That sounds like a reasonable assumption. In your AP4 walk.pz2 we have "hipHeight 0.381689", I will try raising that value to 0.400000, and to 0.430000, and see what happens.

This could have other concequences, like shortening the stride, making the feet slide, or making the legs bend less at the knees, which might make the walk look stiff. We will just have to see.

I think I'm going to have to start looking at feet sliding, have you noticed any so far with your AP4 poses?