Peroneus Tertius Muscle: Anatomy and Function in Animals

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The peroneus tertius is a hindlimb muscle of the cranial crural group whose anatomy varies dramatically across species. In the horse it is almost entirely tendinous, contains no muscle belly, and functions as a passive stay of the hock. In dogs and cats it is a small but genuine muscle that helps flex the hock and extend the digits.

That single sentence explains why this structure causes confusion. A muscle that cannot contract in one species is a contractile muscle in another, and the clinical consequences of injury differ accordingly. This guide compares the peroneus tertius across the horse, dog, and cat, explains its role in the reciprocal apparatus of the equine hindlimb, and outlines what rupture or congenital shortening means for the animal.

What Is the Peroneus Tertius?

The peroneus tertius muscle lies in the cranial (extensor) compartment of the crus, the region of the hindlimb between the stifle and the hock. The name comes from Latin: perone refers to the fibula, and tertius means third, so the muscle was historically named as the third fibular muscle after the peroneus longus and peroneus brevis.

In comparative anatomy, the peroneus tertius has been treated in two very different ways. In humans it is regarded as part of the extensor digitorum longus, sometimes described as its "fifth tendon" [1]. In veterinary species it is generally recognized as a separate structure, although its relationship to the extensor digitorum longus remains debated. A dissection and nerve-staining study in pigs found that the peroneus tertius and extensor digitorum longus share a common femoral origin and receive branches of the same common fibular nerve, suggesting they derive from the same developmental muscle mass but are named separately because of their different morphology [2]. That same study concluded that the pig peroneus tertius and the human peroneus tertius are probably different muscles, a useful caution against assuming that a structure with the same name behaves the same way in every animal [2].

For the horse, the picture is clearer. The equine peroneus tertius is essentially a tendinous sheet rather than a fleshy muscle.

Comparative Anatomy: Origin and Insertion by Species

Horse

In the horse, the peroneus tertius originates from the extensor fossa of the lateral trochlear ridge of the distal femur, sharing this origin with the long digital extensor muscle [3]. From there it runs distally as a flat, broad tendinous band across the cranial surface of the tibia. It does not have a significant muscle belly.

The insertion is complex and divided into several components. Gross dissection of the equine peroneus tertius has described superficial lateral, deep lateral, dorsal, and medial tendons of insertion [4]. These attach directly into the periosteum of the central and third tarsal bones and the third metatarsal bone [4]. That direct attachment to tarsal and metatarsal periosteum is why the structure is relevant to hock lameness, since it places mechanical load on those bony surfaces [4].

Dog

In the dog, the peroneus tertius is a small muscle of the cranial crural group. It arises from the extensor fossa of the femur, the same depression on the lateral condyle that gives origin to the long digital extensor. It inserts on the proximal end of the metatarsus, particularly the dorsal surface of the third and fourth metatarsal bones near the tarsometatarsal joint. Its tendon passes under the extensor retinaculum at the hock.

Because the canine peroneus tertius has a true muscle belly, it can contract actively. Its action is to flex the hock and to assist in extending the digits, which places it functionally in the same group as the tibialis cranialis and long digital extensor.

Cat

The feline peroneus tertius follows the same general plan as the dog. It is a small cranial crural muscle with a femoral origin near the extensor fossa and a distal insertion on the dorsal metatarsus. Cats are digitigrade and rely heavily on rapid hock flexion during jumping and landing, so the cranial crural muscles as a group are important for shock absorption and propulsion. The anatomical arrangement is close enough to the dog that feline anatomy texts describe the muscle in comparable terms.

Comparative Summary Table

The Equine Peroneus Tertius as a Passive Stay

Why a Tendon Can Do a Muscle's Job

Elastic connective tissue can store and return energy, and a tendinous sheet stretched across a joint can resist motion without any active contraction. The equine peroneus tertius works exactly this way. It is part of the reciprocal apparatus, a system of tendinous connections that mechanically links the stifle and the hock so that when one joint flexes, the other flexes automatically.

The reciprocal apparatus has two main tendinous components. On the caudal aspect of the limb, the superficial digital flexor and its accessory ligament connect the femur, calcaneus, and digits. On the cranial aspect, the peroneus tertius connects the femur to the tarsus and metatarsus. Together they convert the hindlimb into a linked mechanical chain. When the stifle flexes during the swing phase of the stride, the peroneus tertius is stretched and the hock flexes with it. When the stifle extends during the stance phase, the tendinous sheet relaxes and the hock extends.

This arrangement saves the horse muscular effort. A horse can protract and retract its hindlimb with far less active muscle work than would otherwise be required, which supports efficient locomotion over long distances.

