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 me 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, with the goal of understanding 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 will generally support themselves on both front limbs and one hind limb, alternating the supporting hind limb. We have long believed that they are able to maintain this standing position through the action of the stay apparatus, which passively stabilises 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 is a series of muscles, ligaments and tendons in the equine thoracic and pelvic limb, that stabilize the limb in a standing position. Let’s look at each of these structures.

In the thoracic limb, the suspensory ligament, working together with the collateral sesamoid ligaments, palmer sesamoid ligaments and distal sesamoidean ligaments, form the suspensory apparatus. These ligamentous structures resist extension of the fetlock, pastern and coffin joins with no muscular effort.

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 two muscles; 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.

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 comprises 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 locking mechanism is activated when the patella is shifted behind the top of the prominent medial ridge of the femoral trochlea, causing flexion of the stifle to be blocked by the medial, intermediate and lateral patellar ligaments. 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, there is the peroneous tertius, and on the caudal aspect, there is 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 are 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 submit 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 Lowri Davies expresses this so well in all of her 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 with which we familiarise ourselves, the better our outcomes will be. Understanding how the locking mechanism of the stifle works – 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