BASIC EQUINE MUSCLE ANATOMY

If you know the basic muscles of your horse’s back and shoulders you can assess if your saddle could be making your horse’s back sore.  If you know the basic muscles of your horse’s back and shoulders you can assess if your saddle could be making your horse’s back sore.

Anatomy can be a little bit overwhelming so I find it’s easier to learn the basics first and then build up from there.  Once you have the basic building blocks you can add more layers of complexity.

ANATOMICAL LOCATION OF KEY MUSCLES

muscle-anatomy-crop1.  TRAPEZIUS MUSCLE

Seen on the outside of the horse as the wither and base of the neck the trapezius muscle is actually divided into two parts – the cervical and the thoracic part.  The trapezius muscle action is to draw the shoulder (scapula) forwards and backwards.  The trapezius is also responsible for the elevation of the shoulder.

The trapezius muscle originates from the nuchal ligament and supraspinous ligament of the second cervical vertebrae and the tenth thoracic vertebrae.

This region of the horse is also the junction point for several major muscle groups of the back and the neck.  The upper (dorsal) branch of the accessory nerve (Cranial Nerve 11) is involved in the innervation (switching on) of the trapezius muscle.

Why is this muscle important?

There is a branch of the accessory nerve (Cranial Nerve 11)  that has a negative impact on the horse’s movement if it is pinched or compressed. This is because horses have one weak spot.  Stay with me as I try and explain.

You may have seen how a lion attacks a zebra in the wild.  The lion will jump onto the zebras back and bite at the base of the neck just in front of the wither.  The action of pressure on the location of the Cranial Nerve stops the innervation (movement) of the trapezius muscle which in turn stops the zebra from fleeing.  It is the weak spot in the defence of the zebra and our horses.

What does this mean for saddle fit you might ask?  If your saddle tree pinches your horse’s shoulders and (Cranial Nerve 11) your horse may not be able to stride out or extend.  They may not stretch into jumps,  jump flat, refuse or evade jumping altogether.  This is sometimes seen as the horse misbehaving or being naughty when all the horse is trying to do is avoid pain.

If you place your saddle on the horse’s shoulder (not behind it) when you saddle up then you are already causing discomfort and pain.  If you see horses with a dip in front of their saddle this can indicate trapezius muscle wasting caused by improper use of the muscle. The dip in front of the wither is caused by the trapezius muscle shrinking and losing strength which in turn affects the horse’s ability to move its shoulder.

muscle-anatomy-crop2.  LATISSIMUS DORSI MUSCLE

The latissimus dorsi muscle is a triangular shaped muscle located just behind the shoulder (scapula).  Its action is to help the horse flex the shoulder joint and move the upper leg (humerus).

The latissimus dorsi attaches into the horses long back muscle fascia (thoracolumbar fascia) and inserts onto the upper part of the front leg (humerus, teres minor tuberosity).

This muscle is innervated (switched on) by the thoracodorsal nerve which runs along the top part of the horses back.

Why is this muscle important?

Imagine a tree that is too tight pinching the horse’s shoulders.  A horse with a tense back is no fun to ride.  What can make their back tense and why do they tense up?  One reason could be your saddle.  If the back has limited elasticity because the horse is trying to cope with a badly fitting saddle it will have a direct effect on how the horse uses its front legs. Its stride will become shorter and it will display what is sometimes called “sewing machine action”.  The front leg is thrown forward from the elbow and the movement becomes stiff and fast.  The horse won’t be able to use it’s back correctly and won’t be able to track up and it’s hindquarters may trail out behind rather than engaging.  This then places more stress on the horse’s muscles as it tries to compensate.  Muscle soreness and injury and ligament damage can occur.

muscle-anatomy-crop3.  TRICEPS MUSCLE

The triceps muscle is part of your horse’s shoulder and is seen as the muscle that forms part of the front leg and is directly in front of the girth.  There are actually several parts of the triceps muscle but we will be looking at them as one group.  The action of the triceps muscle is to extend the elbow joint and flex the shoulder joint.

The triceps muscle attaches on the scapular (shoulder) and also the ulnar (front leg).  It is a fan-like muscle with it’s broadest attachment being on the scapular and a much smaller concentrated attachment on the ulnar.

The triceps is innervated by the radial nerve.

Why is this muscle important?

If the triceps muscle is unable to move smoothly because of tension in the back making the horse move quickly with short strides then you can see that the interlinking relationship of the muscles means they can become sore. Tripping and stumbling can also be indicative of issues with saddle fit and cause muscle tightness and soreness.

With any muscle group pain and pressure can create a negative response which is muscle wasting.  It takes at least 12 months to develop the top line muscles on a horse but it takes a fraction of that for the muscles to disappear or waste.

muscle-anatomy-crop4.  PECTORALIS MUSCLE

The “deep” pectoralis muscle runs from the chest between your horses front legs and under their rib cage. There are also other parts to this muscle known as Superficial, transverse (side to side) and descendants (lower) you can see these when looking at your horse from the front they are commonly referred to as chest muscles.  The action of the deep pectoralis muscle is to support the trunk or abdomen of the horse and assists in stabilizing the shoulder joint.

The deep pectoralis muscle attaches from the horse’s sternum (between the horses front legs) and the thick layer of fascia known as the yellow abdominal tunic! The other attachment is the front leg or top of the humerus (front leg).

This muscle is innervated (switched on) by the cranial and caudal pectoral nerves.

Why is this muscle important?

This is really important to know as your girth can affect how this muscle functions.  It’s really important that the girth buckles never lie on the edge of this muscle. If the girth is too short it will sit just behind the elbow of the horse and can lead to pinching and pain. The pinching of the girth means the horse may not want to extend their front leg as it causes discomfort.  The girth should ideally have double elastic on all the buckles to allow for the chest to expand and contract and be wide enough to be comfortable.  Some girths look lovely and soft but don’t distribute the pressure on the horse’s chest over a wide surface area due to the way they are made.

A note on the girth.  If the girth is tightened severely this will affect the comfort of the horse.  While it will not affect breathing it will have a negative impact on comfort and the ability for the muscles to move.  The girth should be tightened equally on both sides and be well clear of the horse’s elbows (if a short dressage girth) to ensure comfort.

A well fitted and balanced saddle doesn’t need the girth cinched up tightly.