I stumbled upon research saying that around 60% of the vertical movement of the horse is realized by the front legs and around 40% by the hind legs.
If you know physics, you will understand why:
It only says how the weight of the horse is distributed between the front and hind legs.
The natural movement pattern of the horse is “falling down”, which means that the horse moves the centre of gravity outside of the balance, and engages the legs to support the body not to fall down.
If you have children you know that when they learn to walk, they move through the same movement pattern: the upper part of the body moves outside of the balance, and the lower part of the body, which is legs, “catches up” with the upper body to support it not to fall down.
When you learn martial arts, and when you support your horse in discovering how to move in continued balance, you introduce movement in collection, where the vertical movement of the horse is realized more by the hind legs due to the change of the way in which the weight of the horse is distributed between the front and hind legs.
Collection is a new movement pattern, in which the horse is not moving through the movement pattern of falling down, but learns to distribute the weight differently, and because of that engages more the hind legs.
Thanks to that, the body stays in balance, which means that the horse can stop and accelerate in each direction all the time.