When we refer to ‘upper body’ we usually look at head (poll), withers and pelvis (sacrum and tuber coxea) and how these markers move during the two halves of the stride, i.e. during and after the stance of a right limb compared to the left limb.

This evaluation is most frequently done in trot because trot is a symmetrical gait so it is easy to compare left and right ‘half’ of the stride, i.e. the two movement up the upper body between the two trotting diagonals. In addition, the forces acting on the horses limbs are higher than in walk hence magnifying any irregularities which might be related to pain

Normal upper body vertical movement of a horse in trot

In trot, the horse’s body rises and falls twice within the stride. If tracked with the help of gait analysis, the vertical displacement trace of the head, withers or pelvis would be characterised by two distinct peaks and troughs. If the horse is moving perfectly symmetrically, i.e. with even push off and weight-bearing between left and right limbs, these peaks and troughs will be approximately at the same level (as seen in the diagram bellow).

This horse is pretty symmetrical as the two minima (lows) and the two maxima (highs) are more or less level. There does not seem to be a significant deficit in weight-bearing or push-off for either of the hindlimbs.

Equine movement asymmetry and lameness

Horse have been shown to move more symmetrically after successful diagnostic analgesia demonstrating a link between upper body movement asymmetry and pain-related orthopaedic conditions [1-2]. The ‘head nod’ we all look for if we are suspecting forelimb lameness translates into the head dropping more during a sound forelimb stance while the head is held higher during the stance of the lame forelimb.

A typical vertical displacement of head during trot in a horse with forelimb lameness. The ‘head nod’ is indicated by asymmetrical head movement between the two halves of the stride, i.e. greater displacement during and after the sound limb stance and smaller displacement during and after the lame limb stance.

For suspect hindlimb lameness we look at the sacrum or tuber coxae (‘hip hike’). The pelvis drops less during the stance phase of the lame hindlimb or does not rise as much after the stance phase of the lame hindlimb. Similarly, a greater vertical amplitude of hip bone movement will be observed on the side of the lame hindlimb.

A typical vertical displacement of sacrum during trot in a horse with hindlimb lameness. There is an asymmetrical pelvis movement between the two halves of the stride, i.e. greater displacement during and after the sound limb stance and smaller displacement during and after the lame limb stance.

The benefits of measuring equine movement asymmetry objectively

Unfortunately, our eyes are not capable of noticing small asymmetries. In addition, it can be quite hard to remember the exact degree of movement asymmetries when comparing two time points. On the other hand, equine gait analysis can detect very small asymmetries with high sensitivity and has high repeatability between trials [3].

Consequently, measuring movement asymmetry patterns objectively is important because it allows us to quantify what is happening and it helps us understand what is the normal ‘baseline’ asymmetry of each horse. Moreover, gait analysis evaluations can also helps us decide what works and what does not. This can be particularly useful for tracking changes over time during the season or during rehabilitation process.

Are all movement asymmetries a sign of lameness?

Very few horses, just like us, will be perfectly symmetrical so a certain degree of asymmetrical movement is to be expected. One way to assess what movement asymmetry is abnormal is to consider the values of movement asymmetries which can be observed in non-lame and lame horses [3-5]. Asymmetry significantly above ‘reference lameness thresholds’ or a big change in the horse’s individual asymmetry (if tracked over time) can mean there might be an underlying issue which could develop into a bigger problem if unchecked or if the workload is not adjusted.

Other cause of movement asymmetry in horses

It is important to note that there might be other causes of asymmetry and should not be mistaken for lameness:

1. Ill-fitting tack or rider asymmetry: These can have a huge effect on the comfort of your horse and your horse might develop compensatory strategies to deal with an unbalanced saddle or a rider [6-7].
2. Seating style: Rising trot induces asymmetrical loading on the horse’s back (read more here) which means that in rising trot your horse will not move perfect symmetrically [8].
3. Circular motion: Moving on a circle induced systematic changes to the horses pelvis with the resulting asymmetry mimicking ‘inside hindlimb lameness’, this is to be expected and your should ideally have a horse that moves with similar level of asymmetry on both reins [9-11].

Equine Movement Asymmetry: Take home message

• Pain and movement asymmetries in horses are linked.
• BUT most horses will be have a degree of movement asymmetry, just like humans. So we are not necessarily looking for perfect movement symmetry.
• Knowing your horse’s baseline asymmetry and monitoring your horse over time means that deviations from your horse’s normal can be tracked objectively. This can ultimately help you assess how the horse is responding to training or if there are any underlying issues that might need to be investigated by the vet. 

Learning more about equine gait analysis:

I have lots of resources available if you want to learn more about this topic:

• Read my blogs Introduction to Equine Gait Analysis or Practical Guide to Equine Gait Analysis
• Buy a recording of webinar: Asymmetrical or Lame? Where do we draw the line?
• Watch a brief Introduction to Equine Gait Analysis

You can also book a movement asymmetry assessment of your horse with a state of the art equine gait analysis system at your own yard.

References

1 Maliye et al. (2013) An inertial sensor-based system can objectively assess diagnostic anaesthesia of the equine foot. Equine Veterinary Journal

2 Pfau et al. (2014) Identifying optimal parameters for quantification of changes in pelvic movement symmetry as a response to diagnostic analgesia in the hindlimbs of horses. Equine Veterinary Journal 

3 Keegan et al. (2011) Assessment of repeatability of a wireless, inertial sensor-based lameness evaluation system for horses. Am J Vet Res 

4 Rhodin et al. (2017) Head and pelvic movement asymmetries at trot in riding horses in training and perceived as free from lameness by the owner. PLoS ONE 

5 Sepulveda Caviedes et al. (2018) Repeatability of gait analysis measurements in Thoroughbreds in training. Equine Veterinary Journal 

6 MacKechnie-Guire et al. (2020) The Effect That Induced Rider Asymmetry Has on Equine Locomotion and the Range of Motion of the Thoracolumbar Spine When Ridden in Rising Trot. Journal of Equine Veterinary Science.

7 Mackechnie-Guire et al. (2018) Relationship between saddle and rider kinematics, horse locomotion and thoracolumbar pressures in sound horses. Journal of Equine Veterinary Science

8 Persson-Sjodin et al. (2018)  Influence of seating styles on head and pelvic vertical movement symmetry in horses ridden at trot. PLoS One

9 Starke  et al. (2012) Vertical head and trunk movement adaptations of sound horses trotting in a circle on a hard surface. The Veterinary Journal.

10 Pfau et al. (2016) Lungeing on hard and soft surfaces: Movement symmetry of trotting horses considered sound by their owners: Movement symmetry on hard and soft surfaces on the lunge. Equine Veterinary Journal 

11 Rhodin et al. (2016) Head and pelvic movement asymmetry during lungeing in horses with symmetrical movement on the straight. Equine Veterinary Journal.