Sleeve pneumatic muscles have shown significant performance improvements over conventional air muscle design, offering increased energy efficiency, force output, and stroke length, while allowing the actuator to become a structural component. However, there remain comparatively few studies involving sleeve muscles, and current applications have not focused on their potential advantages for joints actuated antagonistically with two muscles or their application to a more general class of pneumatic artificial muscle. This research presents a modular sleeve muscle design using the McKibben type construction, with a separate membrane and braid. To further increase stroke length, an internal pulley mechanism is implemented. The performance of the sleeve muscle is compared to an equivalent unaltered muscle and shows substantial improvements in force output, stroke length, and energy efficiency. Further testing shows that the internal pulley mechanism increased the effective stroke length by 82%, albeit at the cost of reduced maximum force output.