Some riders seem to sit rather well, straight and with their legs in the right position. But then the horse moves and the head of the rider bobs up and down…
What happens is the movement that has to be absorbed in back, hips, knees and ankles comes out in the head. It means there is too much tension in the position of the rider. The spring in the lower back doesn’t work.
Does your head bob? You will feel your neck. As you are balanced, you won’t affect the horse in a bad way. But improvement is possible, as you are tensing up. Start with stretching your horse forward and down into a contact, so his back comes up. Keep your head up, but straight above your spine. It should be the top of a pile of blocks. Don’t push your chin up or out. Look where you are going. Now focus on your lower back. Don’t lose the core stability, but feel that you can move your hips and lower back. Try to absorb the motion in your body, by moving with the horse. Follow him. Your lower back should act like a spring. Try to absorb the motion there. Don’t clench your hips. Good riding is about balance and following movement. If you tense up, you tire out your body and you will block the energy flow.
If you have a bobbing head, ask yourself where you can feel the tension in your body. Try to find out why you are tensing up there. You might need help of a physiotherapist. Are you bracing your back muscles because your horse is pulling? Are you forcing yourself because you want to do well? Are you afraid to lose control? Realising what causes the problem is half of the solution.