I havent read all your replies sorry but have just had a converstaion with my farrier about young horses and the fact I thought that maybe I should be doing a little more with her to build up her muscle as she intermittently gets locking stifle, shes 4. He basically said that in his opinion you get 2 types, those that are physically but not mentally capable and those that are mentally but not physically able, the first often end up with blown minds from doing too much and the second often broken by the time they are 10. My horse falls in the second catagory, so was backed in March as she was basically bored and has hacked lightly for the last couple of months. He thought my light hacking was perfect and that next yr she will be physically stronger to start properly. He said too many people are in such a rush, hunting, jumping and endless circles which often results in a shorter life/work span.