Agreement Between Segmental Bioelectrical Impedance Analysis and Dual-Energy X-Ray Absorptiometry (DXA) for the Magnitude and Direction of Between-Limb Lean Mass Asymmetry in Trail Runners

Background: Between-limb lean-mass asymmetry has been proposed for injury-risk and return-to-sport monitoring. Dual-energy X-ray absorptiometry (DXA), the reference method, requires a fixed installation and ionizing radiation; segmental bioelectrical impedance analysis (BIA) is fast, inexpensive and field-applicable. Agreement for the asymmetry itself, rather than absolute limb values, is unresolved. Methods: In this cross-sectional method-comparison study, 127 trail runners (90 men, 37 women) underwent eight-electrode segmental BIA and DXA in the same session. An asymmetry index, the between-limb difference relative to the limb mean, was computed from raw leg resistance (BIA) and leg lean mass (DXA), and assessed by Bland–Altman analysis, concordance correlation (CCC) and kappa. Results: Mean bias was −0.97 percentage points (pp), but the 95% limits of agreement spanned −6.67 to +4.73 pp, three to four times the observed interquartile range; CCC was 0.414. The same leg was identified in 70.9% of runners (kappa 0.309) against a right-side predominance of 75.6%, and the BIA classification was not significantly better than a constant right-leg rule (p = 0.451). Conclusions: Segmental BIA cannot replace DXA for assessing between-limb lean-mass asymmetry in individual trail runners. The disagreement between methods exceeds the asymmetries observed, and directional agreement must be read against the base rate.