How to prepare young horses to start them under the saddle in a sensible way?

As a rider, we have the responsibility to strengthen the body of the horse enough to be able to carry us. By nature, the horse is not made to be a carrier of loads. The human on the back was not in the blueprint of nature; the constant running in turnarounds neither (each corner of the arena is a quarter of a circle!). That is why we have to form the body of the horse and give it time to develop. Muscles and the carrying capacity do not grow overnight! But not only that: the horse must abandon the body patterns given by nature and learn a new one, to become a good riding horse. Apart from the physical component, this has a mental one which is often not acknowledged.