Photo of a woman (Ebba Zetterberg) standing outdoors next to a large brown horse with an electronic pad in her hand.
Photo: Lisa Chröisty, SLU

When perfect symmetry is not a given – what can cause asymmetry in horses if pain is not the cause?

News published:  03/11/2025

Just like humans, horses can move unevenly, limping, when in pain. But when studies show that as many as 70% of Swedish horses in training move asymmetrically, the question arises as to whether there may be other causes. Important questions for our society, ethics, and for the horses.

What we have discovered has been surprising, raised new questions, and led to some practical advice, says Ebba Zetterberg, a PhD student in this project.

Understanding animals that cannot speak

For many people, both children and adults, who spend time around horses, it is given that the horses should be treated with love and care. At the same time, it can be a big challenge to really know if a horse is feeling well, partly because it cannot communicate with words, and partly because, as prey animals, horses are good at hiding their weakness.

When communication cannot take place through words, we as humans need to find other ways, effective tools, to understand and interpret their signals.

One such tool for detecting pain in a horse in motion is to assess whether it shows signs of lameness, i.e., an uneven, asymmetrical movement pattern. Just as we humans begin to move jerkily and limping when we feel pain in a leg or foot, horses do the same.

We know from many previous studies that horses with genuine orthopedic pain (from the musculoskeletal system) move unevenly—and that when the painful area is anesthetized, the horse's movement pattern becomes even again. Therefore, a horse that moves asymmetrically (unevenly) is often associated with likely feeling pain, says doctoral student Ebba Zetterberg. 

But there are also horses today, quite a few of them, that move asymmetrically without their owners seeing any signs of pain or reluctance to move. Horses that are in full training, and in many cases even performing well in competition – that still present an asymmetric movement pattern.

Does this mean that in many cases we fail to notice that our horses are in pain, or are there other reasons for horses moving asymmetrically than pain?

Photo of a woman in a vest with the text “Equine biomechanics” in a stable environment.
For many people, their interest in horses is based on a love for animals, which is why it is important to be able to determine when a horse is feeling well and when it is not. Photo: Lisa Chröisty, SLU
To obtain more reliable answers to these questions, this research is now being conducted, in which we are trying to figure out what is normal and what is not. What alternative causes could there be for asymmetries, apart from pain?

The research team measures how the horse's head, withers, and croup move up and down when trotting and compares the movement when weight is placed on the left and right legs, respectively. A completely symmetrical horse pushes off and sinks down (absorbs the weight load) completely evenly—like a spring that is both compressed and extended, up and down, step by step. When a horse is lame, one leg is unloaded, resulting in vertical movement asymmetry. This asymmetry can be measured using sensors placed on the horse's head, withers, and croup (see images below), which can detect deviations more reliably than the human eye.

Technical devices, foals, and competition horses at the highest level

To try to understand the asymmetries, Ebba Zetterberg is working with a research group consisting of leading veterinarians and experts in orthopedics, biomechanics, anatomy, and pain assessment.

They are assisted by technical measurement systems that have become increasingly common in recent years at several large animal hospitals, which detect deviations in horses' movement patterns more reliably than the human eye.

Asymmetry or lameness – what is the difference?
Regardless of whether deviations in the horse's movement pattern are due to pain or not, they can be referred to as asymmetries. When pain is considered to be involved, it is instead clinically referred to as lameness.

The first question the research team asked was whether it could be that movement asymmetries in horses are congenital and thus part of “the horse's normal” movement pattern. To find the answer to this question, Ebba and the research team visited farms to measure foals of warmblood trotters and Sweden's most common horse breed, the Swedish Warmblood (SWB).

The results? 45% of SWB foals moved asymmetrically, as did 80% of trotter foals.

So, what does this mean? Could it be as simple as movement asymmetries being genetic?

Probably not exclusively. When adult horses of these breeds have been measured in previous studies, movement asymmetries occur in approximately 70% of warmblood riding horses (35% more than in the same breed as foals) and approximately 80% of trotters. 

This means that there could be a clear genetic influence in trotters, says Ebba Zetterberg, who is surprised that so many foals show asymmetries. This could explain the large proportion of adult trotters with movement asymmetries.

She continues:

But when it comes to SWB horses, more adult horses move asymmetrically than foals – which raises the question: What causes the increased proportion of asymmetries in adults, if not genetics? Is there a training effect, where the way the horse is trained affects how asymmetrical they are?

The research team then chose to measure horses that perform at a high level.

Black and white portrait of a horse in a stable environment.
The high-performance horses were trained and competed on a daily basis to either jump at least 140 cm obstacles, perform at intermediate level in dressage, or achieve three-star (national) level in eventing. Photo: Lisa Chröisty, SLU

However, the results were the same as for horses performing at a lower level: approximately 70% of even the high-performing competition horses moved asymmetrically, and one-third of the horses to the extent seen in horses classified at clinics as having pain-induced lameness.

This is despite the fact that these horses are specifically trained to become more versatile, are surrounded by experienced trainers and riders, and are seen by veterinarians more often than other horses before competitions. On the other hand, however, there is also the fact that this is a group of horses that is subjected to greater stress than horses at lower training and competition levels.

