LAUSR

Background The use of raw bioelectrical variables, such as resistance ( R ), reactance ( Xc ), and phase angle ( φ ), has been advocated for evaluating physiological changes. Methods Before and after 8 weeks of resistance training, adult females were assessed via multifrequency bioelectrical impedance analysis (MFBIA; Seca® mBCA 515/514) and bioimpedance spectroscopy (BIS; ImpediMed® SFB7). Data were analyzed to determine whether cross-sectional estimates and changes (i.e., Δ scores) of R , Xc , and φ differed between devices at 16 shared measurement frequencies ranging from 3 to 1000 kHz. Results Cross-sectionally, strong correlations ( r  ≥ 0.96) were observed for R across all frequencies, although MFBIA produced values 9–14% greater than BIS. Strong correlations ( r  ≥ 0.92) for Xc and φ were observed up to frequencies of ~150 kHz. BIS produced greater Xc and φ values at lower frequencies, while MFBIA produced greater values at higher frequencies. In general, proportional bias was not observed, with the exception of Xc at high frequencies and φ at low frequencies. Δ R did not differ between devices at any frequency and was correlated at all frequencies. Δ Xc and Δ φ did not differ at any frequency and were correlated between devices for frequencies up to ~300 kHz. Proportional bias was generally not observed longitudinally. While individual-level errors were potentially acceptable cross-sectionally, they were concerningly high longitudinally. Conclusion Despite notable differences in the characteristics of the bioimpedance devices and cross-sectional disagreement, strong group-level agreement for detecting changes in R , Xc , and φ was generally observed. However, large errors were observed at the individual level.