Robotic Exoskeletons for legs are able to act as excellent evaluation as well as training devices while reducing physical effort that a therapist needs to expend. For such exoskeletons, assist as needed control strategies are advantageous because these lead to active participation from the user. One such strategy is based on Force Fields where the controller creates a virtual force field around the foot of the user to aid the user’s leg motion. Force field controllers (FFC) presented in literature require the knowledge of parameters such as length, mass, position of center-of-mass and moment of inertia for the combined human-exoskeleton system. This paper presents an “adaptive” assist as needed control strategy that creates a virtual force field similar to the FFC without using any prior knowledge of human inertial parameters like mass, the moment of inertia, the position of COM, etc. A lyapunov stability proof for the proposed controller is also presented along with simulation results demonstrating the controller.