Sensory Innervation of a Tendon

A structure that cannot contract still needs monitoring. Histological study of the peroneus tertius in the horse and ass identified free nerve endings, Pacini-like corpuscles, Ruffini terminations, and Golgi tendon organs within the tendinous tissue [5]. These receptors give the central nervous system feedback about tension and position, which is essential for coordinating the reciprocal apparatus during locomotion [5]. The presence of Golgi tendon organs is notable because these receptors classically monitor tension at the muscle-tendon junction. Their presence in a muscleless tendon underlines that the peroneus tertius is a sensory as well as mechanical structure.

The Peroneus Tertius in Dogs and Cats

A Working Muscle

In small animals, the peroneus tertius is an ordinary skeletal muscle. It has muscle spindles, contracts when its motor neurons fire, and contributes to hock flexion. Because the muscle is small and sits close to larger neighbors such as the long digital extensor and tibialis cranialis, it is rarely the sole target of a clinical examination. Injuries to the cranial crural group are more often recognized by their combined effect on hock flexion and digital extension.

Why Small Animal Clinicians Rarely Name It

The peroneus tertius in dogs and cats is not a common source of isolated lameness. It can be affected by trauma, by penetrating wounds over the craniolateral crus, or by generalized neuropathy of the fibular nerve. When the fibular nerve is damaged, all the muscles of the cranial crural and lateral crural groups weaken, and the animal loses the ability to flex the hock and extend the digits. A clinician assessing a dog with a dropped hock or with knuckling of the paw is evaluating the function of a group that includes the peroneus tertius.

Clinical Relevance, Limitations and Common Mistakes

Rupture of the Peroneus Tertius in Horses

Rupture of the peroneus tertius is a well-recognized equine injury. According to the Merck Veterinary Manual, rupture of the peroneus tertius in horses produces a characteristic gait abnormality related to loss of the reciprocal apparatus [6]. Because the tendinous sheet normally links the stifle and hock, its disruption allows the hock to hyperextend independently of the stifle. The classic presentation is a horse that can extend the hock but cannot flex it normally, or that shows an altered relationship between stifle and hock motion.

Rupture can occur after a fall, a kick, or a period of intense exertion, and it can also accompany trauma to the stifle. Diagnosis rests on physical examination and imaging, and treatment is managed by a veterinarian. Owners often notice a sudden change in gait, swelling around the hock or stifle, and reluctance to move normally.

Congenital Shortening and Flexure Deformity

The peroneus tertius can also be abnormally short at birth. A case report described a foal with a congenital flexure deformity of the right hock that was unable to walk because the peroneus tertius was abnormally short [7]. Tension on the muscle limited the dorsal angle of hock extension to a 70-degree arc [7]. After surgical resection of the peroneus tertius, the hock extended an additional 30 degrees, and two months later the foal had normal gaits with normal extension and flexion of the hock and stifle [7]. This case illustrates how completely the tendinous sheet governs hock motion in the horse, and it also shows that surgical management can produce a good outcome in selected cases [7].

Origin Avulsion

Trauma at the femoral origin can avulse the peroneus tertius along with the long digital extensor. A four-month-old giraffe with stifle effusion and lameness was found to have a fracture of the extensor fossa of the lateral trochlear ridge at the origin of the peroneus tertius [8]. Under anesthesia, dysfunction of the reciprocal apparatus was documented by flexing the stifle while the tarsus remained extended, and an avulsion fracture of the origin of the peroneus tertius and extensor digitorum longus was diagnosed [8]. Surgical removal of the bone fragment was followed by stall rest for three months, and only a mild residual lameness remained at six months [8]. This case is a reminder that the origin of the peroneus tertius is a site of bony attachment that can fail under traction, not just a soft tissue landmark.

The Peroneus Tertius and Hock Lameness

Because the equine peroneus tertius attaches directly to the periosteum of the central and third tarsal bones and the third metatarsal bone, it may be involved in the pathogenesis of hock lamenesses [4]. This means that persistent or recurrent hock pain in a horse should prompt a veterinarian to consider the tendinous insertions as part of the diagnostic picture, not only the joints and bones.

Common Mistakes

A common mistake is assuming that the peroneus tertius is the same structure in every species. In humans it is a variable muscle that is often treated as part of the extensor digitorum longus and is used in tendon transfer surgery [9][10]. In horses it is a muscleless tendon with a mechanical linkage function. In pigs, nerve staining suggests the peroneus tertius and extensor digitorum longus are developmentally related [2]. Applying human anatomical assumptions to a horse or a dog leads to incorrect predictions about function and injury.

