It is of course true that , like many other animals , knuckle walkers like chimpanzees adopt a bipedal crouch like position at the start of fights with their own kind .However ,as such fights progress the protagonists move frequently between an upright stance and a quadruped stance to avoid being overcome by the weight of their opponents ,pinned ,and bitten .

Obiligate bipeds do not have the quadruped stance option . If they are grounded ( knocked over ) they will find it very hard to regain their footing . I just cant see past this disadvantage unless weapons are brought into the equation .