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I want to understand better the mechanism of the Hunt-Crossley sphere forces when used to support the body at the feet. Are the forces bidirectional or uni-directional? I ask because when I apply hip flexion moment to the model standing and supported by Hunt-Crossley forces at the two feet, it has difficulty getting the foot off the ground. As I lower the stiffness and friction values it is just a little easier to lift the foot but the model is less stably supported. If the forces are bi-directional, do I need to kind of switch off the forces whenever I want the model to take a step? I am working with a genuinely inverted pendulum bipedal model that is supported only at the feet. Any insight?

The Hunt & Crossley model is unidirectional. It does include some negative dissipation during separation, but that's combined with positive stiffness forces and the total can never be negative.

Regards,
Sherm