Here is a functional alternative to your posterior tibia glides on the plynth. This athlete cannot walk without an antalgic gait. The athlete performs a LLE pivot lunge while partial weight bearing with the crutches. The R heel must remain on the ground at all times. Frequently check to make sure the R foot does not externally rotate. If it does, it means the athlete is stepping too far. You can easy get down on one knee and use your hands to slow the tibia down/speed up the femur as the L leg comes forward. It is also a great way to re-train gait. Another example of the athlete correcting his own dysfunction using proper appliction of ground, gravity, and momentum.