WHIST THE FORELIMB is mainly concerned with support, the hind limb is for propulsion. It is the structure that drives the horse forwards. Straightened out, the hind limb of the horse is functionally longer than the forelimb because it is the ‘powerhouse’ that drives the horse forwards. Also the hind hooves are a different shape to those of the front limbs. Whilst the front hooves are wider and rounder to help support the weight of the forehand, the hind hooves are longer and more pointed to enable the toes to penetrate the ground giving the horse a firm base against which to propel the body forwards. In this chapter we will illustrate and describe the major muscles of the hind limb and the structures that enable the horse to sleep standing up and rest a hind limb whilst so doing. These are the reciprocal apparatus and the stifle locking mechanism.
ORIGIN 3rd metacarpal, calcaneus and 4th tarsal bone
INSERTION Proximal sesamoid bones
INNERVATION Tibial nerve
FUNCTION Counteracts overextension of fetlock
DEVELOPMENT ISSUES Usually at origin in horses worked on deep rubber surfaces
In the hind limb, the structures below the fetlock have the same configuration as the forelimb including the sesamoidean ligaments (see Chapter 2).
Like the forelimb, the hind limb also contains suspensory apparatus using the same structures as in the forelimb. However, the hind limb also contains two other specialised energy-saving systems which fix the hind limb to enable the horse to sleep standing up: the stifle locking mechanism and reciprocal apparatus.
Stifle locking mechanism
Figure 3.25 The stifle locking mechanism, left hind dorsal view.
The stifle locking mechanism (Figure 3.25) enables the horse to lock the stifle out completely, preventing any flexion during muscle relaxation on rest. The group of muscles that perform this action comprise the quadriceps femoris and tensor fasciae latae, which have insertions on the intermediate patellar ligament. When contracted, with the limb weight bearing, they pull the patella, the parapatellar cartilage and the medial patellar ligament inwards and upwards and ‘hook’ them over the medial trochlear ridge of the femur. This prevents the stifle joint from flexing. The horse will rest the other hind limb by slightly flexing the joints and resting on the toe. To return the stifle to normal movement, the horse contracts the quadriceps and lifts the patella off the trochlear ridge. You can observe this in the resting, standing horse when he is resting one hind limb, and locking the stifle of the other hind limb. However, some muscular effort must be required for this process as the horse will tire of standing on one hind limb after a few minutes and will change to lock the opposite hind limb whilst resting the one that had previously been locked.
Sometimes (particularly in fast-growing warmblood breeds) this can happen during movement. This leads to the condition known as ‘locking stifle’, which is also known as upward patellar fixation, in which the hind limb locks out during normal movement, without conscious effort by the horse. Usually, by making the horse walk backwards, the stifle will unlock, but it can be a distressing condition for horse and owner. Normally, as the horse gets stronger, the syndrome will resolve completely without any intervention, but in some cases surgery may be indicated. Locking stifle is a condition that very often responds to skilled physiotherapy treatment.
Figure 3.26 The reciprocal apparatus
In the absence of trauma, the horse cannot flex or extend the stifle without flexing or extending the hock – the two are inextricably linked by a system known as the reciprocal apparatus (Figure 3.26). This comprises two muscles, the superficial digital flexor muscle (see page 52), and the peroneus tertius muscle (see page 54). Because of the action of these two muscles the movement of the two joints mirror each other. However, neither of these structures are strictly muscles. The peroneus tertius is entirely tendinous and the superficial digital flexor has very little muscle tissue within it; it has evolved to become a passive spring. Therefore the stifle and the hock must move in unison. It follows that if the stifle is locked by the locking mechanism then, because of the reciprocal apparatus, the hock is also locked.
Chapters 2 and 3 should have ensured you now have an appreciation of how the limbs of the appendicular skeleton work, and just how fragile they can be, especially below the knee and hock.