A three-dimensional finite element analysis was conducted to simulate the effects of the varying material parameters on the contraction behaviors of a muscle-tendon complex using an active finite element method. The material behavior of the skeletal muscle was assumed to be orthotropic and the muscle model consists of two parts: the active and the passive parts. An active finite element method was then used for accommodating both the active and passive behaviors of the muscle into the muscle model. In this active-passive muscle model, the active component is governed by an activation level, a time period, a muscle sensitivity parameter and a strain rate. The material property of the passive component was assumed to be viscoelastic and the tendon is assumed to be linear elastic. The effects of activation amplitude and viscoelastic material parameters on the active, passive and total force-length relationship of the cat muscle under isometric contraction were predicted. The predicted results were found to be close to the experimental data reported in the available literature. Hence, the active-passive muscle model was extended to simulate the stress distribution of the cat muscle subject to shortening contraction and different activation amplitude. By varying the magnitude of the material parameters, different muscle behaviors could be generated. The proposed active finite element method lays a good foundation for simulation of human musculoskeletal motion.