We are constructing an anatomically-correct testbed (ACT) of the human hand to understand its mechanisms, function, and neural control. As a part of this effort, we have created an actuator that mimics both the active and passive behaviors of the human muscles. To use it as the actuator of the ACT Hand, it must conserve the tendon-driven structure so that the size or the weight of the hand does not have to be compromised. A custom-made spring composite is used to closely simulate the human's nonlinear passive muscle stiffness (R/sup 2/=0.99). A coreless DC motor is used to simulate the active contraction. The passive and active components interact in parallel and they are attached to a cable tendon that inserts into the bones. In order to study the neural control of hand movements, we incorporate Hill's model in the active contraction component. Therefore, the computer program can simply specify the muscle activation level, and the appropriate tendon tension can be provided given the current muscle length. This muscle actuator allows an accurate realization of neural activation based control of the ACT Hand, and provides a foundation for intimate brain-machine interface.