How do circles develop suppleness in dressage?

Circles are one of the most fundamental exercises for developing suppleness because they require the horse to maintain uniform lateral bend through its body while traveling on a curved path — a gymnastic demand that systematically develops both the lateral flexibility of the horse's spine and ribcage and the activity of the inside hind leg stepping under the body. A correctly ridden circle is not simply a round track but an active gymnastic exercise in which the rider maintains correct bend throughout, uses the inside leg to encourage the horse's barrel to swing away and the inside hind to step under, and uses the outside rein and leg to maintain the shape and prevent the horse from falling out through the shoulder. The size of the circle directly affects the gymnastic demand: a large twenty-meter circle makes relatively modest demands on the horse's lateral suppleness and is appropriate for early training work; a ten-meter circle requires significantly greater lateral flexibility and inside hind engagement; a six-meter volte makes very high demands on lateral suppleness and collection and is only appropriate for more advanced horses. Repeatedly changing the size of circles — spiraling in from a twenty-meter to a ten-meter circle and back out — develops suppleness by continuously varying the degree of bend required, preventing the horse from becoming habituated to a fixed degree of flexion and requiring it to adjust its lateral flexibility in both directions of the size change. Circles are most effective for developing suppleness when they are ridden with genuine attention to correct bend rather than simply as round tracks — a horse that is allowed to maintain incorrect bend throughout circles merely practices its evasion rather than developing the genuine suppleness that correctly ridden circles produce.