Based on these results, I believe that there is a greater likelihood that horses in this group may be in pain, and that the ability to perform does not necessarily exclude the possibility that the horse is in pain. We know that horses are very good at hiding their pain, as they are prey animals, says Ebba Zetterberg.

She continues: 

At the same time, all of us who are looking at these results must remember that we are only measuring asymmetries, which we do not currently know are always caused by pain, or whether there may be other causes than those we have studied so far.

Regardless, these are important issues to take seriously, both from an ethical and animal welfare perspective. That is why horse research such as this is being conducted. 

Practical advice: What you who are involved with horses can take away from this research:

Portrait photo of a woman (Ebba Zetterberg, SLU) with a big dark brown horse in a stable setting.
Horses are much-loved animals, and helping to understand them is the most fundamental reason for conducting this research. Photo: Lisa Chröisty, SLU

Reading about the latest research is one thing, but feeling empowered by the fact that it has contributed to a new understanding is another.

I think it's important to feel that this knowledge can be used in practice, says Ebba Zetterberg, who then offers the following advice:
  1. A horse that moves asymmetrically is not necessarily less healthy or less valuable – but continuity is important. As the study has shown, asymmetries appear to be congenital and thus possibly part of the horse's “natural movement pattern.” However, if a sudden change occurs in the horse, it is more likely to be caused by pain.

  2. Are you unsure whether your horse has an asymmetry that may be due to pain? Seek help. Many veterinarians and clinics today use objective motion systems that can detect even minor asymmetries. Does your veterinarian not have access to such a system? Ask for it. Once you have had your horse “measured” by a veterinarian who uses this type of tool, it is possible to monitor the horse and see changes, even in everyday life at home.

  3. Measure your horse even when everything seems to be fine: This will help you learn what your horse's normal movement patterns are, as research indicates that there may be individual differences in what can be considered “normal.” This is a step you can start with, via a veterinarian, once your horse is old enough to trot by hand without stress. 
The third advice is one that lies really close to my heart: I believe it is one of the best ways we can improve our ability to detect even minor signs that something is wrong with our horses. Think of it as a routine to include movement analyses regularly, just like vaccinations and annual vet checks, says Ebba Zetterberg.

In conclusion, Ebba Zetterberg emphasizes that you don't need to be afraid if your horse moves a little asymmetrically, but at the same time, take it seriously, especially if changes occur suddenly.

It would be dangerous to conclude that ‘well, 70% of riding horses in Sweden are asymmetrical, so it doesn't matter if mine is,’ but rather, each horse should be monitored to see if it is possibly a sign of pain on the horse's part, she concludes.
Portrait photo of a woman (Ebba Zetterberg, SLU) standing indoors on a staircase.
Ebba Zetterberg, PhD student in the research project, is a trained veterinarian herself. Instead of working clinically, she has chosen to use her expertise to contribute to animal health—through research. Photo: Lisa Chröisty, SLU

About this project:

  • Three of the four sub-studies in the doctoral project are funded by the Horse Research Foundation. The project is expected to run from 2022 to the end of 2027.
  • The doctoral student in the project is Ebba Zetterberg, together with a research group consisting of:
    • Main supervisor Marie Rhodin, veterinarian and professor of domestic animal anatomy at the Swedish University of Agricultural Sciences (SLU).
    • Elin Hernlund, co-supervisor, veterinarian and associate professor of biomechanics at SLU.
    • Emma Persson-Sjödin, co-supervisor, veterinarian and researcher in biomechanics at SLU.
    • Pia Haubro Andersen, co-supervisor, professor emerita in large animal surgery at SLU.
    • Johan Lundblad, co-supervisor and researcher in biomechanics and pain assessment in horses.

  • The doctoral project's official title is: “The prevalence of movement asymmetries in horses of different breeds, ages, and training levels.”

The project can be roughly divided into four parts, simply described as:

  1. The occurrence of movement asymmetries in young riding and trotting foals.
  2. Movement asymmetries in high-performance horses and links to possible unilateral movement in horses. (Horses in show jumping, dressage, and eventing)
  3. The occurrence of movement asymmetries in Icelandic horses and possible links to their ability to perform tölt and pass.
  4. Examination of Icelandic horses with clinical lameness: what does the compensation pattern look like?

Links to published articles:

  • Zetterberg, A. Leclercq, E. Persson-Sjödin, J. Lundblad, P. Haubro Andersen, E. Hernlund and M. Rhodin (2023). Prevalence of vertical movement asymmetries in foals; a description of Standardbred trotter foals and Warmblood riding foals in straight line trot. PlosOne. https://doi.org/10.1371/journal.pone.0284105
  • Zetterberg, E. Persson-Sjodin, J. Lundblad, E. Hernlund, M. Rhodin (2024). Prevalence of movement asymmetries in high-performing horses perceived as free from lameness by the owner and riders’ perception of horse sidedness. PlosOne. https://doi.org/10.1371/journal.pone.0308061.

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