Equine Stay Apparatus: Active or Passive, and Why it Matters

For a long time, I have wanted to write about the stay apparatus, but it was not until recently that I finally found the inspiration to jump into this topic.

I received an email, asking my thoughts on disproving the theory of the stay apparatus.

The theory? I had not considered that the equine stay apparatus was a theory – it had always seemed ‘just the way it is’. But I love the idea of re-examining accepted wisdom when it comes to how we think and practice as Vetrehabbers. By re-looking at accepted theories, we may gain new insights and develop a deeper understanding of what we’re dealing with.

So let’s do that with the equine stay apparatus; let’s dive in, to understand deeply and more clearly what we see and think we know.

WHY a stay apparatus in the first place?

Always start with the why. Why would a horse need a stay apparatus; what function would it serve, and how would it benefit the horse? 

In a standing animal, two forces are acting on limbs simultaneously – the weight of the body acting downwards from the shoulder and the hip, and the ground reaction force acting upwards from the hoof. To prevent the limb from flexing under the pressure of these forces, the extensor muscles of the joints need to remain active.

The bigger an animal, the greater the muscle force required to maintain that body posture, putting large animals like horses at a distinct disadvantage (Schmidt-Nielsen, 1984). To compensate for this disadvantage, larger animals will have proportionally larger muscles and will have a more upright limb posture, vs the more angulated limbs of smaller animals (Alexander, 1985).

Horses spend an average of 80% of their time in a standing position, which includes light sleep but not deep sleep (Dallaire, 1986; Boyd et al., 1988). Compare this to dogs, which spend an average of 50% of their day asleep, 30% resting, and only 20% of the day in activity.

In a relaxed standing position, horses can support themselves on both front limbs and one hind limb, alternating the supporting hind limb. We have long believed that they can maintain this standing position through the action of the stay apparatus, which passively stabilizes the limbs and allows the maintenance of posture with minimal energy consumption.

From these two points alone – the increased weight and force carried by the horse, and the increased time spent in an upright position – it seems likely that horses would have developed a mechanism whereby they exert the least possible amount of energy and avoid fatigue while standing.

WHAT is the stay apparatus?

The stay apparatus consists of a series of muscles, ligaments, and tendons in the equine thoracic and pelvic limbs, that stabilize the limb in a standing or weight bearing position. Ligaments and tendons are resistant to fatigue and can lock or hold joints into a weightbearing position. Multiple muscles form part of the stay apparatus, and will either have a tendonous band running through them, or will have a higher percentage of fibrous collagen within them to reduce muscle fatigue.

Let’s look at each of these structures.

Suspensory apparatus

There are 3 parts to the suspensory apparatus:

1. The interosseus
2. Proximal sesamoid bones
3. Sesamoidean ligaments

The fetlock joint together with it’s sesamoid bones and their associated ligaments form the first critical part of the stay apparatus, preventing the fetlock joint from overextending and sinking. These structures are repeated in the fore and hind limbs.

Proximally, the interosseus provides the primary support. It is further supported by the Superficial and Deep Digital Flexor tendons, that attach distally to the phalanges.

The sesamoidean ligaments include a proximal metacarposesamoidean ligament, collateral sesamoidean ligaments, an intersesamoidean ligament, and distally the oblique, straight, short and cruciate sesamoidean ligaments. These ligamentous structures resist extension of the fetlock with no muscular effort.

The metacarpophalangeal and each of the phalangeal joints have collateral ligaments that prevent medio-lateral motion, as well as palmar and impar ligaments that stabilise the individual joints.

The SDFT and DDFT are essential in preventing over-extension of these distal joints, and are balanced by the common digital extensor and common lateral extensor on the dorsal aspect.

The check ligaments of the SDFT and DDFT provide passive tension on these tendons, without the activity of the associated muscles.

Carpus, Elbow and Shoulder

The connection between the biceps, lacertus fibrosis and extensor carpi radialis supports extension of the carpus, elbow and shoulder joint, forming an essential component of the stay apparatus. The biceps muscle has a high fibrous or tendonous component, allowing it to store energy and maintain tension with little fatigue of the muscle. The biceps tendon crosses the cranial surface of the shoulder joint through the intertubercular groove, capping the intermediate tubercle. This connection suggests a locking effect, and is the primary extensor of the shoulder joint. The internal biceps tendon anchors the biceps to the radius, providing extensor tone to both the elbow and shoulder, and further to the carpus through the lacertus fibrosis and extensor carpi radialis.

The shoulder is medially supported by the subscapularis, and laterally by the infraspinatus.

The serratus ventralis, connecting the scapula to the trunk, contains a tendinous layer that suspends the trunk when muscles are relaxed, but acts to increase shoulder flexion. This is counteracted by the biceps brachii and the lacertus fibrosis. 

