Fibrotic Myopathy in Horses
- Horse Education Online
- 1 day ago
- 11 min read
Fibrotic myopathy is a mechanical gait abnormality caused by scar tissue within a muscle. It most often affects the hamstring muscles at the back of the upper hind limb, particularly the semitendinosus and semimembranosus muscles.
Healthy muscle fibers are elastic. They lengthen and shorten as the horse moves, allowing the hind limb to swing forward and return to the ground smoothly. Fibrous scar tissue is much less elastic. When enough scar tissue forms, it shortens and restricts the muscle, preventing the affected limb from completing a normal forward stride.
The result is a distinctive gait in which the hind limb swings forward, stops abruptly, and is pulled down or backward before the hoof reaches the ground. The hoof may strike the ground with a noticeable slapping motion. This abnormal movement is usually easiest to recognize at the walk.
Fibrotic myopathy most commonly develops after an injury to the hamstring muscles. It may also follow repeated strain, infection, intramuscular injections, or, less commonly, a congenital abnormality. Once the initial inflammation has resolved, the horse may show little or no pain. The remaining gait abnormality is caused primarily by the physical restriction of the scarred muscle.
What Is Fibrotic Myopathy?
The term myopathy refers to a disease or disorder of muscle. Fibrotic describes the formation of dense connective tissue, commonly called scar tissue.
Fibrotic myopathy therefore means that normal muscle has been damaged and partly replaced by fibrous tissue. This tissue may form a firm band within the muscle, create adhesions between tissue layers, or cause the entire muscle to become shortened and contracted.
The condition is usually found in the muscles at the back of the thigh. The semitendinosus and semimembranosus muscles are especially important because they help extend the hip and hock and assist with moving the hind limb backward. They must also lengthen as the limb swings forward. When one of these muscles becomes shortened by fibrosis, the limb cannot continue its normal forward movement.
Want to learn more about equine muscles? read Horse Muscle Anatomy Explained: Major Muscle Groups, What They Do, and Why They Matter and use the Interactive Horse Muscles Tool to help you study.
One hind limb is affected in most cases, although both sides can be involved. The severity ranges from a mild shortening of the stride to a dramatic gait abnormality that interferes with riding or athletic performance. According to the American College of Veterinary Surgeons, many chronic cases are not painful, but the resulting mechanical restriction can be performance-limiting.
Did you know? Fibrotic myopathy is called a mechanical lameness because the movement is physically restricted by shortened tissue. The horse may continue to move abnormally even when there is no active pain or inflammation.
What Does Fibrotic Myopathy Look Like?
The classic sign is an abrupt interruption of the forward phase of the hind-limb stride. As the horse brings the affected leg forward, the limb suddenly stops and is pulled down and backward. The hoof then contacts the ground earlier and more forcefully than expected.
Owners may describe the action as:
A slapping hoof
A shortened forward stride
A sudden backward jerk of the lower limb
A goose-stepping appearance
A hitch or catch in the stride
An abnormal step that is most obvious at the walk
The gait may look dramatic, but a horse with established fibrosis may not show the usual signs of painful lameness. It may willingly bear weight on the leg and remain comfortable at rest. The abnormality comes from the restricted muscle rather than an attempt to unload a painful foot or joint.
The exact appearance depends on which muscle or combination of muscles is involved and how much tissue has been affected. Some horses have only a shortened cranial, or forward, phase of the stride. In more severe cases, the leg visibly snaps backward and the hoof slaps the ground.
Early signs can be different from those seen after fibrosis has developed. During the acute injury stage, the horse may have:
Sudden hind-limb lameness
Heat and swelling in the upper hind limb
Pain when the muscle is touched
Muscle stiffness
Reluctance to move
A recent history of a kick, fall, slip, injection, or strenuous activity
As the injury heals, the heat and pain may decrease. A firm area, band, depression, or dimple may become noticeable within the muscle. Muscle atrophy can make the affected side of the hindquarters appear uneven.
