A dual-metric approach: Combining bilateral asymmetry and minimal detectable difference to evaluate movement asymmetry in swimming

The aims of this study were to: (i) examine the association between upper-limb propulsive force asymmetry, quantified using the Bilateral Asymmetry Index 1 (BAI-1), and front-crawl swimming velocity in young swimmers, and; (ii) explore the utility of the Minimal Detectable Difference (MDD) for interpreting contralateral asymmetry. Thirty-six competitive adolescent swimmers (20 boys: 15.9 ± 0.7 years, 16 girls: 15.0 ± 1.1 years; mean ± SD) were recruited. Swimming velocity, propulsive force (PF), and BAI-1 were measured over three consecutive swimming stroke cycles. Boys were 12.6% faster than girls ( p < 0.001, ES = 2.34, very large) and generated greater PF in both limbs (right: Δ = 20.8%, ES = 1.15; left: Δ = 13.8%, ES = 0.80; both p < 0.001, moderate effects). Swimming velocity decreased over the stroke cycles analyzed ( p = 0.009). PF also decreased marginally, with boys less affected than girls. BAI-1 only tended to increase over time, without a significant sex effect. BAI-1 was not retained as a significant predictor of velocity. The MDD indicated that a 2.1 N (5.8%) between-limb PF difference was the smallest difference statistically detectable under the predefined significance and power criteria. These findings suggest that MDD may complement asymmetry indices such as the BAI-1 by helping to distinguish meaningful inter-limb differences from measurement error. In this sample, the data did not provide sufficient evidence of an association between upper-limb PF asymmetry and swimming velocity, reinforcing the need for cautious and reliability-based interpretations of asymmetry in young swimmers.