This paper addresses the problem of heavy load co-manipulation in the context of physical human-robot interactions (PHRI). During PHRI, the resulting motion should be truly intuitive and should not restrict in any way the operator's will to move the robot such he would like. The idea proposed in this paper consists in considering the PHRI problem for handling tasks as a constrained optimal control problem. For this purpose, we introduce a modified impedance control method named Online Trajectory generator-Based Impedance (OTBI) control. This method relies on the implementation of a specific event controlled online trajectory generator (OTG) combined with control structure allowing a good tracking of the generated trajectory with a desired impedance property of the physical interaction. This OTG is designed so as to translate the human operator (HO) intentions into ideal trajectories that the robot must follow, while enabling the HO to keep authority over the robot's states of motion. The key idea of this approach consists in generating a velocity trajectory for the end-effector that stay collinear to the interaction force at every moment. The overall strategy is applied to a two DOF robot.