Staying on your horse should not be a matter of thigh strength. It should be about balance based on your core.
Why not grip with your legs though? Firstly, it drives a horse forward, and this results in horses bolting, or getting anxious because of conflicting cues. Secondly, gripping with your legs makes you LESS secure on the horse. It prevents you sitting closer to the horse, so slight movements left or right, or even sharp changes in speed can result in you being unseated. Thirdly, gripping with your legs means you cannot move freely with your horse. It makes your pelvis brace and gets in the way of a horse’s movement. Lastly, when you grip with your legs just to stay on your horse, you reduce how effective your leg cues are. This is not about any particular discipline. Top jumping horsemen like George Morris teach riders that gripping with your knees will push the legs back, and gripping with the calves makes the legs less effective. Dressage masters like Alais Podhasky wrote in-depth about how gripping with your legs impedes a horse’s free movement. This video is not an in-depth lesson on seat position, which is way beyond a YouTube video’s scope, this is just to get people to start thinking about better ways to balance on a horse. Please excuse the young mare in the video struggling with flies, we have had nothing but rain for three weeks and with the sun today the flies were horrendous. She has been sprayed, but the sheer numbers were too high.