Muscle thickness and muscle quality (echo intensity) are correlated with lower body strength/function in young/middle aged adults

B-mode ultrasonography can be used to determine muscle morphology (i.e., muscle thickness - MT) and muscle quality (MQ - i.e., gray scale). Ultrasound measurements are associated with performance in physical fitness tests; yet most studies perform ultrasound measurements at midpoint (50%) of the muscle, thus not being representative of the entire length of the muscle. Also, studies using a muscle group like the quadriceps femoris, often only perform ultrasound measurements in only one muscle (e.g., rectus femoris - RF). Purpose: To evaluate whether muscle ultrasound measurements (muscle thickness and echo intensity) averaged across the muscle length (30%, 50%, and 70%), from the rectus femoris (RF), vastus lateralis (VL), and vastus intermedius (VI), correlate with lower body strength/function in young/middle aged adults. We hypothesized that the RF would have the greatest correlation with the functional tests. Methods: Participants 18-45 years of age participated in the current study. Ultrasound measurements were performed on the RF, VL, and VI while the participant seated (90° knee flexion). Measures of MT and MQ (gray scale analysis) were used to assess muscle morphology. Study participants were also evaluated on the time to ascend a flight of steps, knee extension torque (KET), vertical jump height, and flight time with the Just Jump ® mat. Bivariate correlation analysis between MT, MQ, and functional tests was performed to determine the Pearson’s correlation coefficients. Results: 18 participants (6 men/12 women; age: 26.1±8.01 years) were included. MT from RF had a moderate correlation with KET (r=0.59; p=0.008), jump height (r=0.66; p=0.002), and jump time (r=0.65; p=0.003). MT from VI had a strong correlation with KET (r=0.79; p<0.0001), jump height (r=0.73; p=0.006), and jump time (r=0.71; p=0.0008), while having a moderate correlation with time to ascend a flight of steps (r=-0.57; p=0.01). MT from VL had a moderate correlation with jump height (r=0.52; p=0.02), and a weak correlation with jump time (r=0.49; p=0.03). VI and VL MQ values had a moderate correlation with KET (VI: r=-0.55; p=0.01/VL: r = -0.54; p=0.02). RF and VL MQ values had a moderate correlation with jump height (RF: r=-0.53; p=0.02/VL: r =-0.64; p=0.003), while VI MQ had a weak correlation with jump height (r =-0.45; p=0.005). RF and VL MQ values had a moderate correlation with jump time (RF: r=-0.52; p=0.02/VL: r =-0.64; p=0.004), while VI MQ had a weak correlation with jump time (r =-0.45; p=0.005). Conclusion: Our results show that muscle morphology from three of the muscles of the quadriceps femoris present association with physical performance. Moreover, VI muscle morphology appears to show the highest correlations with physical performance in our population. SIGNIFANCE/NOVELTY: Results from the study showed for the first time that MT and MQ might influence physical performance in young/middle-aged adults. Additionally, exercise trainers, coaches, and physical therapists can leverage ultrasound measurements to gain a better understanding of how muscles influence physical performance. Supported by Towson University Department of Kinesiology (DAGM) and Towson University College of Health Professions Summer Undergraduate Research Institute (DFM). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.