The UC Davis School of Veterinary Medicine notes that chronic cases may contain hardened areas of muscle and exhibit the characteristic interrupted stride, particularly at the walk.
Fibrotic Myopathy Versus Stringhalt
Fibrotic myopathy can be confused with stringhalt, but the direction and timing of the abnormal movement are different.
In stringhalt, the hind limb is involuntarily lifted or jerked sharply upward, sometimes high toward the abdomen. The movement results from abnormal neuromuscular control.
In fibrotic myopathy, the limb begins to swing forward but is stopped by the shortened muscle. It is then pulled down or backward, often causing the hoof to slap the ground.
Condition | Characteristic movement | Primary problem |
Fibrotic myopathy | Forward swing stops abruptly and the hoof is pulled down or backward | Mechanical restriction from scarred muscle |
Stringhalt | Hind limb is suddenly lifted or jerked excessively upward | Neuromuscular gait disorder |
Fibrotic myopathy must also be separated from shivers, upward fixation of the patella, joint disease, nerve injury, and other causes of abnormal hind-limb movement. A gait pattern may provide an important clue, but it should not replace a veterinary examination.
Students who want to become more confident at recognizing abnormal movement can also review the Horse Education Online guide to gait faults in horses and the downloadable Gait Faults Flashcards.
Did you know? Fibrotic myopathy is generally easiest to identify at the walk. Faster movement can make the abrupt interruption of the stride more difficult to see.
What Causes Fibrotic Myopathy?
Fibrotic myopathy develops when muscle damage heals with an excessive or restrictive amount of scar tissue.
Direct trauma
A kick from another horse, fall, collision, laceration, or impact against a solid object can bruise or tear muscle fibers. A horse that catches a hind limb in a fence may also sustain severe stretching and tearing of the hamstrings.
Hyperextension
A sudden slip or awkward movement may force the hind limb farther forward than the muscles can safely accommodate. This overstretches the hamstrings and may cause partial tearing.
Repetitive strain
Horses involved in activities requiring abrupt acceleration, rapid turns, sliding stops, and forceful engagement of the hindquarters can place considerable strain on the hamstring muscles. Cases are therefore reported in cutting, reining, and roping horses, although the condition can occur in horses from any discipline.
Intramuscular injections
An injection can irritate or damage muscle tissue, particularly if the medication is irritating, the injection is repeated in the same location, or infection develops at the injection site. The hamstring muscles are sometimes used for intramuscular injections, but damage and drainage complications make careful site selection important. Injections should be performed under veterinary guidance, using correct technique and an appropriate location.
Infection
A muscle abscess or bacterial infection can destroy normal muscle fibers. Even after the infection is controlled, the affected area may heal with extensive fibrosis.
Congenital fibrotic myopathy
A rare congenital form has been reported in foals. Congenital means that the condition is present at birth or develops from an abnormality established before birth. These cases are different from the more common acquired form caused by injury or inflammation.
How Scar Tissue Changes the Gait
The development of fibrotic myopathy generally follows a sequence:
Muscle fibers are torn, bruised, infected, or otherwise damaged.
The body initiates inflammation and tissue repair.
Some of the damaged muscle is replaced by collagen-rich fibrous tissue.
The scar matures, contracts, and becomes less elastic.
The affected muscle can no longer lengthen normally.
The forward swing of the hind limb is stopped prematurely.
Several changes may occur within the affected area:
Pathological change | Effect on the muscle |
Fibrosis | Normal muscle fibers are replaced by scar tissue |
Contracture | The muscle becomes shortened and tight |
Adhesions | Adjacent tissue layers become bound together |
Atrophy | Muscle volume decreases |
Dimpling | A visible depression forms over the damaged area |
Ossification | Bone-like tissue occasionally forms within chronically damaged muscle |
Mechanical restriction | The limb loses part of its normal range of motion |
Fibrosis should not be confused with ordinary post-exercise soreness. Mild muscle soreness generally resolves as the tissues recover. Mature fibrous tissue is a structural change and may permanently alter how the muscle moves.
