Consistency of novel ultrasound equations for estimating percent intramuscular fat

Echo intensity reflects both intramuscular adiposity and fibrous tissue content. Recently, equations that allow investigators to estimate the percentage of intramuscular fat for individual muscles via measurements of echo intensity and subcutaneous fat were created. However, as the trial-to-trial consistency of these values has not been reported in the literature, it is unclear if they can be used to track changes during physical activity and/or nutritional interventions. The purpose of this study was to examine the test-retest reliability of rectus femoris percent intramuscular fat.

Nineteen healthy adults (mean ± SD age = 22 ± 3 year; ten females, nine males) with a body mass index ≤ 30 kg/m² participated. On three occasions, B-mode ultrasonography was used to acquire panoramic images of the dominant rectus femoris in the transverse plane. ImageJ software was used to quantify echo intensity, subcutaneous fat thickness and cross-sectional area. Rectus femoris percent intramuscular fat was estimated with sex-specific equations.

Uncorrected and corrected echo intensity, subcutaneous fat and cross-sectional area exhibited excellent consistency (P > 0·05, intraclass correlation coefficients [ICCs] ≥ .900, standard errors of measurement [SEMs] ≤ 7·26%). Percent intramuscular fat for all participants also demonstrated satisfactory reliability (ICC = 0·980, SEM = 3·07%), with similar findings for males (ICC = 0·970, SEM = 3·63%) and females (ICC = 0·968, SEM = 1·41%).

The high ICCs and low SEMs suggest that ultrasonography-derived rectus femoris percent fat may be a reliable tool for tracking changes in lower extremity intramuscular adiposity.