“Percentages of body weight supported by the hands during push-ups at various postures”
A new study in the Journal of Strength and Conditioning Research identified the percent of body weight you lift with four varieties of the push-up. In a regular push-up, you lift 64 percent of body weight, whereas with a knee push-up, you lift 49 percent. If you’re new to training, performing the push-up with hands elevated on a 24-inch bench will allow you to lift even less than a knee push-up, at 41 percent of body weight. Elevating the feet on the 24-inch bench makes it harder and allows you to lift 75 percent of body weight, still not enough to elicit serious strength gains if you can bench press 225 lbs.
Here is the study he cites from the Journal of Strength and Conditioning Research.
Kinetic Analysis of Several Variations of Push-Ups