Did you know? A scar does not have the same contractile ability as normal skeletal muscle. Even when the original injury has stopped hurting, the scar may continue to shorten the stride.
Diagnosing Fibrotic Myopathy
Many cases can be strongly suspected from the characteristic gait and the location of the abnormal tissue. A complete veterinary examination remains important because several orthopedic and neurologic conditions can produce unusual hind-limb movement.
History
The veterinarian may ask about previous injuries, falls, lacerations, injections, infections, and the horse’s type of work. An old injury may be relevant even if it seemed to heal at the time.
Gait examination
The horse is usually observed walking in a straight line on firm, level ground. Both hind limbs are compared carefully. Walking toward and away from the examiner may help reveal asymmetry, while viewing from the side makes it easier to see the shortened forward phase.
Video can be valuable because the movement happens quickly. Slow-motion footage may help show the exact point at which the leg stops and is pulled backward. Videos are useful for documenting the gait, but they do not provide a diagnosis by themselves.
Palpation
The veterinarian examines the hamstring region for:
Firm or hardened bands
A visible or palpable depression
Muscle loss
Heat and swelling
Pain
Reduced flexibility
Differences between the right and left sides
Diagnostic imaging
Ultrasound can show disruption of normal muscle fibers, changes in tissue pattern, adhesions, and the approximate extent of fibrosis. It is especially useful for identifying which muscle has been affected.
Radiographs may be recommended if the veterinarian suspects mineralization or ossification within the muscle, a previous fracture, or another skeletal injury. Additional tests, such as scintigraphy or advanced imaging, may be considered when the diagnosis is uncertain.
A muscle biopsy is rarely required. In selected chronic cases, microscopic examination may demonstrate the replacement of normal muscle fibers by dense connective tissue.
Treatment of Acute Muscle Injury
Early treatment is intended to control pain and inflammation, prevent further damage, and promote organized healing. The exact plan depends on the severity and location of the injury.
Treatment may include:
Rest during the initial stage
Cold therapy for recent heat and swelling
Nonsteroidal anti-inflammatory medication prescribed by a veterinarian
A gradual return to controlled movement
Therapeutic ultrasound
Carefully directed stretching
Passive range-of-motion exercises
A structured rehabilitation program
Aggressive stretching of a newly torn muscle can make the injury worse. Rehabilitation should begin at the appropriate stage and progress according to veterinary findings rather than a fixed schedule.
An acute muscle injury offers the best opportunity to limit further tearing and excessive scar formation. Prompt examination is particularly important when a horse develops sudden swelling or pain in the upper hind limb.
Treatment of Chronic Fibrotic Myopathy
Once dense scar tissue has matured, treatment becomes more difficult. The goal is usually to improve flexibility and function rather than completely restore the original muscle.
Conservative treatment may include:
Controlled exercise
Massage
Supervised stretching
Therapeutic ultrasound
Shockwave therapy
Laser therapy
Underwater treadmill exercise
Cavaletti work
Passive range-of-motion exercises
Progressive strengthening and conditioning
Not every therapy is suitable for every horse. The location and severity of the scar, the presence of pain, the horse’s workload, and the available evidence should all be considered.
Some horses remain comfortable and useful despite a visible gait abnormality. If the horse is not painful and the movement does not interfere with its intended work, treatment may not be necessary.
Surgery
Surgery may be considered when a fibrotic band severely restricts movement or prevents the horse from performing its intended job. Depending on the structures involved, a surgeon may release, divide, or remove part of the scarred muscle, fascia, or tendon.
Possible procedures include myotomy or myotenotomy, fasciotomy, and tenotomy. These operations are intended to release the contracted tissue and allow a longer forward stride.
Results vary. Surgery creates a new injury, and additional scar tissue may develop at the surgical site. UC Davis specifically cautions that surgery does not always restore full range of motion. In a published series of 22 horses treated with a standing semitendinosus myotomy, outcomes were generally encouraging, but individual recovery and performance results depended on the case.
