Impulsion is the movement of a horse when it is going forward with controlled power. Related to the concept of collection, impulsion helps a horse effectively use the power in its hindquarters. To achieve impulsion, a horse is not using speed, but muscular control; the horse exhibits a relaxed spinal column, which allows its hindquarters to come well under its body and "engage" so that they can be used in the most effective manner to move the horse forward at any speed.The concept and term was first written about by practitioners of dressage, but an ability to move with impulsion is a desired goal in most other equestrian disciplines. Impulsion occurs when a horse is under human control and is one of the desired goals in horse training, but it may sometimes be exhibited by a horse in a free and natural state. Impulsion allows any horse gait to be more elastic and light, and also provides the animal with the power needed to perform complex movements, including the piaffe and the airs above the ground. Within the dressage world, there is an unresolved debate whether impulsion can only occur in gaits which have a period of suspension, the trot and canter, or if it occurs at any gait, including the walk and the ambling gaits.