Fourth in a Series

In this series, Dr. Deb Bennett examines the equine hind limb. In this installment, Bennett explores the coordinated flexion and extension of the stifle and hock.

Farrier Takeaways

  • Coordinated flexion and extension of the stifle and hock are mandated in the horse.
  • Extension of the hock joint can occur only when the stifle is extended, and vice versa.
  • No normal horse ever takes a single step with its hind legs without also moving the joints of its back.

Probably the most famous anatomical demonstration that students of equine anatomy regularly see in the dissecting room — and certainly one of the most spectacular and interesting — is the coordinated opening and closing of the central joints of the hind limb, the “classic” hind limb reciprocation.

Coordinated flexion and extension of the stifle and hock are mandated in the horse (and its nearest relatives the zebras, onagers and asses) by a set of tensionally co-adjusted “bands” that parallel the tibia, forming a jointed parallelogram1 in which, if the stifle joint is opened the hock joint must also open and vice-versa (Figures 1 and 2).

The bands are formed by yellow ligaments and by muscles with tendinous cores that have long tendons of insertion. They originate on the femur but insert primarily on the hind cannon bone and calcaneum. They are elastic and thus store energy when, and to the degree that, they are stretched. Thus, not only does the hind limb reciprocating system foster precise coordination of hind limb movement, but allows the horse to economize on the total amount of effort required for movement, especially at high speed.1

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A normal horse has only one of two choices when making any movement of the hind limbs: either open (extend) both hock and stifle, or close (flex) both hock and stifle. This can be seen no matter what the situation, even in oddball poses that might be adopted by trained horses or foals.

Those who have followed this series (and the previous one dealing with the anatomy and function of the equine forelimb) will be familiar with yellow ligaments, which are muscles that have lost their red contractile fibers and function like strong, elastic bungee cords (see Page 66 in the November 2019 issue of American Farriers Journal.) Previous installments of this series have also prepared readers to understand the anatomy and movement capabilities of both the hock and stifle joints, and the stifle locking and unlocking mechanism (see Page 52 of the September/October 2021 and Page 46 of the November 2021 issues of AFJ).

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Carcass dissections demonstrate the parts of the stifle-hock reciprocating sub-system. Above, highlighting the peroneus tertius. Retinacula (retaining loops) are necessary to hold long tendons in place because tensional forces affecting these structures can be large. Below, highlighting superficial digital flexor (SDF), deep digital flexor (DDF) and gastrocnemius. It is important to remember that both extension and flexion of the stifle-hock are caused by the contraction of muscles that originate up on the pelvis or femur. Hock movements are slave to stifle movements.

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Model of the whole hind limb reciprocating apparatus in motion. Self-quiz: 1. Name the joints designated by red arrows. 2. Name the structures designated by letters. 3. Which of these are yellow bands? 4. Which way does the patella (at 6) move when the stifle flexes? 5. Which way does it move when the stifle extends? 6. What important role does the TFL muscle play? Answers on bottom of page.

Movement Constrained by Yellow Bands

The two largest bands that link the stifle and hock joints to create reciprocal motion are the peroneus tertius and the Achilles tendon (Figures 2-5). As usual, there are a couple of cautions to keep in mind with this traditional terminology: in standard anatomy texts, the peroneus tertius is called a “muscle,” which indeed it was many millions of years ago before its contractile belly was lost. It is here classified as a yellow ligament or yellow band.

The Achilles tendon is no individual structure at all in the horse, but a composite made up of the interwound tendons of insertion of the gastrocnemius, semitendinosus and femoral biceps muscles plus the superficial digital flexor (SDF) yellow band (in the hind limb, the SDF “muscle” lacks a red contractile belly and so is classified here as a yellow band; see Page 52). As Sisson and Grossman note, “The term ‘tendo calcaneus’ or ‘tendo Achillis’ is a convenient designation for the aggregated tendons in the distal part of the leg which are attached to the tuber calcis.”2

In humans and most other mammals, the peroneus tertius is a small muscle with a contractile belly. It originates on the tibia and its long core tendon inserts on the lateral side of the metatarsals, and its action is to supinate the foot (i.e., to pull the little-toe side of the foot upward).

People become acutely aware of their peroneus tertius in certain types of ankle sprains, such as may occur when jumping and then coming down on the side of another player’s shoe while playing basketball. Landing on the slanting shoe forces the foot into sudden and extreme pronation (i.e., curling inward and upward) and the intense pain immediately experienced along the lateral side of the leg is because of overstretching and tearing the core tendon of peroneus tertius.

