Fibrotic Myopathy in Horses: Causes, Symptoms, Diagnosis, and Treatment

Fibrotic myopathy is a relatively uncommon musculoskeletal condition in horses that affects the muscles of the hind limb, most commonly the semitendinosus muscle. The condition causes an abnormal gait characterised by a sudden “slapping” or jerking movement of the affected leg when the horse walks.

Although fibrotic myopathy may appear dramatic, many horses with the condition remain comfortable and able to perform light work. Understanding the causes, symptoms, and treatment options can help horse owners recognise the condition early and manage it appropriately.

What Is Fibrotic Myopathy

Fibrotic myopathy is a disorder caused by scar tissue formation within the muscle, most commonly the semitendinosus muscle located in the back of the horse’s hind limb. The development of fibrous scar tissue causes the muscle to lose its normal elasticity and ability to stretch.

Because the muscle cannot lengthen normally during movement, the affected hind leg moves in a restricted, mechanical pattern. The limb may extend normally but then snaps quickly back toward the ground rather than following a smooth arc.

The condition typically affects one hind limb, though rarely both limbs may be involved.

Anatomy Involved

The semitendinosus muscle plays an important role in hind limb movement. It helps extend the hip, flex the stifle, and control the movement of the limb during the stride cycle.

In fibrotic myopathy:

• Scar tissue develops within the semitendinosus muscle
• The muscle becomes shortened and inelastic
• Normal stretching during movement becomes restricted

This restriction leads to the characteristic abnormal gait.

Causes of Fibrotic Myopathy

Fibrotic myopathy most often develops after injury or trauma to the muscle, which leads to scar tissue formation during the healing process.

Common Causes

• Muscle trauma from kicks or accidents
• Intramuscular injections placed incorrectly in the semitendinosus muscle
• Muscle tears or strains during exercise
• Surgical trauma involving the hind limb
• Chronic inflammation of the muscle

Sometimes the original injury may be mild or go unnoticed, meaning owners may not be aware that damage occurred before the abnormal gait develops.

Injection-Related Injury

One recognised cause is injection-related muscle damage. If medications are injected improperly into the semitendinosus muscle rather than recommended sites such as the neck or pectoral muscles, irritation or infection may lead to scarring.

For this reason, veterinarians typically avoid injecting medications into the muscles of the hind limb unless absolutely necessary.

Symptoms of Fibrotic Myopathy

The most distinctive feature of fibrotic myopathy is a mechanical gait abnormality affecting the hind limb.

Characteristic Gait

When the horse walks, the affected limb:

• Moves forward normally during the stride
• Suddenly jerks downward in a rapid, slapping motion
• Appears stiff or restricted in movement

The abnormal motion is most noticeable at the walk and may become less obvious at faster gaits such as the trot or canter.

Additional Signs

Other clinical signs may include:

• Shortened stride in the affected limb
• A “toe-slapping” sound as the hoof hits the ground
• Visible or palpable firmness in the semitendinosus muscle
• Muscle asymmetry in the hindquarters
• Mild stiffness when first moving

Importantly, many horses with fibrotic myopathy do not appear to be in pain, even though their gait looks abnormal.

Diagnosis

Diagnosis of fibrotic myopathy usually involves a combination of gait evaluation and physical examination by a veterinarian.

Clinical Examination

A veterinarian will observe the horse moving in several situations, including:

• Walking in a straight line
• Turning in tight circles
• Backing up
• Trotting

The characteristic downward “slap” of the limb during walking is often strongly suggestive of fibrotic myopathy.

Palpation of the Muscle

The veterinarian may palpate the semitendinosus muscle to assess:

• Thickening or firmness within the muscle
• Areas of scarring
• Reduced elasticity

Diagnostic Imaging

In some cases, imaging may be used to confirm the diagnosis.

Possible diagnostic tools include:

• Ultrasound to visualise scar tissue within the muscle
• Thermography to identify areas of abnormal heat or inflammation
• MRI or advanced imaging in specialised referral cases

These techniques help differentiate fibrotic myopathy from other causes of hind limb gait abnormalities.

Conditions That Can Mimic Fibrotic Myopathy

Several conditions can cause abnormal hind limb movement similar to fibrotic myopathy.

Differential Diagnoses

• Stringhalt
• Shivers
• Upward fixation of the patella
• Stifle injuries
• Neurological disorders affecting hind limb coordination

Accurate diagnosis is essential because treatment and prognosis differ between these conditions.

Treatment

Treatment of fibrotic myopathy depends on the severity of the condition and the intended use of the horse.

Conservative Management

Many horses with mild fibrotic myopathy do not require aggressive treatment.

Management strategies may include:

• Controlled exercise
• Physiotherapy
• Stretching exercises
• Anti-inflammatory medications if inflammation is present

However, conservative treatment often has limited ability to reverse established scar tissue.

Surgical Treatment

In horses with significant gait abnormalities that interfere with performance, surgery may be considered.

The most common procedure is semitendinosus tenotomy or tenectomy.

Tenotomy

A surgical cut is made in the tendon of the semitendinosus muscle to release tension.

Tenectomy

A small portion of the tendon is removed to reduce restriction caused by scar tissue.

The goal of surgery is to restore a more normal range of motion in the hind limb.

Post-Surgical Rehabilitation

Recovery following surgery usually involves:

• Stall rest initially
• Gradual return to controlled exercise
• Physiotherapy to encourage proper muscle use
• Monitoring for recurrence or complications

Rehabilitation typically takes several weeks to months.

Prognosis

The prognosis for horses with fibrotic myopathy varies depending on the severity of muscle scarring.

Mild Cases

• Horses may remain comfortable
• Light work is often possible
• The abnormal gait may persist but remain stable

Moderate to Severe Cases

• Surgical treatment may improve the gait
• Some horses return to athletic work
• Complete resolution of the abnormal movement is not always possible

Even when surgery improves movement, some degree of abnormal gait may remain.

Prevention

Preventing fibrotic myopathy largely involves reducing the risk of muscle injury.

Safe Injection Practices

• Avoid injecting medications into the hind limb muscles unless necessary
• Use recommended injection sites such as the neck or pectoral muscles
• Ensure injections are performed by experienced professionals

Injury Prevention

• Maintain safe pasture environments to reduce the risk of kicks or trauma
• Use appropriate conditioning and warm-up routines during exercise
• Monitor horses for early signs of muscle injury

Prompt Treatment of Muscle Injuries

Early treatment of muscle strains or trauma may help reduce the formation of excessive scar tissue.

When to Contact a Veterinarian

Horse owners should seek veterinary evaluation if they notice:

• Sudden changes in hind limb gait
• A slapping or jerking motion of the leg during walking
• Muscle swelling or injury in the hindquarters
• Progressive stiffness or difficulty moving

Early diagnosis can help determine whether treatment or monitoring is the most appropriate approach.

Conclusion

Fibrotic myopathy is a mechanical gait abnormality caused by scar tissue formation within the semitendinosus muscle of the hind limb. Although the condition produces a distinctive and sometimes dramatic movement pattern, many horses remain comfortable and able to function normally.

Because fibrotic myopathy can resemble other neurological or musculoskeletal conditions, veterinary evaluation is essential for accurate diagnosis. Treatment options range from conservative management to surgical intervention, depending on the severity of the condition.

With proper management and veterinary care, many horses with fibrotic myopathy can continue to live active and comfortable lives.