Galloping is something I know a bit about. When I compete at a horse trial I feel awkward up until the countdown ends and I leave the start box at the gallop. Well actually at the level I am competing at it's really a big canter, but it's close enough to get me into my comfort zone.
I've noticed that lots of eventers - at all the levels, have difficulty with the galloping involved in cross country. It's not the fences that bother most of them, it's the perceived loss of control in the gallop. When you gallop, you are no longer in control of each stride the horse takes. This is not because he takes over, it is because the physics of motion come into play and the horse really can't stop or turn immediately, you have to plan ahead.
If you galloped racehorses as I did - and loved it - galloping is your old friend and you and the horse get to relax. This is probably the reason Jimmy says all eventers should ride at the track. You learn to leave them alone and relax for the ride or you'll never make it through the day. It's one thing if you are going to gallop one horse, it's another if you have ten to go. If you don't learn to relax yourself and the horse, you'll never have the energy to make it through the day. The oldtimer's tell you this right away when you come on the scene. Relax, baby, let him gallop!
Each horse takes his own "hold" when galloping. Some pull hard and others take a light hold. We don't try to develop the connection the horse has with the bit at the track, we let the horse develop his own. Basically we put our hands down and he finds the bit. You can develop a horse's mouth while galloping and that's what eventers seek to do. They want the horse to be able to get off the forehand for the jumping, so they don't want the pulling. Your dressage and show jumping work develop the aids to ask the horse to get off the forehand.
At the racetrack and anywhere else for that matter, the faster a horse is allowed to go, the lighter the contact becomes. If the horse pulls harder upon acceleration, it is the rider, not the horse that is creating the pulling. Either the rider has not relaxed and let the horse gallop, or worse, he is pulling against the forward motion of the gallop. At the track you learn quickly not to pull because you get run off with.
The job of the rider while galloping is to relax into it and stay balanced. This relaxes the horse. All of the joints of the rider assume the function of shock absorber. This keeps the rider in the center of the gravity of the motion. If the horse does make a sudden move, the rider is thrown into the center of the horse, not out of it, if he is relaxed.
There are always periods of time on the XC course where you can relax the horse and let him gallop. Don't try to prepare for a fence when you are thirty strides from it. Very few horses will run out of control at a fence and if you have one of them you should get rid of him. However, if you're pulling and stiffening, don't be surprised if he pulls against you and accelerates. If you are pulling the whole way, you'll be very tired at the end of the gallop and your horse will know you don't trust him. A very wise jockey told me once, "Kim, you gotta trust them." He was riding in races, where you really are going fast.
If you relax and let the horse gallop, he will soften. Upon approach to the fence you employ the same aids and knowledge you use in the stadium, just from the galloping seat. You can extend or shorten the stride or turn or straighten. The half halt is applied by shifting the weight behind the vertical and then moving back into the center of the motion. Never pull back, it's an accelerator in galloping just as it is in dressage and show jumping.
Galloping is a natural gait for the horse. It is as comfortable as walking, only most horses love to do it more. Most horses, Thoroughbreds in particular, do not have to try to gallop. They are waiting to do it. Now it is true, they have to learn to gallop correctly with a human on board, just like everything else.
My experience has been that when you and the horse gallop, he does not seek to take advantage of you. If anything he wants to share the joy of it with you. If you embrace it and relax into it you will find you have a very receptive partner. Too many eventers fail to make the XC time because they fail to embrace the gallop the way most horses do.
Galloping is a perfect time to develop the trust and confidence in yourself and your partner.
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