For people affected by stroke, frequent physical therapy has been shown to be an effective form of rehabilitation. To this goal, several home therapy devices have been developed. Many of these devices may benefit from the use of a bidirectional pneumatic muscle actuator. This work presents the concept and design of the double-acting, compliant, spring over muscle (SOM) actuator. The principle design uses a spring in parallel with a pneumatic muscle actuator. This concept is economical, and easily scalable. Additionally, a design proposal for an ankle rehabilitation device, which incorporates the SOM actuator, is discussed.