How do I select and properly use a lunge line and surcingle?

The lunge line and surcingle are fundamental training tools for groundwork, conditioning, and the development of both young horses and horses in rehabilitation. Used correctly, they allow a trainer to work a horse at various gaits and transitions from the ground, observe the horse's movement without the influence of a rider, and develop the horse's balance, responsiveness, and muscular development. A lunge line of approximately twenty to thirty feet in length provides the radius needed to work a horse on a circle of appropriate size for safe loping while remaining manageable for the handler. Very long lunge lines are difficult to control and create excess line on the ground that can become tangled; very short lines force the horse to work on a circle so tight that it places excessive stress on the horse's inside joints. The surcingle should fit snugly enough that it cannot slide back or rotate but not so tightly that it restricts the horse's breathing or movement. Most surcingles are fitted two fingers snug at the girth, similar to a saddle girth. A surcingle fitted too loosely can slide back toward the flanks during active work, which alarms most horses and creates a dangerous situation. Side reins, when used with the surcingle during lunge work, should be adjusted to allow the horse to carry its head in a natural, working position rather than forcing collection — a horse pulled into a frame by overly tight side reins will hollow its back in resistance rather than developing the correct musculature that proper lunge work is intended to build.