The horse was never designed to carry a rider. The fact that we can ride them, aside from their trainable nature, is due to their structure and the way they move.
Compared to other 4 legged animals horses have a rigid spine making it easier for man to sit on.
The flexible spine of the whippet enables the hind legs to come further under the body to generate maximum speed. In comparison the horse's spine is inflexible - the horse is not as naturally built for speed or jumping .
To put it in simple terms imagine a horse like a rear wheel drive car. The engine is the hind quarters, the horse's back is the driveshaft and the front end is the steering column. If the driveshaft is impaired in anyway the power produced from the engine won't be transferred efficiently and performance will be poor.
The same principle applies to the horse. A badly fitting saddle will create tension in the back muscles and prevent them from working efficiently. This means the power produced by the hind quarters won't be transferred as a direct line of force, the hind limbs will not be able to engage and the horse will be unable to lengthen into a natural outline and develop a correct musculature. This can result in any or multiple effects mentioned in the first paragraph.
A badly fitting saddle may also have secondary impacts as the horse compensates for not being able to use his back muscles effectively. An assessment of the musculature of the horse reveals areas of over and under development - the overdeveloped areas are formed because they are compensating for the weakened areas. The extra strain can result in soreness, fore/hind limb lameness or even foot problems.
There is a complex system of muscles in the back. The main muscle under the saddle is the longissimus dorsi and it is the biggest and strongest muscle in the body. It is a long flat muscle attaching to the scapula and running flat along the back to the sacrum. The saddle must clear the spinous processes and contact evenly along the longissimus dorsi.
An ill fitting saddle (e.g. too narrow, too wide, assymetric, a curved tree, inadequate spine clearance) will cause pinching and pressure points which results in tension in the muscles affecting the horse's ability to move freely in all basic gaits.
The thoracic spine starts low down in the thorax and the withers are formed by vertical bone projections, called spinous processes, from the 2nd -10th vertebrae. They are held in place by ligaments and muscles and these give the spine it's strength and inflexibility. It's essential to preserve these structures and encourage muscular development if the horse is to be able to support the additional weight of the rider.
The spine runs the length of the horse's body and is divided into regions - cervical (neck), thoracic (thorax), lumbar (lower back) and coccygeal (tail). The saddle sits on the thoracic region behind the shoulder i.e. supported by the ribs and should not go beyond the last rib/ 18th thoracic vertebrae .
Look at the position of the serratus and pectoral muscles relative to the girth. Over tightening the girth can cause restriction and problems in these muscles.
Also note these are just the direct muscles affected, it's not the whole picture!
It's the saddle fitter's job to assess whether issues in the musculature or movement of the horse are being caused by an ill fitting saddle or if there are underlying health/ injury issues which should be referred to another equine professional - vet, physiotherapist, farrier, trainer etc. Often problems go undiagnosed if no attention is paid to the saddle fit so it's important to get your saddle checked regularly by a professional saddle fitter.
Please read below for more detail on the impact of the saddle on the horse.
going on the forehand, heavy in the hand
hollowing of the back, raised neck and head carriage
not engaging hindquarters
stiff on one rein
behavioural issues caused by discomfort e.g napping, bucking, bolting, rearing
back injuries and lameness
As soon as we get on his back we compromise the horse and it's our responsibility to minimise how much of a compromise the horse has to make. The aim is for the horse under saddle to move as freely and beautifully as he does without the saddle, just like this 2 year old lusitano.
An ill fitting saddle can cause numerous problems e.g.