You can help your stiff dressage horse bend better by gently doing the opposite of what he wants to do with his body.
Few horses are ambidextrous—meaning they can bend as easily to the right as to the left. So your goal is to make your dressage horse's soft side more "stiff" and his stiff side more "soft" and bendable.
How Do I Make the Stiff Side "Softer"?
Dressage riders tend to think that the stiff side is the "bad" side because it feels harder for them to bend their horses when that side is on the inside. But you need to think outside the box. The stiff side is not the problem. Your dressage horse feels stiff to the right because the muscles on the left side of his body are shortened and contracted.
The solution to this problem is to stretch those shortened muscles on the left side by riding your horse with too much bend when you track to the right. In schooling, you'll live in "right bend" until you feel the muscles on his left side elongate. (You'll know those muscles are stretching because it’ll feel easier to bend your horse to the right.)
So, let's track to the right—the stiff (hard, strong) side. The main reason your dressage horse feels stiff to the right is because the muscles on his left side are shortened and contracted. These shortened muscles limit how much he can stretch his left side and bend around your right leg.
Here's an exercise to gently stretch and elongate the muscles on the left side (the hollow side) of your dressage horse’s body.
If your horse is really stiff, do the exercise in the walk.
• Go on a large circle to the right.
• Pick a point somewhere along the arc of the circle, and turn onto a 6-meter circle.
• While on the small circle, think about your bending aids. (Put your weight on your right seat bone, keep your right leg on girth, place your left leg behind girth, flex your horse to the right as if you’re turning a key in a lock with your right wrist, and support with your left hand.)
• Ride the 6-meter circle a couple of times until your horse’s body conforms to its arc.
• Once he’s bending, keep applying the 6-meter bending aids, but blend back onto the 20-meter circle.
• If it gets difficult for your horse to stay bent this much to the right, blend back onto a 6-meter circle. The idea is to ride the 20-meter circle with a 6-meter bend.
• Once you can do this on a circle, try riding straight down the long side with your horse bent as if he’s on the arc of a 6-meter circle. (The feeling is a bit like doing shoulder-in in front and haunches-in behind at the same time.)
When you go down the long side, bend your horse to the right from nose to tail as if he’s on the arc of a circle. Be sure you bend him behind your leg as well as in his neck.
How Do I Make the Hollow Side "Stiffer"?
The flip side of this "stiff to the right" issue is that your dressage horse will be hollow or soft to the left. You might think his soft side is his "good" side because he feels easier to bend, but the hollow side of your horse needs help as well.
On the hollow side, your horse doesn't have true bend-equal from poll to tail. He usually overbends the neck to the inside and places his inside hind leg to the inside of his line of travel. By doing so, he can avoid bending the joints of his inside hind (engagement), and he also doesn't carry as much weight on it. As a result, that leg gets weaker, and your horse develops unevenly.
My solution for this problem is to ride your dressage horse without any bend at all when the stiff side is on the outside and the hollow side is on the inside. Keep your horse as straight as he is on the long side even when you go through corners and circles. Think that his body is like a bus that can’t bend on turns.
Let's say your dressage horse is hollow (soft, weak) on his left side. When circling to the left, ride without any bend at all. Keep his body as straight as a bus.
• To get a perception of straightness, halt somewhere on the long side. Make your horse’s body parallel to the long side all the way from poll to tail.
• Also, ride him either with no flexion (His chin is lined up with center of his chest.) or in counter-flexion (-1). In counter-flexion, his face will be 1 inch to the right.
• Ride through corners and circles with no bend through his body and in counter-flexion at his poll. If you ride in this position, your horse’s left hind leg will step underneath his body.
• This will make that leg stronger over time. (This exercise is only for schooling– not for horse shows.)
If you use this philosophy of doing the opposite of what your dressage horse would do on his own, and it’ll be easy to get him to bend on his stiff side. You’ll also find that you rarely get stuck solving training issues. Invite your horse to do the opposite of what he chooses until it becomes easy for him. Once that happens, settle back into a happy medium.
A Happy Horse