How do you fix a horse that is stiff through one rein in dressage?

A horse that is consistently stiffer or heavier on one rein than the other — accepting a soft contact on the left but leaning or resisting on the right, for example — is showing the natural asymmetry that all horses have to some degree and that systematic training must address through specific gymnastic exercises rather than through more pressure on the stiff rein. The most common pattern is a horse that bends more easily to one direction — typically the left — and is stiffer in the other direction, with the stiffness often accompanied by more weight in the contact on the stiff rein and a tendency for the horse to fall through the opposite shoulder. The correct approach addresses the stiffness through the inside leg of the stiff rein rather than through the rein itself: the horse that is stiff on the right rein needs its right hind leg activated to step further under the body, which develops the lateral flexibility that makes the right rein soft rather than the rein contact attempting to pull the stiffness away through force. Shoulder-in to the stiff side is the most specifically targeted exercise for this problem because it directly asks the inside hind of the stiff rein to step under and engage, developing the lateral flexibility and inside hind activity that softens the contact on the same rein. Working on transitions specifically on the stiff rein — particularly canter departs to the stiff lead — develops the inside hind leg's activity in a way that transfers to improved contact quality. A temporary use of counter-flexion — asking the horse to flex slightly away from its natural preference while remaining on a circle in the same direction — can release the neck muscles on the stiff side by working them in an unfamiliar direction, but must be used as a temporary tool rather than a sustained position.