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 have designed a new modified impedance control method named Online Trajectory-Based Impedance (OTBI) Control. This method relies on the implementation of a specific event controlled online trajectory generator (OTG) interconnected to a classical structure of impedance control allowing a good tracking of the generated trajectory. This OTG is designed so as to translate the human operator intentions to ideal trajectories that the robot must follow. It works as an automaton with two states of motion whose transitions are controlled by comparing the magnitude of the force to an adjustable threshold, in order to enable the operator 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 would stay collinear at every moment to the PHRI force. The overall strategy is applied to a two DOF robot.