Designing joints with adjustable stiffness is of great interest in humanrobot collaboration tasks. In addition, remote cable actuation allows inertia reduction. Actuating a revolute joint with a pulley and two antagonistic cables is a simple solution, but stiffness modulation is possible only with nonlinear springs or other additional devices. This paper proposes a simple solution to overcome this limitation. It consists in using an eccentric pulley. We show that this solution allows coactivation as in biological joints, i.e. increasing antagonistic forces increases stiffness.