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Model of the complete hind limb reciprocating apparatus. Structures labeled in red type comprise the upper sub-system; structures labeled in blue type comprise the lower or “classic” sub-system. Yellow ligaments (which form “yellow bands”) are highlighted in yellow.

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A: End-on view of an equine femur. The peroneus tertius yellow band originates in an extensor fossa, which is deep and well-defined in horses. B: Oblique lateral view of the femur of Teleoceras fossiger, a rhinoceros of the Miocene period, about 13 million years ago. In this species, the extensor fossa does not present a deep pit, signaling that it did not have peroneus tertius modified into a yellow band and thus did not possess the lower part of the reciprocating system. C: Caudal aspect of a horse femur showing the extremely large and deep fossa from which the SDF band originates. The gastrocnemius originates along the margins of this fossa. D: Caudal aspect of Teleoceras fossiger femur, in which the extensor fossa is neither large nor deep, implying smaller and weaker SDF and gastrocnemius.

The situation is different in horses and rhinos, in which most muscles involved in either pronation or supination of the distal limb are absent or greatly reduced (Figure 6). As anatomist Cyril Etienne and colleagues observe, “This is similar to what is generally observed in ungulates, as active muscle-driven pronation and supination are more restricted than in carnivores, primates or [ancestral] mammals.”3 The conversion of the relatively small and weak ancestral peroneus tertius to a large, strong elastic band is yet another of the anatomical configurations we have studied that is unique to equines.

Besides the Achilles tendon and peroneus tertius bands, there are some smaller bands involved in hind limb reciprocation. Analogous to the lacertus fibrosus in the forelimb, which links the core tendon of the biceps brachii muscle to the core tendon of the extensor carpi radialis muscle (see Page 42 of the May/June 2020 issue of AFJ), in the hind limb there is an accessorius band that links the lower or third head of the femoral biceps to the Achilles tendon and thus to the calcaneum (Figures 3, 4, 6). Osborn4 calls it the “tendo accessorius;” Sisson and Grossman2 recognize it as the “tarsal tendon” of the femoral biceps; Goody5 visualizes “accessory (tarsal) tendons from the hamstrings [that extend the hock]” (see especially his Plate 20).

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Left: Lateral view of the left hind limb of Palaeosyops leidyi, a titanothere (a primitive relative of the horse) from the Oligocene period, about 25 million years ago. This is a reconstruction made in 19291 by Henry Fairfield Osborn, a vertebrate paleontologist who took his anatomical training in the 19th century. I have preserved his antique anatomical terminology (see especially “peroneus longus,” which we would now call “peroneus tertius” and “plantaris perforatus,” which in equines is the SDF). Osborn was certainly aware that separating the upper and lower reciprocating sub-systems is artificial — a mere artifact of the dissecting room. Right: The same view of the living Rhinoceros unicornis, with modern terminology2 (in an animal that retains multiple toes, peroneus tertius manifests as fibularis longus and fibularis tertius). In non-equines, both peroneus tertius and SDF retain contractile bellies and neither is modified into yellow bands.

The “hamstring” muscles are the semitendinosus and the lower head of the femoral biceps that insert on the lateral side of the stifle joint (as opposed to the semimembranosus, which inserts on the medial side, see Figure 2b). The “accessory” linkage lowers the effective point of insertion especially of the femoral biceps and guarantees that its contractions extend the hock. This anatomy is common to all members of the Order Perissodactyla, including tapirs and rhinos and probably also occurred even in Perissodactyl species going back millions of years4 (Figure 6).

The femoral biceps is misnamed in the horse because it has three heads rather than two (in Latin, “bi” means two, “ceps” means head). Its functioning is interesting because the upper head, attached from the posterior surface of the femur to the lateral side of the patella, acts to extend the stifle joint. The middle part, which inserts a little lower down upon the lateral part of the patella and tibia, will flex rather than extend the stifle, especially if it contracts simultaneously with the semitendinosus (see Figure 6a). Because the lower head is attached via the accessory band to the tuber calcis, its action extends the hock. But as Sisson and Grossman remind us, in the horse, “extension of the hock joint can occur only when the stifle is extended and vice versa.”2 This implies complex neurology that permits the upper and lower heads of the femoral biceps to contract together while the middle head rests and vice versa.

