Is the data from the Gait Lab in the form of a force-time profile for each part of the foot striking the instrumented ground?  The first profile is the heel strike, the next profile is the ball of the foot strike, etc?

Are you then trying to apply those forces to the Ankle Foot Orthotic FEM?  Does the FEM include a model of the bones of the foot and the ligaments and flesh around the bones?

Please reply with images of the AFO and other elements of your FEM. Show how you are supporting the FEM, point out what is the fixed support. I expect it is some point along the tibia/fibula.

I can imagine the inversion of the model. Ground is fixed and the foot must strike the ground and roll forward from heel to toe.  While it may be simple to get something working, it may be difficult to get the contact forces to match the lab data, so I can see the attraction of applying the lab force data directly to the AFO, Foot, Anke FEM.