Well, without having read the entire study and their parameters and controls and whatnot, I will say that limb proportion for a modern human is such that being on all fours is awkward for most activities, which would include striking downward. Looks to me like the photos show people down on their knees, which is an awkward position to be in for many activities. Great apes have a different limb proportion so they are still on their feet and not down on their knees when knuckle-walking, and being in that posture is MUCH less awkward for them than for humans. Early human ancestors would have had different limb proportions from modern humans as well, so I'm not sure how this theory would hold up, in terms of human evolution. Early human ancestors and apes of any kind, do not walk around on their knees. Whatever their limb proportions, and whether they are bipedal or quadrupedal, they are on their feet, not their knees. People did not evolve from moving on their knees to moving on their feet. So conducting the study with people doing activities while on their knees strikes me as very problematic for the study.
