Cortez
Tough but Fair
Draught horses pull, leaning into the collar over their shoulders so all the weight goes forwards. Riding horses move the centre of gravity back by lowering their hindquarters and carrying more behind (well, properly trained ones do, at any rate). Opposite uses of the horses power. I don't know of any studies per se (I'm sure there are some), but I rely on what several and various experienced vets tell me; big and very tall horses tend to have more problems, with their hocks, tendons, joints, hearts and wind, etc..