What a pity Sir Isaac Newton is unavailable for comment! Having thought about this before, I had assumed it was down to conditioning as some players do seem better at sustaining a jump. What I am surprised is not seen very often, is a concerted effort by players to use team mates' shoulders to gain height. It almost makes sense for certain players to be nominated as the best headers of a ball at their club and they then use team mates' shoulders at set pieces. Of course, this may be outlawed in football via an obscure regulation Lifting was once illegal in Rugby Union, but it is now legitimate.