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Skeletal muscle ultrasound has recently emerged as one of the imaging techniques to examine muscle quality. Validation of the ultrasound technique and quantification of muscle echo intensity (EI) to units that are consistent with other body composition measures are necessary. Purpose: The purpose of these studies was to validate the ultrasound technique with high-resolution T1-weighted magnetic resonance imaging (MRI). The second purpose was to compare ultrasound-measure % intramuscular fat to other health and fitness indicators. Methods: Two experiments were performed. The first experiment examined four muscles in the lower extremity using both ultrasound and MRI. Muscle EI and MRI-measured % intramuscular fat of each muscle were compared and used to establish calibration equations. The second experiment applied the calibration equations to convert EI to % intramuscular fat and compared the ultrasound-measured % intramuscular fat to some health and fitness measures. Results: In the first study, three types of calibration equations were established based on the strong correlations found between MRI % intramuscular fat and muscle EI after correcting for subcutaneous fat thickness (r ≥ 0.8 in all four muscles). The ultrasound technique also demonstrated high between-day reproducibility (r ≥ 0.8 in all four muscles) and inter-analyzer reliability (r = 0.9 in all four muscles). In the second study, strong correlations were found between ultrasound-measured % intramuscular fat of the four muscles (r ≥ 0.8). Weak to moderate correlations were found when compared % intramuscular fat to BMI, waist/hip ratio, muscle thickness, and muscular strength. Relationships between ultrasound –measured % intramuscular fat, physical activity level, and frequency of fast food consumption were also observed. Conclusion: Muscle ultrasound is a practical technique to examine muscle health when other imaging techniques are not available. Future studies are needed to validate the calibration equations presented in these studies as well as establish more equations for other muscles to enhance its use in both the research and clinical settings. More studies are also needed to investigate the complex relationship between intramuscular fat and lifestyle.