Joints in the human body utilize torque to create movement, where a force F acts at a distance, r, from a rotation point. In biomechanics, F is generated by muscles acting on the joint, and r is the distance between the joint’s center of rotation (COR) and the muscle line of action. The distance r is also known as a geometric moment arm (GMA). Changes in GMAs may be correlated with loss of muscle and hand function in people due to aging, injury, or pathology. To comprehensively evaluate GMAs, efficient and inexpensive in vivo measurement methods are needed. This study explores whether ultrasound is valid for quantifying the GMA of muscles crossing the wrist. The capitate was defined as the wrist’s COR due to its central location within the joint, and the measured GMA was defined between this COR and the flexor digitorum superficialis (FDS) muscle. An agar-gel phantom study determined if distance measurements from 20 ultrasound images were accurate compared to four baseline distances. A subsequent in vivo case study explored whether ultrasound could measure GMAs within a human wrist. Both studies returned data that exhibited low measurement variability over consecutive days and provided proof that ultrasound can accurately measure wrist GMAs.