Flexibility training is probably the most forgotten tool as a rider… muscles shorten in size as they contract. When the body relaxes, that is not the same thing as lengthening the muscle. The positioning has to be correct for a body part to lengthen correctly. What usually happens is the rider will relax OUT of the correct balanced position in the saddle and then, have to quickly organize the seat for position support. This results in the horse having a very inconsistent stability from it’s rider. Some horses show this as inconsistent rhythm or distance to a jump.
Anytime a rider is trying to do things with their position, a horse is vulnerable to negative training. This can lead to a lifetime of bad habits in the horses gait. It is important of a rider to train off the horse for the perfect alignment of the joints in the body. Feel proper muscle contraction to the core and train the muscles to extend by increasing the range of motion in the joints. Do this on the ground and you will have control in the core for position correction on the horse.
If you are trying to get the leg longer in the saddle or sit taller in the saddle, the rider needs to have control as things extend aways from the center of the core. The most secure position of a person is the fetal position, bringing all extremities into the stomach. Muscles want to instinctively shorten into the center of the core when they loose balance in the saddle.