Author : Lisa Agius-Gilibert
Lisa is a Sydney-based Equine Podiotherapist with a Bachelor of Equine Science and over 12 years’ professional experience in hoof care. At Flex Hoof Boots, she creates evidence-based educational content to help horse owners feel confident in hoof care and boot fitting – empowering them to give their horses the best possible lives.
Underrun heels are one of the most common (and most misunderstood) hoof distortions in domestic horses. They can occur in both barefoot and shod horses, and if left unaddressed, they can seriously impact hoof function, comfort, and even boot fit. In this post, we’ll walk through what underrun heels are, how to identify them, what causes them, and what you can do to help your horse grow healthier, straighter heels.
What Do Normal Heels Look Like?
A healthy heel is more than just the right height. It also needs to be in the right position and aligned with the rest of the hoof.
Normal, healthy heel features:
- The heel platform is located at the widest part of the frog
- From the side, the heel angle matches the dorsal wall, with a straight hoof-pastern axis
- Heel tubules run straight down from the hairline to the ground
- Bars are straight, pointing toward the frog apex without curling
This heel structure supports caudal hoof function, allows for proper weight distribution, and promotes strong digital cushion and lateral cartilage development.
What Are Underrun Heels?
Underrun heels (also called run-forward heels) occur when the heel grows forward, rather than straight down. This causes the heel platform to migrate out of position and distorts the caudal hoof.
- Heel platform sits in front of the widest part of the frog
- From the side, the heel angle is lower (more sloped) than the dorsal wall
- Heel tubules are curved or collapsed instead of straight
- Bars are curved or laid over the sole
Extreme under run heels
This distortion often results in weak heel structure, poor frog stimulation, and compromised caudal support. Importantly, heel length alone doesn’t define underrun heels. They can be long or short, it’s the angle and position that matter.
Same hoof, extreme under run heels. Note how far forward the heel platform is, when compared to the back of the foot, and the pointy heel bulbs.
Same hoof again, different day. This oblique angle shows how run forward the heels are. There is no support for the back of the hoof here.
Less severe underrun heels
Underrun heels aren’t always so obvious. Here is a less severe underrun heel, that you might not think had anything wrong. Note that the back of the heel platform is not at the widest part of the frog.
Run forward and uneven heels on a hind hoof. Notice how the outside heel (on the left of the photo) is further forward than the other heel.
Underrun heel on a hind hoof – note the angle of the heel is shallower than the angle of the wall at the toe.
Healing process
Underrun heels at the beginning of rehab
The same hoof as the photo above, 10 months later. The heels are significantly improved from correct trimming and heels first landings.
Why Do Heels Become Underrun?
There are multiple causes of underrun heels, and they often overlap. Key contributing factors include:
- Long toes, which delay breakover and push the heels forward
- Weak caudal hoof structures (digital cushion, lateral cartilages)
- Long trim cycles, allowing the heels to grow forward unchecked
- Leaving heels too tall in an attempt to preserve height
- Failing to straighten heel tubules and bars during trimming
- Low palmar/plantar angles that promote heel collapse
- Displacement of the caudal hoof backward and down, leading to overhang
- Conformation – horses with long pasterns and pedal bones are particularly prone when not trimmed on a short, tight schedule
- Long-term musculoskeletal imbalances or injuries – causing overuse of the back half of the hoof
How Do Underrun Heels Affect Boot Fit?
Underrun heels can complicate hoof boot fitting. Some boot fit issues are:
- The back of the hoof may not seat properly in the boot shell
- The heel bulbs may hang over the back edge of the boot
- The heel region may be unsupported, increasing the risk of rubbing or pressure
- Boots may appear too big at the heel (or too tight!) due to distorted proportions
We recommend discussing fit challenges with your hoof care provider or a boot fitter familiar with caudal hoof distortion.
Under run heels in a Flex Boot on the left. Note that the heels bulbs are hanging out the back of the boot shell, and the TPU gaiter is too tight. Compare to the right diagram – a healthy heel sitting well within the boot shell, with the TPU gaiter fitting correctly.
What Can You Do About It?
Correcting underrun heels takes time and consistent trimming. The goal is to restore heel position and alignment while maintaining comfort and function.
Trim strategies:
- Trim heels back to the widest part of the frog
- If this reduces height too much, slipper the heels: bevel or relieve the curled tubules while preserving vertical height
- Trim the toe to create 50/50 heel/toe balance around the centre of the hoof
- Keep short trim cycles: ideally every 1-2 weeks for touch-ups, or every 4 weeks professionally with in-between maintenance trimming by the owner.
- Ensure the horse is landing heel-first to encourage caudal stimulation
Underrun heels don’t happen overnight, and they won’t be fixed in a single trim. But with the right strategies and consistent care, they can be reversed – restoring a stronger, more functional back-of-foot.
Stay tuned for upcoming posts on heel-first landings, palmar angle rehab, and how to use boots and pads to support caudal development.
