Spinal muscular atrophy (SMA) and Duchenne muscular dystrophy (DMD) are serious neuromuscular disorders that occur in 1 in 11,000 live births and 1 in 6,000 boys, respectively. Recent pharmaceutical advances have revealed the lack of sensitivity in current clinical assessments to functional changes in patients. Ultrasound has become a common imaging modality to measure skeletal muscle architecture and quality due to its non-invasiveness and low cost; however, a link between ultrasound measurements of muscle architecture and quality and muscle function has not been well established. Previous studies have demonstrated the use ultrasound to measure quality of skeletal muscle in neuromuscular disease based on the echogenicity or brightness to estimate disease progression. However, other studies report no correlation between muscle force per unit area (specific tension) and echogenicity, though these studies were conducted in small age range groups of healthy subjects. The goal of this study was to examine the relationship muscle strength and ultrasound measurements across a range of individuals with SMA and DMD, as well as age-matched controls. We used ultrasound and dynamometry to measure biceps brachii structure and function in children with SMA and DMD and age/sex matched healthy controls. The dominant arm biceps brachii muscle of forty-two subjects (patients w/ SMA: n=10, patients w/ DMD: n=19, healthy controls: n=13) were imaged in the midsection in the longitudinal and transverse planes by an experienced musculoskeletal sonographer. An image processing algorithm was developed to measure the cross-sectional area (CSA) and average echogenicity of the muscle. Maximum voluntary elbow flexor moment was measured using a handheld dynamometer placed on the forearm with the elbow positioned at 90 degrees. Estimated specific tension was calculated by dividing the elbow flexor moment (Nm) by the CSA (cm2). We found that estimated specific tension hyperbolically correlated with echogenicity of the muscle in each group of subjects (R2=0.4151). We also found the average echogenicity in subjects with SMA and DMD to be significantly higher than healthy controls using an ANOVA test with Tukey’s HSD post-hoc test (DMD: p=0.00008, SMA: p=0.00002). Our findings suggest that echogenicity measured from ultrasound relates to the functional capacity of the muscle. We plan to track subject measurements over time to develop a novel, objective, and sensitive functional assessment.