Interference: Equine Limb-to-Limb Contact in Movement

Interference Contact Points on Hind Limb

Perhaps you hear a repetitive popping or clicking sound while you are riding or exercising your horse, or you notice mysterious wounds, sores, or hair loss on your horse’s lower limbs. Any of these observations could result from “interference” – one limb contacting another limb during some phase of a horse’s stride.
There are many types of interference, each resulting from specific causes. It is important for riders and equine caretakers to detect and understand interference, because it can be a sign of an underlying problem and in itself also can result in limb injuries.
Conformation, lameness, poor fitness and training, mechanical gait deficits, neurologic disease, improper shoeing or trimming, management and riding are all factors that may cause a horse to interfere. In addition, a perfectly healthy horse can also take a single misstep and interfere, resulting in injury.
Interference has been around as long as people have had horses, and horse people have developed colorful names for the different types of interference. Like many horseman’s terms though, there is confusion and inconsistency in the naming. Certain types of interference are more common in horses of different breeds, types and disciplines, and shoeing styles. The names for these different types actually may differ between different disciplines, further confusing the issue.  That said, it is still useful to consider the different types of limb-to-limb contact.
“Brushing” refers to contact between opposite (right and left) fore or hind limbs. This most commonly takes place in the lower limb, pastern or fetlock and rarely above the fetlock. Brushing happens as a consequence of a lateral gait deficit, meaning something affecting side-to-side movement of the limbs in flight.

Interference Brushing

Brushing happens more frequently in the forelimbs of narrow-chested horses with base narrow, toed-out conformation. The more extreme this conformation, the worse the interference usually is. These horses break over on the inside of the hoof and tend to “wing-in” at the walk and trot. In the hind limbs, brushing is seen in horses that are base narrow and travel close. Brushing is most common at the trot but may occur at other gaits. The speed and energy level of the gait affects the tendency to interfere. Some horses tend to interfere more at higher speeds while others will improve as they go faster.
“Forging” refers to contact of the sole of the front foot by the toe of the hind foot.  Forging can take place in any of the three gaits but is most common at the trot. It usually refers to a hind foot hitting the front leg on the same side. Forging is more common in horses with short backs (short-coupled), with relatively long hind legs.

Interference Forging

“Over-reaching” or “grabbing” is a type of forging in which the toe of the hind foot contacts the lower forelimb on the same side, usually causing a heel bulb wound or catching the heel of the shoe.

