Tune parameters during simulation on SimMechanics

I'm working on a structural optimization code for a non linear compliant beam. The system could be synthesized as a square beam with lumped parameter approach ( Flexible Bodies on SimMechanics )

Since the beam needs to achieve a certain desired geometry, the material behaviour needs to be updated as the bending moment changes.

To do so, the spring stiffness on the revolute joint on Sim Mechanics 2nd Generation needs to change during the simulation. After reading the user manual (page 5-43), I found that the parameters on Body Spring and Damper are "Tunable".

I tried many approaches, either by updating on the Matlab's workspace, Model workspace, or either by hand pressing on pause, but the change doesn't happen.

Any help on the subject would be really appreciated.

Thanks a lot in advance.

Bruno.