Side reins are one of the most widely used pieces of training equipment in the equestrian world, employed by dressage riders, showjumpers, event horses, and vaulters across disciplines. They serve as an introduction to contact for young horses, a gymnastic tool for horses in work, and a means of establishing a consistent frame during lunging. Used correctly, they are an asset to any training programme; used incorrectly, they can create tension, resistance, or physical discomfort. Understanding what side reins in horse training actually do — and what they don’t do — is essential before using them.
What Are Side Reins?
Side reins are adjustable straps that run from the horse’s bit rings to the girth or the rings of a roller, on both sides of the horse simultaneously. They are used primarily during lunging or long-reining work, not while the horse is being ridden, though some in-hand trainers and vaulting coaches use them differently. Their function is to provide a consistent, stationary contact point that the horse can yield to and reach toward, developing an understanding of the bit and encouraging a rounder, more engaged way of going.
The most common types are plain leather or synthetic side reins and elastic-insert side reins, which include a rubber ring or bungee section to allow some give. Both have their advocates. Plain side reins are firmer and give clearer feedback; elastic types are more forgiving of sudden head movements and are often recommended for young, green, or sensitive horses that might brace against a fixed contact.
How Side Reins Work
When a horse works freely on the lunge, it may carry its head high, hollow its back, and disengage its hindquarters — all of which are counter-productive to building the muscle and suppleness needed for ridden work. Side reins, correctly adjusted, provide a boundary: the horse can lift or extend its neck slightly but meets gentle resistance when it tries to invert. Over time, most horses learn to seek the contact and carry themselves in a more rounded, balanced frame rather than bracing against or evading the reins.
The key word is “correctly adjusted.” Side reins fitted too short prevent the horse from working through its back and often cause the horse to get behind the vertical — a position that is actively detrimental to training progress. Side reins fitted too long provide no useful contact at all. A general rule of thumb is to begin with a length at which the rein creates light contact only when the horse adopts a natural, relaxed head carriage. Adjustments should be gradual and always begin loosely, tightened incrementally over multiple sessions as the horse develops confidence and suppleness.
Using Side Reins During Lunging
Side reins are attached only once the horse is warmed up — typically after five to ten minutes of free lunging in each direction. This allows the horse’s back and muscles to loosen before the contact is introduced. They should be fitted to matching holes on each side to ensure equal contact, unless a horse has a known asymmetry that requires compensatory adjustment. During the session, the lunger should monitor the horse’s way of going constantly: a horse that is tense, above the bit, short-stepping, or swishing its tail is communicating that the side reins are too tight or that more warm-up is needed.
Side reins are always removed before the horse is ridden. Leaving them attached during mounting or early ridden work creates a safety risk (the horse cannot raise its head to balance if surprised) and removes the rider’s ability to influence the horse through the rein. A young horse transitioning from lunge work with side reins to first ridden work benefits from the standing martingale or other aids only once the trainer has determined they are appropriate. For a broader view of training progression, see our entry on posting trot, which addresses ridden basics for horses in early education.
Side Reins Versus Other Training Aids
Side reins are distinct from draw reins, which run from the girth through the bit rings back to the rider’s hands and are used during ridden work. Draw reins allow dynamic adjustment but require considerable skill to use without over-flexing the horse. Side reins have a fixed length and are passive — they do not pull; they simply provide a consistent endpoint for the horse to work into.
A chambon is another lunging aid that acts differently, working on the poll and encouraging the horse to stretch forward and down rather than into a contact. For horses that need to stretch and develop their topline from the bottom up, a chambon may be preferable to side reins. The choice of training aid always depends on the individual horse’s conformation, stage of training, and specific needs as assessed by a qualified trainer. The British Horse Society recommends that training aids of all kinds are used under the guidance of an appropriately qualified instructor.
Frequently Asked Questions
At what stage of training should side reins be introduced?
Side reins are typically introduced once a horse is comfortable lunging freely in both directions, accepting the bridle and bit without distress, and moving in a reasonably relaxed and forward manner. For most young horses, this is between three and four years of age, though individual readiness varies. The horse should be physically mature enough to carry itself without strain, and the trainer should already have established trust and forward movement before adding any restricting equipment.
Should side reins be equal on both sides?
For most horses, yes — equal length on both sides ensures equal contact and avoids creating or exacerbating asymmetry. Some horses have a natural tendency to be stiffer on one side, and an experienced trainer may adjust the stiffer side very slightly shorter to encourage more even bend. However, this requires careful assessment and is not something to do as a general rule; unequal side reins applied without proper diagnosis can make asymmetry worse.
Can side reins be used on a horse in work at all levels?
Yes. Side reins are not just for young horses. Many professional riders and trainers use them during lunge warm-ups for horses at all levels, including advanced dressage horses. For an established horse, they provide a consistent warm-up routine and can be used to supple a horse before ridden work or to address specific training issues under the guidance of a coach. They are a tool that remains relevant throughout a horse’s training life when used with appropriate knowledge.