetrical force production-the characteristics and performance implications of strength asymmetry remain insufficiently understood, particularly regarding contraction velocity and sex. Objective: This study aimed to: (1) characterize shoulder and hip isokinetic strength asymmetry in competitive swimmers at 60°/s and 180°/s; (2) explore sex differences in asymmetry profiles; and (3) examine associations of absolute strength and strength asymmetry with sprint performance. Methods: Thirty-two swimmers (20 males, 12 females) aged 18-22 years completed isokinetic assessments of shoulder flexion/extension, shoulder internal/external rotation, and hip flexion/extension at 60°/s and 180°/s. Asymmetry indices were calculated using the percentage difference method. Swimming performance was evaluated via 15-m start, turn, and 100-m freestyle times. Between-session reliability was established (ICC = 0.988-0.998; CV = 1.85%-6.25%). Mann-Whitney U tests, Wilcoxon signed-rank tests, and Spearman rank correlation were performed. Results: Males demonstrated significantly greater absolute strength and faster performance times than females (all p<0.001). Mann-Whitney U tests revealed a significant sex difference in hip extension asymmetry at 180°/s, with female swimmers exhibiting greater asymmetry than males (p = 0.019). Asymmetry indices were generally higher at 180°/s than at 60°/s, with significant velocity-dependent increases only for shoulder external rotation in males (Z = -2.373, p = 0.016, r = 0.531). Spearman rank correlation showed that, in females, hip extension torque at 60°/s was significantly positively correlated with turn time (left: rho = 0.679, p < 0.05; right: rho = 0.618, p < 0.05). In males, left shoulder extension at 60°/s and 180°/s showed significant negative correlations with turn time (rho = -0.470 and -0.531, respectively, both p < 0.05). No significant correlations were observed between any asymmetry index and performance in either sex. Conclusion: Strength asymmetry in swimmers is movement-, velocity-, and sex-specific, with female swimmers exhibiting greater hip extension asymmetry at high velocity. Absolute hip extensor strength is significantly associated with turn performance in females, whereas in males, shoulder extensor strength correlates negatively with turn time. Strength asymmetry is generally unrelated to swimming performance, supporting the view that asymmetry should not be universally deemed detrimental. These findings provide evidence-based guidance for individualized strength diagnostics in competitive swimming.