How do you develop longitudinal suppleness through the horse's back?

Developing longitudinal suppleness — freeing the horse's back to swing and transmit energy — is the foundational physical task of early dressage training and one that continues throughout the horse's development because tension or stiffness through the back can resurface at any level when training demands exceed the horse's current physical or mental capacity. The primary tool for developing longitudinal suppleness is forward, rhythmic work at the trot that encourages the horse to reach forward and swing through its back — the working trot on large figures, ridden forward with a consistent rhythm and into a soft, following contact, develops the back's elasticity more effectively than most specific exercises when ridden with genuine attention to the horse's topline quality. Long and low work, in which the horse is encouraged to stretch its neck forward and down while maintaining rhythm and impulsion, specifically targets longitudinal suppleness by requiring the horse to release the muscles of its neck and back in a way that is often the first clear indication of whether the back is genuinely free or chronically tense. Transitions — particularly repeated transitions that ask the horse to swing from one gait to another — develop longitudinal suppleness by repeatedly asking the back to change its movement pattern in a way that reveals and addresses stiffness. Serpentines and changes of rein that repeatedly ask the horse to bend first one way and then the other also develop longitudinal suppleness indirectly by requiring the back to be flexible enough to follow the horse's changing direction. Massage, appropriate warm-up, and avoiding cold work that asks too much before the muscles have had time to warm and loosen support the development of longitudinal suppleness from the management side, as does ensuring the saddle fits correctly and does not create pressure points that cause the horse to brace its back in self-protection.