A horse that is heavy in the hand — that leans on the bit, pulls the reins out of the rider's hands, or maintains constant downward pressure on the contact — has developed this habit through a combination of training patterns that inadvertently rewarded leaning or failed to develop the self-carriage that lightness requires. Understanding the causes directs the fix, because heaviness that comes from one source requires a different correction than heaviness that comes from another.
The most common cause of heaviness in a ridden horse is a rider who provides constant backward rein tension that the horse learns to lean against. When the rider always maintains the same backward pressure, the horse finds a balance point against that pressure and settles into it — leaning becomes its normal way of going. The fix requires the rider to stop providing the constant resistance, which initially feels like losing control but is actually what teaches the horse that there is nothing to lean against and it must carry itself.
A second common cause is insufficient engagement of the hindquarters. A horse that falls on its forehand — that carries too much weight on its front legs — becomes heavy in the hand because the weight that should be carried by the hindquarters is falling onto the forehand and through it onto the bit. The fix requires more hindquarter engagement through transitions, half-halts, and lateral work that shifts weight rearward rather than more rein pressure that simply creates more resistance to lean against.
For Western riders, a horse that is heavy in the hand often reflects a transition from direct rein work to neck rein work that happened before the horse was ready — the horse has learned to lean on the contact as a steering guide rather than responding to the lighter cue-and-release system that true neck reining requires.