The stifle and hock are not the only joints of the hind limb that open and close in coordination, for passive elastic bands also invest the fetlock and coffin joints and cause them to move with the joints above (Figures 3, 4). In the horse, the SDF is a yellow band lacking a contractile belly. It originates in a deep pocket (the supracondyloid fossa) on the lateral side of the caudal aspect of the femur, a pocket that (like the pocket for origin of the peroneus tertius band) is much deeper in horses than in other mammals, even in rhinos, tapirs, brontotheres, or other ancient perissodactyls (Figure 5).

The SDF is intimately attached to the lateral part of the gastrocnemius muscle so that its large, fleshy contractile belly can pull on the SDF band and thus regulate its tension. About two-thirds of the way down the tibia, the SDF tendon wraps around the gastrocnemius so that at the point of the hock, the SDF is on top (Figure 2b). There it widens out to form a cap over the tuber calcis, which is anchored down on either side by strong, short ligaments.

The hind limb SDF, like that of the forelimb, inserts upon eminences on either side of the upper end of the short pastern and upon the center of the lower end of the long pastern, tucked between the collateral ligaments of the pastern joint. Sisson and Grossman note, “on account of the exceedingly small amount of muscular tissue, the action [of the SDF] is to be regarded chiefly as a mechanical effect which results from the action of other muscles on the stifle joint.”2

Such an effect would not be possible, however, if the SDF were rigidly fixed in place; it must be able to slide up and down. (In the September/October 2021 issue of AFJ, we studied the tarsal sheath that carries the deep digital flexor tendon (DDFT) over the hock joint, likewise permitting it to slide up and down). There is a large synovial bursa interposed between SDF and the tendon of the gastrocnemius that passes down to the middle of the cannon bone (Figure 2). This tough sac, filled with synovial fluid, helps the SDF glide through the whole range of flexion and extension so that when the stifle causes the hock to flex, flexion of the hock, in turn, causes flexion of the fetlock and coffin joints (and vice versa). Likewise, the DDFT inserts upon the coffin bone and acts to flex the coffin joint when the hock is flexed.

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The phases of a jump provide an excellent opportunity to study the full range of hind limb motion in a normal horse. See text and Tables 1 and 2 for details.

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Who can flex their knee (stifle) while stretching out their hock? A, It’s perfectly normal for dogs (A) or elephants (B) to adopt this posture, but when it’s seen in a horse (C), it is the diagnostic clinical sign of rupture of the peroneus tertius yellow band. In this and all the movement studies that follow, blue dots and lines summarize the bones and joints of the hind limb, while the red line summarizes the position of the lumbar spine.

Hind limb Reciprocation Models

In this article, I present a very straightforward model of the functioning of the hind limb reciprocating apparatus (Figures 3, 4). Since, however, no body system can be understood out of context — as if it functioned without influencing or being influenced by other body parts — I present the stifle-hock “classic” reciprocating system as merely the lower sub-system of a larger biomechanical linkage that coordinates movement of the stifle and hock below with movement of the hip and sacro-lumbar joints above. Neither the stifle nor even the hip joint is the uppermost moveable joint of the equine hind limb. That distinction belongs to the lumbo-sacral joint, a subject we will cover in much more detail in our next installment. By the laws of physics, movements made at the uppermost joint in any limb govern movements made at all the more distal joints.

That the “reciprocating apparatus” is part of a larger system has usually been ignored since the end of the 19th century — in other words, since nearly every person knew what a live, moving horse felt like under saddle. No normal horse ever takes a single step with its hind legs without also moving the joints of its back, but this is not so obvious to those whose knowledge of equine biomechanics comes exclusively from carcasses and whose “feel” for movement comes primarily from riding in an automobile.

Another distortion that can easily slip into the student’s mind from working with dead horses is that movements of the hock cause movements of the stifle. This wrong idea insinuates itself because it is much easier for the instructor to move the carcass limb from the lower end, while it is more difficult to realistically replicate the opening and closing of the stifle from points above the stifle, especially if the specimen happens to be rather stiff, either from rigor mortis or because it has been pickled in salt or formalin.

In the living animal, it is contractions of the big stifle flexors (lower biceps, semitendinosus, semimembranosus) and extensors (upper biceps, rectus femoris and the three vasti) that open or close the stifle. Thanks to the fact that the stifle is linked to the hock by means of the yellow bands above mentioned, movements of the stifle cause movements of the hock and never the other way around.

This, of course, begs an obvious question: if movements of the stifle (the joint above) cause or govern movements of the hock, fetlock and coffin joints (those below) — what governs movements of the stifle? The answer is “movements of the joints above it,” the important implications of which we will explore in our next installment. Meanwhile, I encourage the study of the model of the stifle-hock sub-system (Figures 3 and 4), along with the self-quiz provided.


