Most horses are stronger on one side than the other--just like humans.
With people, the preference for one side shows up in right-handedness or
left-handedness. Most horses are "left-handed."
For a horse, a preference for one side means that he can bend in that
direction more easily, his balance is better when turning in that
direction, he finds it easier to pick up that lead in canter, and he is
just generally better at EVERYthing on his preferred side.
A very strongly one-sided horse may not be able to flex *at all* in his
"bad" direction. He may be unable to turn in a tight circle in his "bad"
direction. He may not be able to pick up the "bad side" canter lead at
all, or to keep the "bad" lead in a circle even if he can pick up the lead
in the first place.
As riders, we really quite prefer our horses to be reasonably competent at
most things we request of them--whether it's right or left. A strongly
one-sided horse makes that difficult. Here's what you can do:
Stretch.
The one-sided horse is stronger on his good side, which means that the
muscles on that side are tighter. The weaker muscles, on the "bad" side,
are looser. As a result, it's easier to bend toward the strong, tight
muscles, and away from the looser, weaker muscles.
Stretching will help tone both sides. When you ask the horse to stretch
those muscles, you help to tighten the loose ones and to loosen the
tight ones.
Exercise:
If your horse's strength or balance to one side is questionable, then he
needs to work on that side. It can be helpful if he has a chance to
build up his balance/strength without a rider. Lunging helps a great
deal. The horse can feel his own way, and not have to worry about the
additional balance of his passenger. If you're not very familiar with
lunging, you may wish to visit my article on
lunging.
At first, you will be asking something that the horse is specifically
bad at doing. That's the whole idea, after all, to help him learn and
develop by practicing. Remember not to ask too much at once. Since he's
not good at it, he'll be using muscles in ways he's not used to; he will
get sore very quickly. So make sure that you start slowly, you ask for
short sessions, and you give frequent breaks.
For example, if your horse is really bad at cantering to the right, then
you will want to lunge him to the right.
Make sure you warm up well at walk and trot and that you have asked
him to go both right and left.
Now ask for a right canter.
If he strikes off on a left canter, you may need to stop him and ask
for the right lead. He may be able to change to the right lead on his
own. Changing leads is not as desirable as striking off on the correct
lead, but for now, take what your horse offers. As he gets more sure
of himself, he will more reliably strike off on the correct lead.
Let him go around once, twice, three times. Then ask him to trot
again. If he starts breathing very heavily or looks like he is getting
tired, ask him to trot earlier.
Give him a short rest; you may have him walk, or or you may ask him
to walk, trot, or canter in the other direction. Whatever you do, give
him a few minutes where he's *not* cantering to the right.
Ask for the right canter again, and have him circle two or three
times. Then slow again, and then give him another break.
You can use about three short spurts at first, then either work him
in four short sessions or three slightly longer sessions. As he gets
more sure of himself, you will be able to gradually extend the
sessions...until finally you see little difference between the right
and the left.
As with all lunging, remember that circles are hard on the horse's
joints, and he should not ever have to really *work* on the line for
more than about half an hour at a time.
In everything you do, be sure not to focus on one side. If you over-do it
on one side, you can make the horse one-sided again...in the other
direction. If you stretch and strengthen both sides equally, they will
eventually even out.