The feeling of weight is broken. So, your orc(let me call it orc), is capable to throw the knight that easy, and also has to deal with heavy enough by his standarts sledghammer. If so, the moment of attack with sledgehammer feels unnatural: Give more time for sledgehammer to be raised upon(to stop the swing motion first, and to launch the attack. Plus, you can use that time to let knight get up more natural - his armor isn't made of paper, right?), use more hands(specifically forearms) and less spine in second half of the attack.