Why is  rhythm so important?

The base for all successful riding is rhythm and balance.
Rhythm is important wether you are a dressage rider; a show jumping rider or just ride for fun. As a dressage rider, you need to be able to follow the horse’s swinging back without disturbing his/her rhythm while the show jumping rider should be able to keep the same rhythm when jumping a whole course.
Riding is like dancing since it is about your ability to adopt to the rhythm of your horse. The rhythm is also the first step in the German scale of education and is defined as the structure of movement in terms of time, space, and dynamics. It is closely connected to balance since a lack of balance in the rider can cause rhythm irregularities in the horse. Each time the balance of the equipage is out of order, it is a sign that the rider is not in balance.
The rhythm refers to the regularity of the horse’s gaits and can be measured in the number of steps that a horse takes per minute.

Thus, rhythm is not the same as speed.
Due to the differences in the rhythm, each gait presents a distinct challenge to the rider: The walk is a four-beat, the trot a two-beat, and the canter a three-beat gate. Each gait should have a clear rhythm that is consistent.
The goal should be that your horse can maintain the same rhythm on both straight and bended lines as well as in transitions between different gaits and tempos. When the horse changes its gaits, it is a special challenge for you as a rider. During transitions, you must learn to quickly adjust to the changing motion sequences of your horse. 

To interact in harmony with the horse
You need to learn to align yourself to the horse’s rhythm. You should be able adopt to the rhythm of the horse you are riding and bring your horses rhythm into your own body. Only when you can move in identical rhythm with your horse will you be able to, in the next step, positively influence and change the horse’s rhythm through your body. It is all about “melting into” the horse’s movements and be one with your horse. 

Every riding session should start with the rider and horse finding their natural rhythm and synchronize it.
A rider beginner should first learn to feel the rhythm on the moving horse in different gaits, and to maintain her balance and rhythm. S/he needs to be relaxed enough to permit herself to be moved by the horse. Later, s/he will gradually learn to consciously influence the horse with her body.
If the rider for instance rides in a rhythm that is too fast for a horse, the rider-horse combination will not be able to find a good balance and move as one. If the horse moves in an irregular rhythm, the rider should immediately ask herself why. It might be an indication of injury or problems with the horse’s legs or back.

What if the rider is not able to adopt to the horse’s rhythm?
Since rhythm and balance are connected, the rider and horse will be unable to move together in balance.

How can you, as a rider, improve your ability to ride in rhythm?
A good exercise is that you vary the seat in posted trot, and for instance stand during two strides and sit during one (or the opposite). This will give you possibility to fully concentrate on aligning to the rhythm of your horse’s movements.
Riding over poles or cavaletti in trot or canter is also a good way of training the rider’s rhythm.

Tips from Mads Hendeliowitz 

When passing my friend and Grand Prix rider Mads the other day, we discussed the rhythm of rider and horse. His advice is to ask the rider to loudly count in the rhythm of the horse’s movements, for instance 1-2 in trot.
Mads told that he has ordered a metronome, a device used mainly by musicians, to help maintaining a good rhythm. I remember myself that many years ago, I saw that Major Hans Wikne, at that time head teacher in Strömsholm, had placed a metronome in the riding hall to help his riders to maintain a steady rhythm.