(46) RELATIONSHIP BETWEEN RAW BIOIMPEDANCE VALUES AND MUSCLE MASS, SIZE, AND QUALITY IN EUMENORRHEIC WOMEN

Purpose: Assessing muscle mass, size, and quality is important for tracking performance and recovery in women. While ultrasound (US) is commonly used to assess muscle characteristics, bioelectrical impedance spectroscopy (BIS) may offer an accessible alternative. Prior research in men has shown that BIS-derived phase angle is negatively correlated with echo intensity (lower echo, higher muscle quality), suggesting an association between bioelectrical properties and muscle characteristics. However, the direct relationship between raw BIS values (impedance [Z], resistance [R], reactance [Xc], and phase angle [PhA]) and muscle characteristics (mass, size, and quality) remains largely unexplored in women. Therefore, the purpose of this study is to evaluate the relationship between raw BIS values and muscle characteristics—mass (fat-free mass index [FFMI]), size (cross-sectional area [mCSA]), and quality (echo intensity [EI])—in the leg and thigh segments of eumenorrheic women. Methods: Fifteen eumenorrheic women (age: 21.2 ± 1.0 yrs; %BF: 26.4 ± 1.2%; BMI: 23.9 ± 0.6 kg/m²) completed a single visit during the low hormone phase ([LHP]: 1–7 days from the start of menses). Raw BIS values (Z, R, Xc, PhA) were obtained for both the full leg and thigh (vastus lateralis [VL]) segments. Leg FFMI (kg/m²), was calculated based on leg fat-free mass (FFM; via a separate multifrequency bioimpedance analysis device) and leg length [FMMI=Leg FFM/Leg Length (m)²]. Size (mCSA) and quality (EI) of the VL were assessed via panoramic US scan. Two BIS and US measurements were taken and averaged for analysis. All BIS data were analyzed at 50 kHz using time-delay corrected values. Pearson’s correlation coefficients were used to assess associations between BIS and muscle characteristic outcomes. Results: Higher leg Z and R were associated with lower FFMI (Z bivariate: r= -0.526, p=0.044; R bivariate: r=-0.527, p=0.043). However, no significant associations were observed between FFMI and Xc (r=-0.286, p=0.301) or PhA (r=0.151, p=0.592) for the leg. There were no significant associations between leg Z, R, Xc, or PhA and mCSA (bivariate: r=-0.277 to -0.169, p=0.318–0.548) or EI (bivariate: r=-0.035 to 0.173, p=0.381–0.902). Similarly, no significant associations were observed between thigh Z, R, Xc, or PhA and FFMI (bivariate: r=-0.073 to -0.052, p=0.797–0.852), mCSA (bivariate: r=0.024 to 0.329, p=0.231–0.931), or EI (bivariate: r=-0.223 to 0.138, p=0.425–0.631). Conclusions: These findings suggest that leg BIS-derived Z and R are inversely associated with leg FFMI but are not significantly related to muscle size (mCSA) or quality (EI). The inverse relationship between Z and R with FFMI is consistent with the physiological expectation that lower FFM corresponds to higher R due to FFM and muscle's positive association with intracellular water. In contrast, thigh BIS-derived bioimpedance showed no significant associations with muscle mass (FFMI), size (mCSA), or quality (EI). Practical Applications: BIS may serve as a quick, non-invasive tool for athletes and clinicians to estimate and monitor leg muscle mass trends of FFMI in eumenorrheic women. However, the weak correlations between BIS values of the leg and thigh with ultrasound-derived muscle characteristics suggest that its utility as an independent method for assessing muscle size and quality require further investigation. Acknowledgements: None