ScienceDaily (May 18, 2011) — A University of Utah study shows that men hit harder when they stand on two legs than when they are on all fours, and when hitting downward rather than upward, giving tall, upright males a fighting advantage."
Why have we, of all the apes, evolved an upright stance? David Carrier, a biology professor, thinks it was because it made us better fighters, and to back up this idea he has conducted a study to show how. Using male boxers and martial arts specialists he measured how hard they could hit while on their hands and knees and while standing. I think it comes as no surprise that the hardest blows came in the standing position. While of all the directions the downward hit is by far the most forceful, more than three times harder than an upward blow. Just think of those wildlife documentaries most of you must have seen of gorillas, chimps, bears and even horses fighting for mates or territory, and all of those animals rise up on their hind legs to fight.
All this, David propose, explains why women are instinctively attracted to taller men whilst men find average or short women sexually attractive. Women are unconsciously drawn to men who give the impression they can protect their offspring and partner effectively.
So the conclusion for all males out there must be - stand up straight, and if you do get into a fight and are knocked down, get straight up - fighting from the ground just isn't effective.
In detail here:
David R. Carrier. The Advantage of Standing Up to Fight and the Evolution of Habitual Bipedalism in Hominins.PLoS ONE, 2011; 6 (5): e19630 DOI:10.1371/journal.pone.0019630