I decided to do some research on the chair seat and gaited horses. This is some of the information I discovered:
What is it? The chair seat is exactly what the name implies, the sort of seat you have when you are sitting on a chair, knees bent, feet well in front of your hips. If the chair vanished, you would fall on your behind on the floor!
What is wrong about it? This seat puts the rider's weight behind the balance point of the horse/rider combination, making the rider unstable in the saddle. A person riding in this seat can easily lose balance with the horse and fall off, or end up gripping with the upper legs to stay on. It puts the rider's weight closer to the weakest part of the horse's back (the loin) than the straight balanced seat does, and encourages the horse's back to sag downward under that weight (ventroflex) often resulting in a more lateral gait. Ventroflexion obtained this way can cause the vertebrae of the spine to impinge on one another, leading to neurological problems.
Why do people use it? The chair seat is easy. A person with poor body condition, tight leg muscles, and stiff hips will find it much easier to sit in a chair seat than to stretch his legs down around his horse's back. - It makes it easy to put your heels down, a precept that is often pounded into rider's heads with explanation other than it is just "done that way". - Many saddles are built to encourage this seat and are often bought because they are comfortable for the rider. Lane Fox cut back saddles, western saddles with build up pommels, some plantation style saddles, some "all purpose" English saddles are designed to put the rider's legs in front of his hips, feet on the "dashboard" and practically guarantee a chair seat. - It throws the rider's weight toward the loin of the horse's back, encouraging ventroflexion and making a horse more likely to work in one of the ventroflexed gaits, such as the rack, saddle rack or stepping pace, or incline him to high action in the front legs in other gaits, a desirable trait in some trotting horses.
What alternative is there? The balanced, straight, classical seat is a very good alternative to the chair seat, provides superior balance, puts less stress on the horse's back, and is safer for a rider to use. On those horses that require a slight amount of ventroflexion to perform their gait, it is possible, using this seat, to slightly tip the pelvis, brace the lower back, maintain the same shoulder, hip and heel alignment, encourage some ventroflexion, and still stay in balance with the horse.
This rider is sitting a very slight chair seat on her Saddlebred, in a rack. Her hand and arm position are correct for his head and neck carriage. Over all, an almost ideal position for riding a horse in the rack in saddle seat classes ..
Would this be a good seat for a 20+ mile trail ride over all kinds of terrian?