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The paper introduces an active variable stiffness exoskeleton robotic system (AVSER) with the active variable stiffness elastic actuator (AVSEA), which improves the safety for human-robot interaction and produces unique adjustable stiffness capacity to meet the demand for safe active-passive elbow rehabilitation. The AVSEA consists of two DC-motors. One is used to control the position of the joint, and the other is used to adjust the stiffness of the system. The stiffness is generated by a leaf spring. By shortening the effective length of the leaf spring, the AVSEA is able to reduce the stiffness automatically, which makes the AVSER from active (assistive) motion to passive (resistance) rehabilitation during the process of therapy. In the paper, the mechanical design, modeling, and control algorithms are described in details. The capacity of the proposed AVSER with electromyogram (EMG) signal feedback is verified by rehabilitation exercise experiments for the subject to demonstrate the efficacy of the developed system.