Musculus extensor digitorum longus
The long digital extensor originates from the extensor fossa of the distal femur. Its tendon passes under the extensor retinaculum at the front of the hock and runs down the dorsal metatarsus to insert on the extensor process of the distal phalanx (P3). It extends all joints of the hind digit and flexes the hock. It is innervated by the peroneal (fibular) nerve (roughly L5–S2).
Origin: Extensor fossa of the distal femur (between the peroneus tertius and lateral digital extensor)
Insertion: Extensor process of the distal phalanx (P3)
Action: Extends all joints of the hind digit; flexes the hock
Innervation: Peroneal (fibular) nerve (L5-S2)
The tendon can be involved in lacerations or trauma on the dorsal cannon or hock. Isolated strain is less common than in the flexor tendons.
## Overview Effusion of the extensor tendon sheath on the dorsal hock produces a soft, fluctuant swelling. It is often idiopathic and cosmetic, but can accompany distal tarsal osteoarthritis or other
Stringhalt is a condition characterized by involuntary, exaggerated flexion of one or both hind limbs during movement. The limb is jerked upward toward the belly in a spasmodic manner, most noticeably
Swelling or heat on the front of the hind cannon after trauma should be evaluated.
## Overview Effusion of the extensor tendon sheath on the dorsal hock produces a soft, fluctuant swelling. It is often idiopathic and cosmetic, but can accompany distal tarsal osteoarthritis or other hock pathology when lameness is present. ## Causes Idiopathic synovial effusion or secondary to joint disease or strain. ## Diagnosis Palpation and ultrasound. Lameness examination determines whether the effusion is clinically significant. ## Treatment Observation if the horse is sound. Drainage, injection, or treatment of underlying hock disease if lame. ## Recovery and Prognosis Excellent for cosmetic, non-painful effusion. Depends on associated joint disease if lame. ## Prevention None specific. Signs include: soft fluctuant swelling on the front of the hock; usually painless; may accompany hock lameness
Stringhalt is a condition characterized by involuntary, exaggerated flexion of one or both hind limbs during movement. The limb is jerked upward toward the belly in a spasmodic manner, most noticeably at the walk. It can be idiopathic, acquired (often linked to toxic plants), or occur after trauma or surgery. ## What it is Stringhalt involves abnormal function of the nerves and muscles that control hind limb flexion, most classically the lateral digital extensor muscle and its tendon. In acquired stringhalt, damage or irritation to this muscle-tendon unit leads to the characteristic hyperflexion. The Australian form is associated with ingestion of the weed *Hypochaeris radicata* (catsear), while other cases may follow trauma, surgery, or occur idiopathically with no clear cause. The condition is usually more obvious at the walk than at faster gaits and often improves when the horse is turned or backed. It can be unilateral or bilateral and ranges from mild to severely debilitating. ## Signs and symptoms - Exaggerated, spasmodic upward flexion of the hind limb during walking (snapping toward the belly) - Worse at walk than trot; often improves at canter - More noticeable when turning, backing, or in cold weather - Can be unilateral or bilateral - In severe cases, the horse may hit the abdomen or chest with the hoof - Australian stringhalt often follows ingestion of toxic weeds and may affect multiple horses in a group ## Causes and risk factors Stringhalt has several forms. The acquired form is often linked to ingestion of toxic plants (especially *Hypochaeris radicata* in Australia and New Zealand). Other cases follow trauma to the hind limb, surgery in the area, or occur idiopathically. There is no strong breed or age predisposition, though it can appear at any age. ## How it is diagnosed Diagnosis is primarily based on the characteristic gait abnormality observed during movement. A thorough neurologic examination helps rule out other causes of abnormal hind limb movement. In some cases, electromyography or ultrasound of the lateral digital extensor muscle and tendon is used. Ruling out other neurologic diseases is important. ## Treatment and management Treatment depends on the type and severity. In mild cases, many horses can continue light work. For more severe or acquired cases linked to toxic plants, removal from the offending pasture is essential. Surgical removal of a section of the lateral digital extensor tendon (myotenectomy) can provide significant improvement in many horses. Medical management with phenytoin or other medications has been tried with variable success. ## When to call the vet Contact your veterinarian if you notice sudden or progressive exaggerated hind limb flexion. While stringhalt itself is not an emergency, it is important to differentiate it from other neurologic conditions that may require urgent attention. ## Prevention For acquired stringhalt, prevention involves avoiding pastures containing known toxic plants such as *Hypochaeris radicata*. Regular pasture management and monitoring horses for early signs after moving to new properties can help reduce risk. ## Frequently asked questions **Is stringhalt painful?** Most horses with stringhalt do not appear to be in pain, though the gait abnormality can affect performance and, in severe cases, cause the horse to hit itself. **Does stringhalt get worse over time?** Idiopathic cases may remain stable or slowly progress. Acquired cases linked to toxic plants can improve once the horse is removed from the source, though some horses retain a degree of abnormality. **Can stringhalt be cured?** Surgical treatment (lateral digital extensor myotenectomy) provides good improvement in many horses, though complete resolution is not guaranteed in all cases. This information is for educational purposes only. Always consult with a licensed veterinarian for diagnosis and treatment of your horse. Signs include: Exaggerated, spasmodic upward flexion of the hind limb during walking (snapping toward the belly); worse at walk than trot, often improves at canter; more noticeable when turning, backing, or in cold weather; can be unilateral or bilateral; in severe cases, the horse may hit the abdomen or chest with the hoof; Australian stringhalt often follows ingestion of toxic weeds and may affect multiple horses in a group
The tendon can be involved in lacerations or trauma on the dorsal cannon or hock. Isolated strain is less common than in the flexor tendons.
Swelling or heat on the front of the hind cannon after trauma should be evaluated.
See the Long Digital Extensor on our interactive 3D horse model. Open 3D Model