zLineOfAction will try to contract the muscle based on curve length. You can adjust how sensitive it is by adjusting pos sensitivity. (A higher value will cause the muscle to contract on a smaller variation in curve length)
Material stiffness is represented by the Young's modulus attribute on the zMaterial node.
Young's modulus is just the engineering term. Incidentally, you can look up values for different materials on Wikipedia and plug them into our system and it should behave as expected for that material.
When you disconnected the node piping into excitation, did you also set excitation to zero? Do you have a Ziva cache in your scene?