Physics of a pushup

Push ups are probably the most common form of a workout you will see.  They are easy and can be done almost everywhere, but how good of a workout are they actually giving people?  There are different ways to do a push up but when done correctly, most people will be lifting around 65% of their body weight.  This would be when someone is doing the standard position of having their feet kicked out.  When a push up is performed on the knees instead of feet extended, we see that the amount of body weight they are lifting goes down by around 15%.  These percentages show how push ups can also be conformed around what each person wants.  Some can be putting in more of a workout while others can adjust their position in order to suit their needs or put in more reps.