How do you fix a horse that is behind the leg in dressage?

A horse behind the leg — one that does not maintain its gait without constant driving from the rider's leg, that falls back in pace when the leg is removed, or that requires increasingly strong aids to maintain the forward energy the gait requires — is showing either a training problem in which the horse has learned that ignoring the leg eventually causes the leg to stop, or a temperamental tendency toward sluggishness that requires systematic work to overcome. The correction for a horse behind the leg begins with establishing a prompt, genuine response to the lightest leg aid: applying a very light leg aid, waiting briefly, and if no response comes immediately escalating to a more definitive aid — a tap with the whip behind the leg or a sharper leg aid — followed by an immediate complete removal of all leg pressure the moment the horse responds with genuine forward energy. The escalation must be immediate and definitive enough to produce a genuine response, not a gradual increase that the horse learns to wait out; the release must be equally immediate so the horse can clearly connect the forward response to the removal of pressure. The key is then to maintain the lighter leg and allow the horse to sustain the gait without constant driving — if the horse falls back, applying the light leg and if needed the immediate escalation again trains the horse over time that the light leg means go and stay forward, rather than that the leg is background noise that can be ignored until it becomes stronger. Testing self-maintenance by riding actively and then removing the leg completely for several strides reveals immediately whether the horse is carrying itself forward or waiting for the leg to drive it. A horse behind the leg cannot develop genuine impulsion or the quality of expression that dressage rewards, making this correction one of the most fundamental priorities in training.