Side Reins: Helpful or Harmful?

There is no shortage of training tools in the horse world. Bits, spurs, gadgets, specialty tack, all of them marketed as solutions to training problems. You could write an entire book on them. But one tool that has come up again and again for me, especially in English disciplines, is the side rein.

Side reins are auxiliary reins that connect the bit to the girth or surcingle (you can see some examples in the picture below). They're typically used during lunging with the goal of improving straightness, getting the horse to accept contact, developing topline muscles, and encouraging a correct head and neck position. On the surface, they can seem like an easy way to help your horse use their body better and move toward collection.

Image: Gehlen et al., 2021

Why I started questioning them

When I first started riding English after many years in the world of Western riding, side reins were a new concept for me. As I learned to use them while working under other trainers, I started noticing that something wasn't sitting right with me. The horses I worked in side reins were stressed, and their movement was tighter than it had been before adding the side reins. I also noticed that many people who used them couldn't tell me exactly how they work, beyond getting the horse "on the bit." Rather than relying on that gut feeling alone, I started looking into what existing research and experienced, welfare-focused professionals actually say about side reins and how they affect the horse physically and mentally, so I could make up my mind on whether they have a place in my training.

What the research does (and doesn't) say

Unfortunately, there is not yet much research on the effects of side reins. There is some research showing that head and neck position does influence a horse's back movement and hindlimb mechanics. In other words, the horse's posture matters. However, there is not much research directly on side reins and their effect on movement and welfare. As of 2026, only six studies directly assessed the biomechanical effects of training aids. The results of those studies indicate that they do influence posture, coordination, and load distribution during movement, but they don't consistently improve activation of the neck and trunk muscles or improve biomechanical efficiency, or the way the horse holds itself through movement. In other words, these studies do not prove that side reins do what they are marketed to do.

The problem with "fixing" head and neck position

The biggest issue with side reins is the forced head and neck position. Side reins are primarily used to create the appearance of the horse working "on the bit" or collected, which often just results in a shortened, flexed neck. The problem with this is that correct head carriage is supposed to be the result of correct movement, not something we impose on the horse to get correct movements. It is impossible to get self-carriage if we do not allow the horse to find it himself.

Jec Aristotle Ballou mentions one of the issues with this in 55 Corrective Exercises for Horses, writing that side reins can make a horse look better by securing his head, but they do not ensure the horse is moving with correct form. They can even make it more difficult for the handler to see compensation and often allow the horse to tuck his nose in, avoiding the bit and creating tension through his body. According to a presentation from the 4th International Equitation Science Symposium, incorrect usage of side reins can also result in the horse leaning on the forehand, neck pain, and lessen the quality of the horse's natural gaits.

At the end of the day, correct posture is not one that's held in place by force. Good training improves a horse's balance, coordination, and strength over time. That process requires free movement, the ability to adjust, and it should include messy moments as the horse learns how to move properly.

Side reins and learning theory

On top of the physical drawbacks, there's also a learning theory issue with side reins that often gets overlooked. One of the core ideas of the International Society of Equitation Science is the correct use of cues and signals, part of which is making sure each signal only has one meaning.

How does this apply to side reins? Rein pressure is supposed to function as a clear signal. When we use the reins to ask for something, typically deceleration (slowing down, stopping), the pressure needs to have a distinct beginning and end. There needs to be a release from pressure to reinforce the behavior we're looking for. With side reins, however, the horse has to deal with constant pressure on the bit without a person on the other end to release the pressure. Over time, this dulls the horse's response to rein aids. If the pressure is always present and sometimes means to shorten the neck and other times means to slow down, it begins to lose its meaning.

What about welfare?

Even when adjusted correctly, side reins restrict the natural movement of the horse's head and neck. This restriction can limit the horse's ability to find their on balance, create compensation in other parts of the body, and create frustration if the horse cannot escape the pressure of the side reins. While not all horses will show signs of distress, more subtle indicators like tension or lack of forward movement are important to look for.

Where I stand on side reins

At the end of all my research on this tool, I am pretty firm in my belief that it doesn't serve a purpose in my training program, although I'll always be on the lookout for new research. There is no evidence that side reins improve biomechanics, and there are legitimate concerns about the effects of side reins on learning, physical health, and welfare. While they may have their uses, I have not found any that are not outdone by allowing the horse to find his own balance, strength, and carriage through correct training.

References and Further Reading

Cottriall S, Ritruechai P, Wakeling JM. The effects of training aids on the longissimus dorsi in the equine back. Comparative Exercise Physiology. 2008;5(3-4):111-114. doi:10.1017/S1478061509342346

Jec Aristotle Ballou's 55 Corrective Exercises for Horses

Gehlen, H.; Puhlmann, J.; Merle, R.; Thöne-Reineke, C. Evaluating Horse Owner Expertise and Professional Use of Auxiliary Reins during Horse Riding. Animals 2021, 11, 2146. https://doi.org/ 10.3390/ani11072146

McClean, A., McGreevy, P. (2010). Horse-training techniques that may defy the principles or learning theory and compromise welfare. Journal of Veterinary Behavior. 2010;5;4:178-195. https://doi.org/10.1016/j.jveb.2010.04.002.

Siqueira, R. F., Figueiredo, I. M. (2026). Biomechanical effects of equine training aids: A systematic review. Equine Veterinary Education. https://doi.org/10.1111/eve.70095