This paper proposes a modified Theo-Jansen mechanism to develop an exoskeleton for people with muscle weakness in the lower extremities. The synthesis of this modified mechanism is performed using genetic algorithms to adjust its movement to a conventional human walking or jogging. The movement patterns of real leg’s trajectories are experimentally obtained through image processing. According to their relevance, these patterns are analyzed and weighted, to optimize a weighted cost function. Furthermore, the kinematic and kinetic analysis of the mechanism are briefly shown.