something like Timing for Animation. Bits of physics calculations and how scenes are placed... but especially important would be how to properly know, estimate, generate poses according to the orientation and position of limbs during walk cycles, so that it loops in exact symmetry and counterposes againsts each others, or at least not produce too much unwanted artifacts, that doesnt even closely resemble its variations like limping, gaits or running... simply has to model the cycle to be as similar and convincing in a mechanically ordered way.