Muscles act as motors that perform positive mechanical work when fibers, which are comprised of sarcomeres and are organized into bundles encased by connective tissue sheaths, shorten across a joint. Here, we examine the relationship between muscle structure and function in a fusiform muscle. We model the muscle as a constant volume barrel to predict regional strain patterns throughout the muscle, and test the model empirically with a fusiform muscle, palmaris longus (PL) in two species of frogs, Rana pipiens (R. pipiens) and Rana catesbeiana (R. catesbeiana). We allowed the muscle to contract against a servomotor at submaximal force conditions while filming with a high-speed camera, then analyzed the images taken at the extremes of the muscle length to measure strain in the inner and outer fibers. Both species of frogs exhibited significantly less strain in the outer muscle fibers compared to the innermost fibers, in broad agreement with predictions from our model. We then added an external constrain to the PL muscle of R. catesbeiana and measured force and length output under otherwise identical conditions. Adding this radial constraint reduced the length the muscle could contract by 50%, resulting in a decrease in the amount of mechanical work it could perform of 50%. We discuss how regional strain variations may similarly affect the positive work output and implications for these findings on natural constraints to radial expansion.