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The actuators used in wearable robots must behave compliant. This, however, is not easy mainly due to two reasons: lack of force measurement capability and lack of backdrivability. Both of above needs to be realized, but the latter issue lies in the actuation principle of conventional robot actuators. We have been investigating in the electro-hydrostatic actuators to realize backdrivability by minimizing friction due to the reduction. Still, the way to design actuator, considering backdrivability, including the one due to the internal leakage, was an open problem. We introduced the actuator model “Series Dissipative Actuator” to explain the actuator behavior. This model enables us to understand the roll of internal leakage in the hydraulic system and it shows the design guideline considering the backdrivability. In this paper, we show the method to apply SDA (Series Dissipative Actuator)concept on EHAs (Electro-Hydrostatic Actuators). Development of the actuator prototype targeted for use in lower limb is also presented and preliminary results in backdrivability evaluation is performed.