Why does my reining horse drop its shoulder?

Dropping the shoulder means the horse is collapsing through its inside front leg and losing balance through the front end rather than staying upright and carrying weight through the turn, circle, or approach. It is one of the most pervasive problems in reining training because the shoulder is involved in every maneuver — circles, spins, stops, rollbacks, and lead changes all require the horse to carry its front end correctly, and a horse that habitually drops the shoulder will show that weakness in each of them in slightly different ways. In circles it causes the horse to cut inward and tighten the arc involuntarily. In spins it causes the horse to fall rather than sweep through the turn. In the stop it causes crookedness as the dropped shoulder pulls the slide to that side. In lead changes it causes the horse to drift through the change and often produces a late change behind. Shoulder control is therefore one of the most important foundational skills in reining rather than a refinement added later, and building it requires specific exercises that teach the horse to move its shoulder away from leg pressure independently and to carry it upright through movements that would otherwise invite the collapse. The inside leg at the girth is the primary tool: it lifts the inside shoulder by creating forward energy and preventing inward drift simultaneously. The outside rein is the secondary support, preventing over-bending that collapses the outside shoulder the other direction. Shoulder-in work, counter-bending, and exercises that specifically isolate the movement of the front end from the hindquarters build the horse's awareness of its shoulder position and the muscular strength to hold it correctly through the demands of high-speed reining work where the temptation to drop is greatest.