Muscle resistance and affect on the horses mouth

“Once the resistances of the muscles are overcome and the animals are balanced, all horses have soft mouths, as the school horses prove, often with very flat bars. If the muscles resist with full force, if the hindquarters thrust more than the forehand supports, etc., the horse will always seek his lost balance in the hand. And then all of them have hard mouths, like race horses who take an arm-numbing contact in spite of the sharpest bars and bits. Thus, the hard-mouthed horse becomes soft-mouthed, when he develops the strength to carry his neck, and the soft-mouthed horse becomes hard-mouthed under a weak rider.” ~ Friedrich v.Krane