A second mistake is expecting the peroneus tertius to be a common cause of isolated lameness in dogs and cats. It is not. The muscle is small, and when it is affected it is usually part of a broader cranial crural or fibular nerve problem. A dog with an isolated peroneus tertius strain would be unusual, and a clinician should look for other explanations when hock flexion is weak.

A third mistake is over-interpreting surface anatomy. The peroneus tertius is highly variable in humans, with reported prevalence ranging widely across populations and with numerous insertion types [11][12][13]. Surface palpation methods can identify its presence in some individuals but not others, and the muscle may be absent entirely. A cadaveric study found the peroneus tertius absent in 26.6 percent of specimens and directly originating from the extensor digitorum longus in another 23.3 percent [12]. These figures belong to human anatomy, but they illustrate a general principle: a structure that varies this much in one species should never be assumed to be constant in another.

Limitations

Anatomical and clinical knowledge of the peroneus tertius in companion animals is far thinner than in horses. Most published veterinary work focuses on the equine reciprocal apparatus and on the human muscle, and the canine and feline literature tends to describe the peroneus tertius only as part of the cranial crural group. Individual animals vary, and a veterinarian examining a specific patient will rely on that patient's history, physical examination, and imaging rather than on general anatomy alone.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Frequently Asked Questions

What is the peroneus tertius muscle?

The peroneus tertius is a hindlimb muscle of the cranial crural group. In horses it is a tendinous sheet that links the stifle to the hock and metatarsus, and in dogs and cats it is a small muscle that helps flex the hock and extend the digits.

Does the horse peroneus tertius have a muscle belly?

No. The equine peroneus tertius is essentially tendinous. It has no meaningful muscle belly and cannot actively contract. It functions as a passive stay within the reciprocal apparatus of the hindlimb.

What happens if the peroneus tertius ruptures in a horse?

Rupture disrupts the mechanical link between the stifle and the hock, producing hyperextension of the hock and a characteristic gait abnormality. A veterinarian should evaluate any horse with sudden hindlimb gait change.

Is the peroneus tertius the same in dogs and horses?

No. In dogs and cats it is a small contractile muscle of the cranial crural group. In horses it is a muscleless tendinous sheet with a mechanical stay function. The same name covers very different structures.

What nerve supplies the peroneus tertius?

The peroneus tertius is supplied by branches of the fibular (peroneal) nerve in mammals, consistent with its position in the cranial and lateral crural compartments.

Can the peroneus tertius cause hock lameness in horses?

Yes. Because its tendons attach directly to the periosteum of the central and third tarsal bones and the third metatarsal bone, the peroneus tertius may be involved in the pathogenesis of hock lamenesses.

Why is the peroneus tertius described as part of the reciprocal apparatus?

Because its tendinous sheet mechanically couples stifle flexion to hock flexion. When the stifle flexes, the peroneus tertius is stretched and the hock flexes with it, so the two joints move together without requiring separate muscle activation.

Are peroneus tertius problems common in dogs and cats?

No. The muscle is small and rarely injured in isolation. When it is affected, it is usually part of a broader problem involving the cranial crural muscles or the fibular nerve.

Related Articles

• Equine Muscular System: Major Muscle Groups and Function
• Equine Larynx Anatomy and Function in Respiration
• Canine Lymphatic System: Anatomy and Function
• Equine Hoof Anatomy: Structures and Function
• Fish Gills: Anatomy, Function, and Common Health Issues
• Muscle Protein Synthesis

Sources

1. An unique variation of the peroneus tertius muscle.
2. Comparative study of peroneus tertius muscle in pigs based on origin, course, insertion and innervation.
3. Computed tomographic anatomy of the equine stifle joint.
4. Anatomy of the tarsal tendons of the equine tibialis cranialis and peroneus tertius muscles.
5. Sensory nerve endings in the peroneus tertius muscle of the ass and horse: a functional hypothesis.
6. Rupture of the Peroneus Tertius in Horses - Musculoskeletal System - Merck Veterinary Manual
7. Anatomy and therapeutic resection of the peroneus tertius muscle in a foal.
8. Surgical resolution of an avulsion fracture of the peroneus tertius origin in a giraffe (Giraffa camelopardalis reticulata).
9. Insertional Characteristics of the Peroneus Tertius Tendon: Revisiting the Anatomy of an Underestimated Muscle.
10. Absence of the peroneus tertius muscle: cadaveric study with clinical considerations.
11. Variations of Peroneus Tertius Muscle in Omani Population: A Surface Anatomical Study.
12. Clinical and variational evaluation of peroneus tertius muscle.
13. Morphological Variations of Peroneus Tertius: A Cadaveric Study with Anatomical and Clinical Consideration.