"at least some of angular momentum generated in the kicking leg seems likely to be generated by the vertical deceleration of the pelvis cause by the standing /support foot meeting the ground after the final approach stride."

As the standing or supporting foot meets the ground and produces deceleration , the tissues in the supporting leg will store energy as they are stretched . Energy return will start as deceleration ends giving vertical acceleration of the pelvis and further angular acceleration of the swing leg .

I believe that the same system may be play during running . Measurement is required to confirm though .