The two most essential facts to consider choosing a saddle are:

1. The spinal column of the horse is a horizontal stack of vertebrae and shims that at the front hangs suspended in shoulder ligaments and at the rear through the hip joints is firmly supported by the hind legs.
This lying column of loose elements is neither meant nor suited to carry a load and only has a fulcrum at the rear.
Loading the spinal column with a significant weight means overloading the joining surfaces and will damage the construction by compression. To minimise this damage it is essential to distribute the weight over as many ribs as possible to make the connected vertebrae share the load as evenly as possible. It is also important to shift as much of the load to the solid rear support. Hereby last complete rib of the horse defines the far most point the back end of the saddle can be.

Illustration: both a saddle with a seat further back and collection on the hind quarters shifts a load more to the solidly supported end of the spinal column

2. The skin and muscle tissue covering the ribs and spinal column on the back of the horse can only stand a limited pressure.
Pressure is scientifically expressed in Pascal = Newton per meter², which easily calculates to kilogram/cm² or pound/inch²: the weight of the rider divided by the effective load bearing surface of the saddle on the horses' back.
The critical value above which the soft tissue rapidly looses functionality and becomes stressed is surprisingly low: only 109 gram/cm². Please note that this is not the average but the máximum.
A problem is that the surface that can support the saddle is limited by the appropriate space on the ribcage.

The saddle space
To allow sufficient freedom for the shoulders of the horse to move this means, standing at rest, that the space for the saddle is limited at the front to some 5 -7 cm. behind the rearmost edge of the shoulder blade. At the rear the saddle obviously has no supporting rib beyond the rearmost rib.

Photo: saddle space on Arabella

Another limit is the width as it is clear that the vertical side of the ribcage cannot support weight and the spinal column itself should be left clear from any load by a well free channel of about 7 cm. wide. That is 3 average male finger or 4 - female.

The resulting weight bearing surface is very limited and therefore the load needs to be distributed as evenly as possible. The actual pressures must be as close to the average as possible.
To distribute the load, the weight bearing panels of the saddle, the bars, must be stiff and follow the shape of the back as closely as possible.
A nasty problem is that the back changes shape when the horse moves and thus the panels will never follow the shape accurately under all circumstances. Luckily the horseback is relatively stiff; incomparable with a dogs let alone a cats back, so the problem cán be addressed by using a well designed saddle.
This stiffness in the back makes the travelling gaits of the horse a lot more energy effective and this is an evolutionary advantage at the cost of stride length thus top speed at the gallop.

Illustrations: movement of the horseback. Biomechanical Riding and Dressage Web Site

A horse that has been schooled to properly use its back by using the belly-muscles, slightly curves its back upwards from static resting and the panel shape should take this into account. Good padding should be sufficient to equalise the rest.

Padding
Between the weight bearing, load distributing panels and the back a layer of padding is essential. Since ‘soft’ doesn't support weight and ‘thick’ isolates the back, the thickness of the padding has a narrow margin. Depending on the properties of the material(s) used 1½ cm. will be the minimum and more than 2½ leads to other problems.
The padding should be able to damp heat and moist, and be elastic in order to 'absorb' peak shocks and spring back to compensate for small differences between saddle panel shape and the back.
Ideally padding should compress to about 50-60% of its thickness under the calculated average load and it should quickly restore its thickness when not loaded.
Suitable padding materials are pads out of true fleece, quality synthetic fleece, open cell foam, hair felt, synthetic felt…

A saddle that evenly distributes the rider's weight over the saddle space on the back and helps shift the centre of gravity rearwards is a MUST.

The remaining criteria range from WANTS to luxury criteria.
The aesthetics of the model and the comfort of the seat I personally put way back at the bottom of the list as luxury criteria.
True wants are:
- the saddle should not hinder the horse in its freedom of movement needed by the intended use
- the saddle should allow for as much cooling as possible
- the saddle should retain its position and distribution firmly and stable under the intended use

The freedom of movement needed is depending on how gymnastic the horse is and on what is asked of it.
The ideal saddle space leaves the shoulder well free so that should no longer be a point of attention.
A saddle featuring less wide panels will allow for more lateral movement of the back then a saddle with panels extending well downwards to the sides. The effect on the weight bearing surface speaks for itself.
The less time you spend in the saddle the less time you compress the tissue the less critical the load on it.
This allows for a small margin for compromise between the health of the horse and the gymnastic freedom of movement. As I see it between a must and a want...
When trail riding the gymnastic possibilities of the horse are a non-issue and therefore there should be no compromise with an as large as possible weight bearing surface.

Now we can actually apply a bit of rocket science to saddling a horse: the thrusting power of a rocket is expressed as a product of the force pushing it forward and the time it is pushing = Newton x second. This can be calculated to kilogram x hour: the length of time the weight of the rider is pushing on the back of the horse!
The larger the product, the greater the thrusting load on the back. This in itself is evident enough but now we have the possibility to measure it, calculate it, thus compare.

The soft tissue on the back of the horse is worth some extra attention. The saddle rests on the skin and muscle which lie over the ribcage; are squished between the panels loaded by the rider and the ribcage tat keeps it all up.
The difference in fluid pressure between the feeding and returning plumbery is low, only about 140 gram/cm². It will be clear that this soft tissue can easily be squished sufficiently to impair the fluid circulation and thus the function of the tissue cells. The threshold value of 109 gram/cm² now starts to make sense; this leaves only 30 gram/cm² to pump the fluid through. Just a bit more load on the tissue and the pumping stops.
The higher the pressure or the longer the duration the greater the damage done by the disruption of the fluid circulation.
This pumping difference between in and out helps to explain a well-known but rarely understood phenomenon: the slight swelling of the back tissue after saddling off. The capillary system has all but collapsed under the pressure by the saddle and the tubing with the highest pressure reopens the quickest thus leading to fluid build up in the tissue = the swelling. This swelling will go away when the returning system recovers but it should be crystal clear now that the swelling is a definite signal of tissue overload.

