In this blog, I will describe the horse’s chest and shoulder region focusing on how they differ from our own and the implications this has for locomotion and performance.
Locomotion is defined for humans.
People are bipedal – we walk on two legs with our bodies in an upright posture. During the transition to bipedal locomotion, the forelimbs evolved into arms and hands with opposable thumbs enabling us to perform tasks that require dexterity and fine motor coordination. We don’t use our arms and hands for weight-bearing during locomotion though if our arms are not busy doing something else, we can swing them in a diagonal pattern with our leg movements as a way to control body rotation.
The Human Collar Bone.
The shoulder region also underwent some significant changes to accommodate bipedalism. Our ribcage is narrow from front to back but wide from side to side. The triangular shoulder blades (scapulae) lie flat on the back of the chest with the shoulder joints forming the widest point at the top of the chest.
The collar bone (clavicle) crosses the front of the chest above the first rib. You can easily feel it as a bony ridge that connects the sternum to the scapula close to your shoulder joint.
Figure 1: Human chest viewed from in front with the collar bone shown in blue.
The impact your clavicle could endure when falling off a horse.
One of the functions of the clavicle is to transfer forces from the arm to the ribcage and the rest of thebody. If you fall with your arm outstretched to break the fall, the impact of landing is transmittedthrough your arm to the clavicle which is a weak link. Most riders know someone who has broken theirclavicle in a horse–related fall, often in the manner shown in Figure 2.
Figure 2: Falling onto an outstretched arm will likely fracture the collar bone.
Let’s define what locomotion looks like in horses!
Horses are quadrupeds, meaning they use all four limbs for locomotion. The hind limbs are the motor that provides most of the propulsion. The forelimbs are responsible for braking and turning, which will be discussed in a future blog.
The horse’s ribcage is flattened from side–to–side and the chest is deep but relatively narrow. The scapulae lie on each side of the chest and the shoulder joint is close to the point of the shoulder at the front of the chest. The humerus connects the shoulder and elbow joints (Figure 3).
Figure 3: The horse has a deep chest with the scapula and humerus
on the side of
the chest at the front of the ribcage. The red line indicates the angle of the scapula.
The anatomical difference between horses and people is defined.
A major anatomical difference between horses and people is that horses do not have a clavicle. This means no bones connect the forelimb to the ribcage and the rest of the body. Instead, the horse’s forelimb is attached by strong muscles, tendons and ligaments. The lack of a clavicle allows the horse’s scapulae to slide and rotate on the side of the chest.
Let’s test out how the equine shoulders affect performance!
Try this: Pick up one of your horse’s forelimbs and lift it forward as high as possible then pull it back as far as possible underneath the horse’s body. Watch how the angle of the scapula changes. If two people are available, one person should move the limb while the other puts their hand on the skin over the scapula to feel how it moves beneath the skin.The entire scapula rotates around a point just above the middle of the bone. When the top part of the scapula rotates back and down, the lower part rotates forward and up (Figure 4).
The foal in Figure 4shows the scapula maximally rotated and illustrate show this elevates the shoulder joint and allows theelbow joint to be pulled forward so the lower limb can swing more freely.