I recently exchanged lessons with the barn manager where Shorty lives. Jennifer is awesome and takes great care of the ponies at her farm. She teaches beginner level riding lessons and carriage driving, so we thought it would be fun to exchange lessons. She got a more demanding dressage lesson from me, and pretty soon I'll be driving a carriage!
One of the things Jen and I worked on was the sitting trot. Jen rides a Norwegian Fjord named Jerry who adorably pudgy and very sweet. Jerry is a bit of a bouncy ride, so when I asked Jen what sitting his trot felt like, she responded that she indeed had problems moving with her horse. This is pretty common with OTTBs too, who can occasionally feel like jack hammers instead of horses especially when they just come off the track.
I started Jen on the longe line and did some gymnastic exercises to warm her up and get her stretched out. One sufficiently warmed up, I had Jen drop her stirrups, cross them (making sure she pulled down the buckles a bit so they didn't pinch her thigh) and stretch her leg down while still keeping her heels slightly lower than her toes.
From there, I had Jen hook both hands under the gullet of the saddle (the reins were knotted and fastened to the front D ring to ensure they didn't slip or get tangled) and pick up a sitting trot. I instructed her to stay loose through the hips and pelvis and instead draw her abdominal muscles in towards her spine to absorb the shock of Jerry's lumbering trot, all while sending her weight down to her heels.
The first attempt went well, but I noticed she was still pinching a bit in her leg, so I had her stop, shake her legs out and do a few stretches. "This time, " I said, "Keep her your legs relaxed and lightly draped around the horse."
Success! Jen was able to move and follow Jerry using her abs to control her upper body. Once comfortable, I had her take her inside hand off the saddle and place it where her hand would be if she were holding the reins. Finally, she took her other hand off and sat the trot without using her hands as a brace. She started to get a bit "grippy" in her leg, so I had her widen her hip angle and imagine pulling her legs just a tiny, tiny bit away from the horse's side. While this change is not noticeable to a layman standing on the sidelines, this prevents the rider from pinching the horse's side and becoming stiff.
To round the lesson off, I sent Jen out to the rail with her stirrups back and off the longe line. We did a sitting trot one more time. While I noticed significant improvement, and I'm really proud of her, she knows she still has to work and practice to develop her muscles to adapt to this slightly different, more open, method of riding.
To sum it up, Jen said, "I'll be sitting the trot furiously for the next few months."
Riders trying this at home can modify this exercise if they are unable to be on the longe line or don't have a helper. I still suggest tying your reins in a knot and buckling them to the inside front D ring (meaning you have to switch sides when you change directions) just in case you lose your grip on the reins.
Ride on the rail putting your outside hand on the knot in your reins and hook your inside hand under the gullet, pulling up. Pick up the sitting trot, keep reminding yourself to breathe, sit up tall, draw your abs in towards your spine, and relax your legs down long with your heels lower than your toes. Try not to let your toes point in an outward direction, as this will pull your legs away from the horse's side and put you behind the vertical; instead, swivel your ankle a bit so your toes are parallel with the horse's side, giving you maximum connection and surface area with the horse.
Switch your hands when you change directions, so your inside hand holds the saddle and the outside hand keeps the horse on the rail. Don't be too concerned if your horse is a bit confused at first, keep your leg on and use your soft seat, inside leg and plenty of encouragement to get a nice relaxed, rhythmic trot from your horse.
Once you're comfortable (which could take a few sessions) try riding the sitting trot with no stirrups and no hand under the saddle. Then go back to stirrups. You may find that the stirrups feel too short all of the sudden, so it may be prudent to lower your irons a hole on each side to keep that nice long dressage-y leg if that's what you're going for. Hunter riders, jumpers and eventers should probably raise them back up for jumping though!