How can I use external force function in RST?

I am trying to build a robot which has 4 revolute joint. I am applying force/payload at the end of robot i.e. at the end-effector using externalForce function in Robotic System Toolbox(RST). But when I am coputing the joint torques I am observing change in torque in just one joint not in all of my 4 joints. What might be the possible reason that I am getting wrong answer ?(Note: I have also defined the home configuration.)