Stroke is the leading cause of adult disability. Many robots have been developed to administer movement therapies or provide physical assistance to stroke survivors suffering from movement deficits. One effective approach has been to support the weight of the arm, offloading shoulder abductor muscles that have become coupled to elbow muscles. However, patients have limited access to such robots due to the robots' complexity, cost, and bulk. To counter this problem, we developed a lightweight (350 g), inexpensive external actuator, which we call an exomuscle. We constructed a prototype exomuscle by reinforcing a plastic bladder with a fabric bag that is sewn to supporting straps. The bladder can then be inflated with pressurized air to provide expansive forces between the user's torso and arm, supporting shoulder abduction. A seam acting as a hinge joint connects the exomuscle to the torso. We demonstrate that our exomuscle reduces muscular effort by 74% in isometric tasks and 72% in dynamic reaching tasks while minimally affecting the range of motion of the shoulder and elbow (average 4% reduction) on three users ranging from 165 to 188 cm tall. Future studies will evaluate the exomuscle with users who have post-stroke motor impairments.