Surface Electromyography (SEMG) signals have been widely used for the functional assessment of muscles. On the other hand, sonography has been commonly used to detect the morphological information of human muscles in both static and dynamic conditions. In this study, synchronized ultrasound images and SEMG signals were continuously collected from the brachialis during the process of isometric contraction and subsequent relaxation together with the generated torque measured by a dynamometer. Then the relationships among the root mean square (RMS) of SEMG, the pennation angle, and the torque were investigated. The results of the contraction phase showed an exponential relationship between the RMS and the pennation angle, and a linear relationship between the torque and the pennation angle. The results suggested that SMG together with SEMG may potentially provide a more comprehensive assessment for the muscle functions.