Range of Motion Studies

Since the object of learning equine anatomy is to apply it to living and moving horses, it is of interest to survey some of the movements that normal horses can make, as well as movements that injured horses make. Here I provide numerous views of normal postures and movements that demonstrate stifle-hock reciprocation because what is normal must be understood first (Figures 7, 8a, 8b, 9a, 11, 12, 13, 14, 15). Five of these (Figure 7) are sequential stages in jumping an obstacle. They provide a good example of the normal range of motion of all the joints of the hind limb, while the others represent individual moments in time. Measurements of joint angles both normal and abnormal are outlined in Table 1 and summarized in Table 2. From the study of the tables, we learn the following:

  1. The range from tightest flexion to widest-open extension is 68 degrees at the stifle, but much greater at the hock (115 degrees). The hock thus goes through 47 more degrees of movement than the stifle. Although reciprocation forces the stifle and hock to open and close simultaneously, the range through which the individual joints travel is not necessarily equal. This is to be expected because a horse is not a machine made from metal; the yellow bands and muscles that tie stifle and hock motion together are elastic. Thus, at different phases in the jump sequence, for example, the stifle can be wider open than the hock or vice-versa.
  2.  Another way to think about the same thing is to identify what happens at the stifle vs. hock joint in different actions of the hindlimb. In all actions which primarily involve flexion (setting for the jump for example), the angle formed at the stifle is always equal to or wider than that formed at the hock (highlighted yellow in tables). This implies greater potential range of motion at the stifle joint, shown by the fact that the average stifle movement during flexion actions is 100 degrees, larger by 17 degrees than the average hock movement during flexion actions. Conversely, in all actions which primarily involve extension (springing up for the jump, for example; highlighted pink in tables), the hock joint is always open wider. This implies that it can extend farther than can the stifle joint; this is shown by the fact that the average stifle movement during extension actions is 101 degrees, which is smaller by 43 degrees than hock movement during extension actions.
  3. Abnormal or pathological movement tends to produce measurements outside the normal range. First, the limb of an abnormally moving horse characteristically exhibits reversal of the stifle-hock relationship, i.e., in a flexion action, for example, when the stifle normally would be open wider, it will be open less wide than the hock, or vice versa. Range of motion is diminished at all four joints (most diminished at the hip, least at the hock), while the difference between stifle and hock is greatly increased (from 44 degrees in normal extension or 35 degrees in normal flexion to 70 degrees in abnormal action). This again implies a de-coupling of stifle and hock movement that is normally more closely linked.

The table and illustrations also reinforce the fact that it is a mistake to leave the hip and especially the lumbo-sacral joint out of consideration because the average movement during either flexion or extension of the hind limb is greater at the lumbosacral (L-S) joint than at any other hind limb joint. The L-S joint is hugely important in equine movement and this has important implications for hoof growth, hoof wear, performance and figuring out optimum regimens for farriery.

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Analysis of hind limb function in extended movements. A: Passage. B: Extended trot. Most riders do not think of passage as an extended movement, but analysis proves it to be so. When extension of stride is correctly executed, the muscles and yellow bands of the reciprocating apparatus are not stressed; as in correct collection, there is harmonious expression of all hind joints from the L-S down to the coffin joint.

Answers to Self-Quiz in Figure 3

1. 1: Lumbo-sacral (or Sacro-lumbar or L-S) joint; 2: Hip (or acromio-femoral) joint; 3: Stifle (or femoro-patellar-tibial, or true knee joint); 4: Hock (or tibiotarsal or true ankle joint); 5: Coffin (or distal interphalangeal joint).
2. A: Accessory tendon; B: Gastrocnemius; C: DDF; D: SDF yellow band, upper part; E: Semitendinosus, lower or primitive part; F: Semitendinosus, upper or “new” part; G: Sacro-sciatic ligament; H: Peroneus tertius yellow band; I: Patellar “ligaments”; J: SDF yellow band, lower part; K: DDF tendon of insertion; L, Suspensory ligament; M: SDF insertions on long pastern; N: DDF insertions on coffin bone; O: Longissimus dorsi muscle; P: Ilio-Psoas complex of muscles. 
3. SDF, Peroneus tertius, and Accessory “ligament”. The core tendons of Semitendinosus, TFL, Tibialis anterior and Gastrocnemius also function as yellow bands. 
4. and 5. Patella moves down when the stifle joint flexes, up when it extends.
6. The TFL is crucial in stifle unlocking and in preventing stifle locking during locomotion.