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.

Background: Between-limb lean-mass asymmetry has been proposed for injury-risk and re-turn-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, 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 predomi-nance of 75.6%, and BIA did not outperform a constant right-leg rule (p = 0.451). Conclusions: Segmental BIA and DXA are not interchangeable for between-limb lean-mass asymmetry in trail runners: the disagreement between methods exceeds the asymmetries observed, and directional agreement must be read against the base rate.

Background: Converting skinfold thicknesses into body fat percentage (%BF) assumes that subcutaneous adipose tissue (SAT) represents a fixed fraction of total fat, an assumption untested at extreme leanness. This study assessed whether skinfold–dual-energy X-ray absorptiometry (DXA) disagreement is random or structured by regional SAT. Methods: Thirty-five elite (Tier 4) female trail runners underwent anthropometry (Withers equation) and DXA in one session. Agreement was evaluated via Bland–Altman analysis and Lin's concordance correlation coefficient (CCC). The inter-method difference (%BF_GAP = %BF_WITHERS − %BF_DXA) was regressed on android SAT, adjusted for total adiposity, and benchmarked against DXA-derived negative controls. Results: The methods correlated closely (r = 0.866, 95% CI: 0.749 to 0.931) but showed poor concordance (CCC = 0.311). Skinfolds systematically underestimated DXA by 5.50 percentage points (pp; total error 5.74 pp), displaying proportional bias that widened with adiposity. Android SAT pre-dicted %BF_GAP (R² = 0.489). Residualized on total fat mass, SAT retained an independent contribution (ΔR² = 0.106), which no negative control showed. Propagated skinfold imprecision (SD = 0.19 pp) accounted for only ~1.3% of the observed difference variance (SD = 1.66 pp). Conclusions: The disagreement is systematic, phenotype-dependent, and structured by regional SAT, reflecting accuracy rather than precision limitations. Con-verted skinfold %BF should not be reported alone or compared against DXA reference data; raw skinfold sums are preferable, as a variable to be observed rather than a target to be reduced.