Background: A new test for assessing supraspinatus muscle (SM) injury, the Diagonal Horizontal Adduction test (DHA), has been developed which demonstrates more isolation of the SM and the similar widening of cross-sectional area (CSA) visualized through musculoskeletal ultrasound (MSK). Purpose:The purpose of this study was to determine whether the resistance changes in CSA of the SM using DHA testing are consistent with the forces generated by this muscle under voluntary contraction using Isometric Dynamometry (ID). Study Design:A prospective cohort study involving 44 subjects (mean age of 24.2 ± 1.9) with no previous history of shoulder surgery or pathology. All subjects performed graded resistance and no controls were necessary. Methods: Subjects were observed and measured while providing isometric contraction in the DHA position using Biodex ID during an isometric contraction at resistance levels of 0%, 50%, and 100% of maximum contraction. Cross-Section Area (CSA) of the SM was captured and measured using a Terason t3200 MSK ultrasound unit.Results: One-way Analysis of Variance with repeated measures examining differences of CSA at each level of resistance were significant at p < 0.05. Post-hoc testing revealed a significant difference in CSA between 0% and 100%, 0% and 50%, and 50% and 100% maximal voluntary contraction of the muscle. Conclusions:The CSA of the SM using MSK is consistent with the amount of force generated by the muscle during contraction. Greater force led to significantly greater CSA. Clinical Relevance:The change in CSA during increased resistance suggests previous comparisons of SM isolation using the DHA technique are useful in determining strength of contraction and amount of activation during manual testing and may be able to be used in place of fine-wire EMG.What this Study Adds to Existing Knowledge: The DHA test for the supraspinatus shows comparable results with CSA and MSK Ultrasound as compared to other SM testing methods such as local and fine wire EMG.