Side reins are straps that connect your horse’s bit to a surcingle or girth during lungeing, providing a steady contact that encourages the horse to seek the bit and develop balance without a rider. They’re one of the most commonly used training aids in flatwork, but their effectiveness depends entirely on how you fit, attach, and introduce them. Done well, side reins build topline muscle and teach self-carriage. Done poorly, they create tension, restrict movement, and can cause real physical harm.
What Side Reins Actually Do
Side reins restrict and guide the position of the horse’s head and neck, which in turn influences how the back moves and how the hind legs engage. When a horse works into a correct contact with side reins, the abdominal muscles activate to support the spine, and the hindquarters carry more of the horse’s weight. Over time, this strengthens the muscles along the topline, the chain of muscle running from the poll down the neck, over the back, and into the hindquarters.
This only works when the horse is moving forward willingly into the contact. If the reins are too short or the horse’s head is pulled behind the vertical, the mechanics reverse. The horse braces against the pressure rather than working through it. Instead of building muscle by suspending the head and neck naturally, the horse fights to raise its head against aversive pressure. Taken to the extreme, this forced downward position tightens the nuchal ligament, shifts weight unevenly to the hindquarters, and can reduce fetlock joint angles enough to contribute to lameness over time.
Choosing Between Elastic and Solid Reins
Side reins come in two main types. Solid reins, typically leather or another non-stretch material, provide a fixed contact with no give. These are best suited to experienced horses that already understand how to accept contact and move forward into it. Elastic reins look similar but include a section of elastic or a rubber ring insert that stretches slightly with the horse’s movement. This small amount of give makes them more forgiving, which is why they’re the better choice for horses being introduced to side reins for the first time. The stretch mimics, in a limited way, the slight give and take of a rider’s hands.
Where to Attach Side Reins
Side reins clip to the bit rings on one end and to the surcingle (or saddle girth) on the other. The height of the surcingle attachment determines the frame you’re asking the horse to work in, and this should match the horse’s current level of training.
For horses early in their training, or during warm-up when you want a long, low stretch, attach the reins to the middle or lower rings of the surcingle, roughly at the horse’s chest height. This encourages the horse to reach forward and down, which opens the back and promotes relaxation. As the horse progresses and you begin asking for more collection, you can move the attachment point up to the middle or upper rings. Raising the point of attachment asks the horse to carry more weight behind and lift through the withers.
Sliding side reins work slightly differently. They attach to the girth, run along each side of the horse through the bit rings, and fasten back to the girth on the same side or between the front legs. This design allows more freedom for the horse to stretch forward and down, making them a good option for encouraging a relaxed, seeking contact.
Getting the Length Right
Correct length is the single most important variable. The goal is a light, steady contact when the horse is working in a natural frame for its level of training. The horse’s nose should be at or slightly in front of the vertical. You should never ask the horse to carry its head behind the vertical.
A useful reference point: compared to the neutral resting length of a horse’s neck, side reins need to be shortened by about 10 centimeters to approximate the feel of a rider’s contact in trot. Start longer than you think you need. You can always shorten by a hole or two once the horse is moving forward and relaxed. Both reins must be exactly the same length to avoid pulling the horse’s head to one side.
Signs the reins are too tight include the horse working behind the vertical (nose tucked toward the chest), shortened strides, visible tension in the neck and jaw, and reluctance to move forward freely. If you see any of these, lengthen the reins immediately.
When to Put Them On and Take Them Off
Side reins should never be attached while the horse is standing still, walking to the arena, or being mounted. They’re a tool for active work on the lunge, not for tying down a horse’s head.
Start every session by lungeing without side reins for at least five to ten minutes. Let the horse warm up, stretch, and find its rhythm at walk and trot before you introduce any restriction. Once the horse is moving freely and relaxed, halt and clip the side reins to the bit. Begin with them loose enough that the horse barely feels them, just the weight of the straps hanging along the neck. Then gradually shorten to your working length.
At the end of the session, or anytime you need to stop, halt the horse and unclip the side reins from the bit before the horse stands for any length of time. You can snap them together over the withers or hook them onto the D-rings of the surcingle so they’re out of the way. The United States Dressage Federation recommends detaching side reins at the bit before a rider mounts or dismounts during lunge lessons as well.
Introducing Side Reins to a New Horse
For a horse that has never worn side reins, use elastic or rubber ring reins and attach them so loosely that there is no restriction on the head and neck at all. The first goal is simply to let the horse get comfortable with the weight of the straps and lunge happily without reacting to them. Some horses will fuss, shake their heads, or try to root against the contact. Give them time.
Over several sessions, shorten the reins one hole at a time. Only shorten when the horse is moving freely and calmly at the current length. Rushing this process teaches the horse to brace against pressure rather than yield to it, which defeats the entire purpose.
Safety During Lungeing
A horse in side reins has limited ability to use its head and neck for balance, which makes falls more dangerous. Keep your lunge line carefully organized in the hand closest to the horse’s head so it can unravel quickly and cleanly if something goes wrong. Never wrap the lunge line around your hand.
If the horse panics, rears, or trips, you need to be able to release the line or reach the horse quickly. Work on a surface with good footing, avoid lungeing in side reins over poles or jumps unless you’re experienced, and keep the circle large enough (minimum 15 to 20 meters) that the horse isn’t struggling with balance on a tight turn while also navigating the contact.
Never leave a horse unattended in side reins, and never turn a horse out or let it graze with side reins attached. A horse that steps on a rein or catches it on something while its head is restricted can injure itself seriously.
Common Mistakes to Avoid
- Starting too short. The most frequent error. Begin long and shorten gradually. If the horse looks restricted or tense from the start, the reins are too tight.
- Uneven length. Even one hole of difference between the left and right rein pulls the horse’s head to one side and creates asymmetric muscle development.
- Skipping the warm-up. Attaching side reins to a cold horse is like asking someone to sprint before stretching. The muscles need to be warm and loose before they’re asked to work in a frame.
- Using side reins as a shortcut. Side reins don’t replace correct riding or training. They supplement it. A horse that only goes well in side reins but falls apart under saddle hasn’t actually learned self-carriage.
- Overuse. Side reins add physical demand. Fifteen to twenty minutes of focused work in side reins is plenty for most horses. Longer sessions lead to fatigue, and a fatigued horse compensates with poor posture, which builds the wrong muscles.