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​Equine Insights: Mechanics of the Horse's Neck (Part 2)

​Equine Insights: Mechanics of the Horse's Neck (Part 2)

Posted by Dr. Hilary Clayton on 7th Jun 2024

How the Horse’s Neck Works

My last blog described how the seven cervical vertebrae are anchored together by ligaments, muscles, and tendons. This arrangement helps to support the weight of the head and neck while allowing a small amount of movement at each of the intervertebral joints. However, most of the movements of the neck occur at its base and at the poll with relatively little movement of the joints in the middle part of the neck during everyday activities.

The joints at the base of the neck move the entire neck and head in all directions, from the downward posture during grazing to the elevated posture when watching something in the distance to the lateral movement required to reach around and dislodge a fly from the flank.

The joints close to the poll provide independent movements of the head so that the eyes and vestibular apparatus in the ears can be stabilized, for example when the horse is doing a spin.

The limbs have specific coordination patterns that are different in each gait, and this is associated with an equally characteristic movement pattern of the trunk and the neck due to its attachment to the front of the ribcage. Imagine a horse moving in a round pen with a dense hedge obscuring the limbs and most of the trunk, but the neck and head are visible above the hedge. Observing the oscillations of the neck and head makes it easy to identify the horse’s gait. Think about that for a moment and form a mental image of how the neck and head move in walk, trot and canter.

The base of the neck attaches to the front of the ribcage with the length of the neck cantilevered out in front of the horse’s body where it is acted upon by the forces of gravity and inertia. Gravity is always pulling the neck down while inertia tends to continue moving the neck in its current direction. The following section will describe neck motion in the gaits of walk, trot, and canter/gallop.

The Walk

In the walking horse, the head and neck are carried at or just above a horizontal position (Figure 1). Oscillations above and below this position occur twice per stride, with the head rising in synchrony with the croup but out of synchrony with the withers. The croup and head are high at forelimb contact and low in mid-stance of the forelimb. The range of vertical motion of the poll is about 10 cm.

Figure 1: Walking horse. The left diagram shows the moment in the stride when one forelimb is vertical, which coincides with the elevation of the withers and lowering of the croup and the poll. The right diagram shows the moment when one hind limb is vertical, raising the croup to its highest point. This coincides with the elevation of the poll and the lowering of the withers.

The Trot

The neck is held about 20 cm higher in trot than in walking with small vertical oscillations of only 4 cm in each stride. The head, withers, and croup bounce up and down together as they follow the diagonal rhythm of the steps. The entire body is highest in the suspension phases and lowest in the middle of the diagonal stance phases (Figure 2).

Figure 2: Trotting horse. The head, neck, and trunk rise and fall in synchrony. The left diagram shows the suspension phase when the entire body is highest. The right diagram shows midstance with the diagonal pair of limbs vertical when the entire body is lowest.

The Canter and Gallop

Excursions of the neck and head are largest in canter and gallop with a range of vertical motion of about 15 cm. Unlike the walk and trot which have two oscillations per stride, the canter has a single oscillation per stride. The croup is lowered as the hind limbs swing forward, then the withers are lowered as the horse’s weight passes over the diagonal limb pair and onto the leading forelimb. The horse rolls forward from the croup over the withers to the poll.

Figure 3: Cantering horse. As the horse moves forward, its body rotates. On the left, the croup is low relative to the withers as the hind limbs swing forward and the horse is in an uphill balance. In the center, the croup and withers are at similar heights, and the horse is in a level balance. On the right, the withers are relatively low as the weight passes over the forelimbs, and the horse is in a more downhill balance.

In all gaits, the effects of gravity and inertia cause the cantilevered neck and head to nod down slightly at a certain point in the stride, which the rider may feel as an increase in rein tension. The downward motion of the head is arrested partly by muscles and partly by the nuchal ligament that acts like a bungee cord.

The nuchal ligament runs from the top of the withers where it detaches from the spinous processes and continues forward up the neck. There are separate left and right nuchal ligaments, each consisting of funicular and lamellar parts. The funicular part is an elastic cord that runs from the withers to the back of the skull. The lamellar part consists of sheets of elastic tissue that descend from the funicular part to the spines of the cervical vertebrae (Figure 4). The lamellae to the last two cervical vertebrae are not well developed.

Figure 4. Diagram of the nuchal ligament on the right side of the horse’s neck. Th green cord represents the funicular part arising from the spinous processes at the highest point of the withers and inserted on the back of the skull. The dark blue bands represent the lamellae that descend from the funicular part of the ligament to the cervical vertebrae with strong connections from C2 to C5.

The nuchal ligament is loaded in tension when the head and neck descend during locomotion. Due to its elasticity, the ligament lengthens, and, at the same time, tension in the ligament resists and slows the downward motion of the neck. After the poll reaches its lowest point, the stretched nuchal ligament recoils and helps to raise the neck. The beauty of this mechanism is that the stretch and recoil cycle in the nuchal ligament occurs passively and does not require energy expenditure by the horse.

The contribution of the nuchal ligament should not be underestimated. Its elastic recoil provides 55% of the energy needed to raise the neck in each stride of walk, 33% in trot, and 31% in canter. This amounts to a considerable energy saving!

Next time, we’ll look into the nuchal ligament in more detail.