In this paper we present a method to stimulate multiple muscles in a human arm to perform interaction tasks, using an implanted Functional Electrical Stimulation (FES) neuroprosthesis. The unstable effect arising from interaction tasks is considered, and the arm stability is directly treated as one of the control objectives in the controller design. By exploiting the kinematic and muscular redundancy of the system, we can control the interaction force and the arm's stiffness property simultaneously, thus ensuring the stable execution of interaction tasks. A representative example of such interaction tasks, namely the 'pushing with a stick' task, is simulated. It is found that using our proposed controller, as compared to a previously developed feedforward FES controller that does not consider arm stiffness or stability, the stability of the arm is guaranteed while the task of force control is correctly achieved.