The biceps brachii has a collagenous tendon that extends through the length of the muscle and divides near the elbow to form the muscle insertion.

The lacertus fibrosis connects to the extensor carpi radialis and the fascia of the forearm. This allows the biceps muscle to relax while maintaining extension of the shoulder, elbow and carpus. 

To counteract extension of the carpus and fetlock without muscle activity, the superficial digital flexor muscle extends into a thick tendon below the carpus, with the superior check ligament attaching here to reduce the muscle activity required. The deep digital flexor muscle also extends into a tendon from the carpus, with the inferior check ligament joining the tendon and reducing the muscle activity required in a standing position. This aids in preventing extension of the fetlock, pastern and coffin joints.

Pelvic Limb

In the pelvic limb, with much greater angulation than the thoracic limb, a different mechanism is employed to allow the limb to remain in a weight bearing position. The pelvic limb stay apparatus includes patellar locking, the reciprocal mechanism and the check apparatus.

The patellar locking mechanism is the key to the success of the stay apparatus in the hindlimb. With the stifle stabilised, the hock joint is also stabilised through the action of the reciprocal apparatus, which couples the movement of the two joints. The distal joints are stabilised passively through the tendons and ligaments, as in the forelimb.

The patellar locks when the patella is lifted over the top of the prominent ridge of the medial trochlea of the femur. Once locked, the stifle is unable to flex. 

The vastus medialis muscle inserts directly onto the parapatellar fibrocartilage, and is essential to maintaining the locked position with tonic, low-level activation. The level of activation found in the vastus medialis is estimated to be less than 2% of the force that would be required in the absence of a locking mechanism (Schuurman et al., 2003).

The reciprocal apparatus is made up of largely tendinous muscles on either side of the tibia: on the cranial aspect, the peroneus tertius, and on the caudal aspect, the superficial digital flexor tendon.  These two tendinous muscles ensure that the stifle and hock move as a unit, ensuring that when the stifle is locked, the hock is also immobile, without any additional muscle activation.

Active or passive?

Looking at the structures involved and their functions, it is clear that the stay apparatus is both active and passive. The mechanisms involved in maintaining the horse in a standing position are also integral to locomotion. Muscles, tendons and ligaments are involved, and so is the fascia of the body. The biceps brachii, lacertus fibrosis and extensor carpi radialis all connect with the fascia of the forearm. Similarly, the patella tendons form the attachment directly to the four portions of the quadriceps muscle, as well as indirectly through the fascia to the sartorius, gracilis, tensor fascial latae and biceps femoris.

As with many things related to the body, I certainly don’t think we have a complete picture of what exactly is involved in these structures, either in a normal biomechanical model or a pathological model. I encourage you to share your understanding of these structures and how they operate – what is your perspective and experience?

I also think that while this is a great example of the interconnection of the body – different systems impacting each other and working together to achieve one goal – these structures are still working within the body, the interconnections of which we are only beginning to understand.

Dr Elizabeth Ulh and Michelle Osborn expresses this so well in all of their lectures – the body resembles a tensegrity model, where all parts are connected and impact all other parts. The body does not move or stabilise robotically like a machine with gears, but smoothly, gracefully, one movement blending into the next, no beginning or end. There is a seamless continuation of movement and flow. And so I would argue that the stay apparatus is both active and passive. 

Why is this important?

Because questioning what we believe to be true is essential! Regularly question, test and try to disprove the very foundation on which you build your practice and clinical reasoning. This is how we deepen and broaden our understanding, our effectiveness and our skill.

 The better we understand the body, the more perspectives we deeply understand, and the more research we familiarise ourselves with, the better our outcomes will be. Understanding how the locking mechanism of the stifle works, for example – the tone present in the vastus lateralis while the patella is locked – will help us clinically reason and effectively treat patella fixations, abnormal movement of the stifle, weakness and more. 

References

1. WikiVet: https://en.wikivet.net/Stay_Apparatus_-_Horse_Anatomy
2. Schuurman, SO, Kersten, W and Weijs, WA. The equine hind limb is actively stabilized during standing.
3. Schmidt-Nielsen, KS. 1984. Scaling Why Is Animal Size So Important? Cambridge, MA: Cambridge University Press.
4. McNeill, AR. 1985. Body support, scaling and allometry. In: M Hildebrand, DM Bramble, KF Liem, DW Wake (editors). Functional Vertebrate Morphology. Cambridge, MA: Harvard University Press.
5. Boyd, LE, Carbonaro, DA, Houpt, KA. 1988. The 24-hour time budget of Przewalski horses. Applied Animal Behavioural Science.  21:5–17. 10.1046/j.1469-7580.2003.00166.x. 
6. Dallaire A. 1986. Sleep as behavior. Equine Practice. 2:591–607. 10.1046/j.1469-7580.2003.00166.x