This paper presents the importance of endpoint stiffness and its role in improving the interaction stability of a human-robot collaborative task. A low effort collaborative task is simulated with the help of an admittance controlled robot. The performance of this robot for different levels of grasp stiffness are compared and a solution in the form of a Variable Stiffness Mechanism is provided. This mechanism provides an opportunity to modify the stiffness at the port of interaction based on two measures, an instability index in the frequency domain, and human muscle contraction in the time domain. Experimental results show an improvement in the performance and stability for the system with high stiffness vs low stiffness. Human muscle contraction provides a time instant at which the stiffness has to be modified and the instability index value provides information about the direction in which the stiffness has to be modified.