fmincon and forward dynamics

Hey! I have a pendulum with three bars. The first and second joint are controlled via fmincon optimal control. The third one is supposed to be "free". Now I want to add constraints, since the third has to act like a torsion spring (around a certain angle) and must not exceed the angles -pi/4 and pi/2.

How can I fix the constraints for the third bar into the fmincon function without adding a torque to the joint?

Thanks! Best wishes V