This paper proposes a novel technique for the real-time estimation of the joint torques variations in humans while performing heavy manipulation tasks. To achieve this, the method is based on the deviations of the Centre of Pressure (CoP) and Ground Reaction Force (GRF) in the presence of interaction forces. The CoP and GRF variations are calculated from the difference between the estimated values (assuming no interaction forces) using a pre-identified statically equivalent serial chain (SESC) and the measured ones (with the effect of interaction forces) using an external device. The calculated variation vectors and the measured joint angles of the human body are then used for the estimation of the overloading joint torques in real-time. We evaluated the efficacy of the proposed method both in simulations and experiments, in various poses of the human and interaction force profiles.