Soft exosuits are wearable robotic devices that assist or enhance the human muscle performance. A human machine interface simulation platform based on MATLAB-OpenSim interface is developed in this paper for closed loop dynamic simulation with feedback control strategy and to study its effect on human physiology. The proposed simulation model is based on Computed Muscle Control (CMC) algorithm and is implemented using the MATLAB -OpenSim interface. A Gravity Compensation (GC) controller has been implemented on the external device and the resulting decrease in the physiological torques, muscle activations and metabolic costs during a simple load lifting task with two different speeds is investigated.