Measuring and calculating thrust on the back sheds light on unexpected facts; covering long distances at a slow pace for example is wróng. But obviously any horse used for trail riding should be fit and healthy and thus be well up to long periods of trotting, so we can put this to the side as given fact.
Whether the horse travels at the walk, trot, canter or gallop, whether you rise in the saddle or sit quietly, this is irrelevant for the compressing force on the soft tissue since you remain to be as heavy as you are. The more you move the greater the amplitude around the average but this average equals your weight on the scales.
Let us do some calculating now:

A 60 kilo rider walking his horse 30 kilometres at 5 km. per hour = 360 kilogram hour.
An 80 kilo trotting these same 30 kilometres at 15 km. per our = 160 kilogram hour.
A heavier and faster riding rider is A LOT easier on the back of the horse than the lighter but slower one.
If you want to do it even better, trot the horse some 3/4 of the hour, loosen the girth and walk beside the horse for 10 minutes. A drink and relief, put the girth back up, and continue at the trot.
This way you keep the average speed up and the tissue can recuperate during the ride. If you are even better you use the walking intervals as true intervals and rewards; after a climb at a fast trot or gallop you might as well change the timing and reward the horse for this effort by getting off and give its body two or three minutes to recover before you girth up again and continue to trot.

The cooling of the horse is critical. For radiating heat the horse has an unfavourable relation between surface and volume. Added to this the horse does not have our sophisticated system to slow down dehydration through sweating. Whereas we loose fewer electrolytes the more we sweat the horse keeps losing electrolytes linearly at an unfavourable rate.
To make the disaster complete the skeleton muscles of the horse generate a lot of heat and the largest of those are the back muscles which are isolated by the saddle.
The riders' weight will add about 20% to the load, to the energy the horse needs to generate. Coupled to the poor cooling system it is easy to understand how easily a horse can get dehydrated and/or overheated. Both these conditions are potentially lethal as once overheated or dehydrated the horse is very slow to recover, if at all.
This means we will have to do what we can to avoid nearing the critical zone and the saddle can help in two areas:
- isolating as little of the back as possible and allowing for maximum ventilation
- wicking the sweat away from the skin so it can it can perform its cooling role by extracting evaporation heat instead of isolating the back muscles by acting like the fluid film in a wet suit.

A moving saddle causes friction and causes rubbing spots among other things.
With a well designed tree and good fitting panels a properly constructed girthing system keeps the saddle on the spot without the need of buckling up the girth to much.

A properly constructed girthing system allows for an adjustment of the girth position to the natural girthing spot of a particular horse. Also it keeps both the front and rear of the saddle firmly connected with the horse.
A good example is the so-called McClellan- or V-girth. The lower the V has its point attached to the girth the better. The Y-systems are easier to girth but geometrically less favourable. Still a lot better than an English system or centre rigging though.

I ride in the mountains and thus have to manage a lot of very steep inclines. This asks for special attention. Under normal circumstances, even over mild jumps, a proper fitting and well girthed saddle will not move. Under extreme inclines however the riders' weight will push or pull the saddle forwards or backwards. This will put considerable stress on the girth and thus the belly of the horse. To alleviate this a breast collar and crupper are definitely functional.
Of these two descending is the worst, as the front quarters get to carry the most of the load exactly when the saddle will impair the movement of the shoulders when these optimally would need a bit móre room. Whereas the shape of the ribcage helps limiting the movement of the saddle and thus the breast collar, the shape of the horse does nothing to prevent the saddle moving onto the shoulders, making the proper adjustment of the crupper quite important..

The seat of the saddle is not a WANT but not unimportant, its comfort however is.
The seat should have a centre well behind the middle, ideally at 2/3 and must allow for a stably independent seated rider position with the legs stretched well downwards.
Thy- and leg supports are conflicting with an independent seat; support from the saddle equals leaning on the horse.
The same importance have the stirrups; these should simply allow the rider to maintain his seat. The rider should not need to mind his feet and thus the design must be inherently safe, like the Camargue-type cages.

Personally I prefer a saddle of the McClellan type or a Portugesa. The McClellan offers optimal cooling but at the price of a reduced weight bearing panel surface and you will need to address this.
I like the Malibaud Gardian with leather side flaps to keep my legs cleared from the sweat and put this saddle on a CorrecTOR in a thin felt 'sock'.
This combination puts me as low on the horse as possible with only 4 mm. leather between my legs and the horse, distributes the weight over a sufficiently large surface without impairing the freedom of movement, has a rearward weight bias and features an almost ideal girth construction.
The CorrecTOR and sock feature an open slit running all the way parallel over the spine allowing for optimal ventilation. The felt covering the CorrecTOR and the felt of the sock provide real padding and draw the sweat from the skin.

Photos: Malibaud on CorrecTOR

My love prefers an different compromise. She likes the flexible panels of the Orthoflex design best. The Patriot model meets the criteria well and the panels provide fit and freedom of movement at the price of a slightly imperfect load distribution. The panel surface however is generous and she weighs about 30 kilo less than I do thus remaining well within the safe zone.
The booties enveloping the panels do not cover the spine and leave a completely open channel above the skin thus allowing optimal ventilation. The felt-fleece combination of the good, thin, padding wicks the sweat away.

Photos: Ortheflex Patriot

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