Designing optimal pneumatic muscles for a particular application requires an accurate model of the hyperelastic bladder and how it influences contraction force. Previous work does not fully explain the influence of bladder prestrain on actuator characteristics. We present here modeling and experimental data on the actuation properties of artificial muscles constructed with varying bladder prestrain and wall thickness. The tests determine quasi-static force–length relationships during extension and contraction, for muscles constructed with unstretched bladder lengths equal to 55%, 66%, and 97% of the stretched muscle length and two different wall thicknesses. Actuator force and maximum contraction length are found to depend strongly on both the prestrain and the thickness of the rubber, making existing models inadequate for choosing bladder geometry. A model is presented to better predict force–length characteristics from geometric parameters, using a novel thick-walled tube calculation to account for the nonlinear elastic properties of the bladder. It includes axial force generated by stretching the bladder lengthwise, and it also describes the hoop stress created by radial expansion of the muscle that partially counteracts the internal fluid pressure exerted outward on the mesh. This effective reduction in pressure affects both axial muscle force and mesh-on-bladder friction. The rubber bladder is modeled as a Mooney–Rivlin incompressible solid. The axial force generated by the mesh is found directly from contact forces rather than from potential energy. Modeling the bladder as a thin-walled tube gives a close match to experimental data on wall thickness, but a thick-walled bladder model is found to be necessary for explaining the effects of prestrain.