I asked that over on the ABC bodybuilding website a while back and this is the reply I got:
"When you do a pushup, you are exerting a moment about your point of rotation (where your toes touch the ground) that's equal to the moment (torque) gravity exerts using your bodyweight. If you looked at your body as rotating about where your toes touch the ground (what happens when you do a pushup), then you could analyze the forces pretty easily. If you weight 150 lbs, assume your center of gravity is about in the middle of your stomach. Say that location is 4'from your toes when you do pushups. As gravity acts downward through your CG (you can get the same moment by using a point load at the CG as the integrated moment over whole body), it exerts a moment of about 150 lbs*4 ft or 600 ft-lbs. about the pivot point. So when you do pushups, if the line of action of your pushing force is 5' from your toes, you have to exert the same torque in the opposite direction to raise your body (rotate about where your toes touch the floor) against gravity. So you have to exert 600 ft-lbs. You have a 5' moment arm, so you have to exert 120 lbs. of force."
Evidently no one esle knew what that meant either so after much discussion everyone decided it was about 75% of your body weight....
Pushups