Prognosis
The prognosis depends on:
The amount of muscle affected
Which muscles are involved
How long the fibrosis has been present
Whether one or both limbs are affected
The horse’s intended use
The severity of the gait restriction
The response to rehabilitation or surgery
Whether the original injury is repeated
Many horses live comfortably with fibrotic myopathy because chronic fibrosis is often not painful. Mildly affected horses may return to useful work, although the altered gait can remain visible.
UC Davis describes the general prognosis as good and reports that many affected horses return to full work. However, that statement should be interpreted in the context of the individual case. A horse with a small, stable lesion and modest performance demands has a different outlook from one with extensive bilateral fibrosis or severe contracture. The gait may also make some horses unsuitable for disciplines that demand precise hind-limb action.
The condition does not usually continue to worsen unless the muscle is reinjured or subjected to the same damaging strain.
Did you know? A horse can be comfortable while still showing a pronounced abnormal gait. Soundness, pain, mechanical function, and suitability for a particular job are related, but they are not identical measures.
Can Fibrotic Myopathy Be Prevented?
Not every case can be prevented, but several practical measures may reduce the risk:
Warm the horse up gradually before strenuous work.
Maintain regular turnout or appropriate daily exercise.
Condition the horse progressively for fast turns, stops, and demanding hindquarter work.
Avoid sudden increases in exercise intensity.
Keep footing as safe and consistent as possible.
Address muscle injuries promptly.
Follow veterinary guidance for intramuscular injections.
Avoid repeatedly injecting the same muscle.
Monitor injection sites for heat, swelling, pain, or discharge.
Use a structured rehabilitation program after a hamstring injury.
Any horse with a sudden, painful upper hind-limb injury should be evaluated early. Proper management during the initial healing stage may influence how much restrictive scar tissue ultimately develops.
Frequently Asked Questions
1. Is fibrotic myopathy painful?
The initial muscle injury may be painful, warm, and swollen. Once mature fibrosis has formed, many horses show little or no pain. The remaining gait abnormality is often caused by mechanical restriction.
2. Is fibrotic myopathy a neurologic disease?
No. It is primarily a muscular and mechanical condition. Neurologic disorders can produce unusual hind-limb movements, so a veterinary examination may be needed to distinguish between them.
3. Which muscles are usually affected?
The semitendinosus and semimembranosus muscles of the upper hind limb are affected most often. Other muscles can be involved depending on the original injury.
4. Why does the hoof slap the ground?
Scar tissue prevents the affected muscle from lengthening normally. The forward swing of the limb is stopped abruptly, causing the lower limb and hoof to be pulled down or backward before normal foot placement occurs.
5. Is fibrotic myopathy the same as stringhalt?
No. Stringhalt produces exaggerated upward flexion of the hind limb. Fibrotic myopathy stops the limb’s forward swing and pulls the foot down or backward.
6. Can fibrotic myopathy affect both hind limbs?
Yes, although one-sided involvement is more common. Bilateral cases may be more difficult to recognize because there is no normal hind limb available for comparison.
7. Can a horse still be ridden?
Some mildly affected horses remain comfortable and useful. Suitability for riding depends on the severity of the restriction, the horse’s balance, its intended work, and veterinary assessment.
8. Does fibrotic myopathy get worse over time?
It is generally considered nonprogressive once the scar has stabilized. It can worsen if the muscle is reinjured or repeatedly exposed to the original cause.
9. Can stretching remove the scar tissue?
Stretching may help maintain flexibility and range of motion, but it cannot reliably turn mature scar tissue back into normal muscle. Improper or aggressive stretching can also damage healing tissue.
10. Can surgery cure fibrotic myopathy?
Surgery can improve movement in selected cases by releasing restrictive tissue, but it does not guarantee a completely normal gait. Recurring scar formation and incomplete restoration of muscle length are possible. A veterinary surgeon should evaluate the expected benefits and risks for the individual horse.
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