Interference Overreaching

In contrast to contact to the front limb by the hind limb, there are several terms referring to contact wounds to the hind limb from the same side front limb.  “Scalping” refers to contact with the coronet band,  “speedicutting” refers to a strike to the fetlock area, “shin hitting” to the cannon area and “hock hitting’ to the hock region.  A confusing use of the term “speedicut” can also be given to wounds high on the inside of the carpus or hock from contact from the opposite limb. This is mostly seen in trotters and racehorses.
 I have always found the term “cross firing” to be confusing. Many people interchange the term with “cross cantering”, which refers to a horse that is on opposite leads on its fore and hind limb at the canter.  When thinking about limb contact (interference) though, cross-firing refers to contact between diagonal limbs (ex: right fore and left hind), which usually occurs at the pace in harness horses. Cross firing usually is a result of underlying training/riding mistakes, lameness or neurologic conditions causing abnormal limb movement, or it can simply be caused by a single misstep in an otherwise healthy horse at speed.Interference Crossfiring at the Pace
Limb to limb contact often leads to injury. Over-reaching can cause shoes to be pulled off, which can wrench and injure joints, tendons or ligaments. Bruising and minor wounds from hoof (shoe) to skin contact are common, and can range from minor to very severe.
Beyond the obvious wounds and injuries, interference affects performance at a more subtle level. If a horse knows that certain maneuvers are likely to cause limb contact and pain, it will begin to avoid those maneuvers or perform them poorly.
With a single bad step, a normal healthy horse may interfere in any of the manners described above, resulting in injury. A classic example is the horse that is turned out for the first time in a long time, plays hard, and ends up with a wound of the lower limb from some sort of interference.
Interestingly, performance horses allowed to exercise free without a rider, are more likely to interfere. They may simply be unaccustomed to exercising at speed without the weight of the rider and so over-reach and make limb contact.
Even horses that are accustomed to running free may interfere in rough ground or poor footing. A classic example is the horse that pulls its front shoe off in the mud. What happens in that case is that the front foot is delayed slightly in take-off in the mud, and the rear hoof/shoe reaches up and grabs the heel of the shoe, yanking it off. This is the real cause of what horse people sometimes describe as “the mud sucking the shoe off”.
Interference also happens in horses turned out in groups, during rough play or other interaction.  In some cases, other horses’ limbs and hooves are the cause of a lower limb wound.
Horses that are exhausted are also more likely to accidentally interfere, as their limb movements become less coordinated. Fatigue from over-exertion, especially in deep footing, is a common cause of interference.
Young horses being started under saddle take time to adjust to the weight of the rider. They lack limb coordination and tend to interfere. This often gradually improves as they gain strength, experience and coordination.
Finally, rider factors play a big role in causing a horse to interfere.  Some horses will tend to interfere when being ridden by one rider but not another. And it can be hard to know specifically what is causing the horse to interfere when ridden by one person and not another.
In contrast, some interference happens consistently or frequently in certain horses at certain gaits. Consistent interference can be the result of basic limb or back conformation traits such as short back with long hind legs, incorrect shoeing or trimming, lameness, back soreness, generalized weakness, or neuromuscular disease.
Any of these or combinations of them may cause a horse to move abnormally, leading to interference. Rarely, after all other causes ruled-out, some horses are simply sloppy, inattentive movers, and more frequently make limb contact.
In many cases a horse has an underlying disorder causing frequent or consistent interference.  But in some cases, consistent interference does result (or is made worse) by particular shoeing or trimming. There are many ways this can happen:
• Medial-lateral hoof imbalance can cause or worsen changes in the trajectory of the feet, and cause brushing.
•  Long toes (affecting break-over) of either hind limbs or forelimbs, or both, can lead to forging and over-reaching by changing the timing and arc of flight of the limbs.
• Accessories like toe grabs or calks may interfere with movement and break-over, worsening interference.
•  Extreme shoeing techniques demanded by certain disciplines may predispose to interference.
•  When consistent interference begins after shoeing, it is usually best to let the horse have a day or two to adjust to the shoeing before insisting on a shoeing change. In some cases, horses will adjust to the new shoeing and stop interfering.
Prevention of both accidental and consistent interference begins with selection of sound horses of proper conformation for your intended use. Know the demands of your discipline and the ideal conformation for it, and select horses that meet your expectations.
•  When purchasing a horse, always have a veterinary pre-purchase examination.  The examining veterinarian may find evidence of interference at the purchase exam such as conformation predisposing to interference, limb contact during the exam, special shoeing that seems intended to prevent interference, or scars from prior interference events.
•  Understand the basic mechanics and practice of balanced trimming or shoeing.  Select a shoeing or trimming style that favors the well being of the horse. Avoid unnecessary traction devices and un-natural angles and styles.
• Stay on a schedule of 6-8 weeks shoeing or trimming interval in most cases. Do not allow horses to grow excessively long hooves between trimming or shoeing, as this exaggerates the arc of flight and predisposes to interference of all kinds.
•  Do not shoe young horses unless you must, or shoe them only in the front to begin with, as they tend to over-reach or forge.
•  In horses that have a history of frequent interference, protect the lower limbs from contact injury by using bell boots and other protective lower-limb gear.
•  Horses that interfere as a consequence of poor conformation (or any other underlying problem) may need special shoeing and/or trimming and will need to wear lower limb protective gear (bell boots, splint boots).
•  Avoid exhausting horses during performance by never asking for more than what they are capable of and trained for. Ensure that horses are essentially “over-trained” for the performance expected.
•  Keep turnout consistent and frequent for all horses. Avoid long periods of layup followed by sudden free turnout.
•  Keep in mind that shoeing and trimming may or may not be the cause of interference. Be mindful that there may be other causes.
•  Frequent, repetitive interference or shoe loss from limb contact requires a veterinary exam to rule out underlying conditions.
•   Proper and consistent, balanced riding is always important in reducing interference.
Be mindful of and on the watch for interference in your horses.  Always consider the possibility of interference as a cause for lower limb injuries. Remember that any horse can interfere as a single accidental episode, but consistent or frequent interference should always prompt an investigation for underlying causes.
By Douglas O. Thal DVM Dipl. ABVP, Board Certified in Equine Practice

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