Muscles are biological actuators with unique properties compared to traditional mechanical actuators. The ability to simultaneously modify stiffness while providing power as well as the potential for self-repair make their use desirable when considering the design of robotic systems. Improving our understanding of how muscles work as actuators, struts, and springs is essential for both robotic design and for understanding animal locomotion. Unfortunately, our understanding of the energetics of muscles is incomplete, and traditional methods for studying muscles have inherent limitations. Our group has developed a novel apparatus that allows us to test muscles in unique ways by tethering the muscle to a movable platform coupled to a computer-simulated load. With this setup, we can vary the parameters of the "virtual load" to address more complex and relevant questions, and examine the dynamic interactions between muscles.