"I'm having trouble schooling the canter. My horse will canter in dressage shows, or throughout a jump course, but when I try to school the canter on the flat she consistently breaks after 4 or 5 strides. She's checked out okay with the vet and the dentist, her saddle fits (it's custom made for her), and she gets weekly massages. Thinking that it was most likely a problem with me, I've tried riding other horses, but haven't had any problems cantering them. If I use my whip just before the break, she pins her ears, and either continues for a few more strides and then breaks, or kicks out and bucks. How can I overcome this problem so that I can school the canter?"

If your horse is willing to maintain her canter in situations that encourage her to be excited, energetic and forward, but not in her daily schooling sessions, the problem seems to be with her responsiveness to your aids rather than actual physical limitations, especially since you’ve been so responsible about evaluating the possible sources of pain that can lead to training issues. However, I suspect there is some reason why you started giving your horse weekly massages, and wonder if she is generally tight in her back and hind end? You may want to evaluate with your massage therapist whether there are muscle spasms that consistently indicate pain in an area that would affect your mare’s ability to maintain her canter, and whether they are due to conformation or strength issues.  Also, does your mare hold her canter more easily immediately after she’s had a massage and gradually become less willing as her muscles go into spasm again, or is there no apparent difference?  Evaluating these factors would help you determine how large a part discomfort plays in her unwillingness to hold her canter.

I applaud your willingness to investigate the problem as “pilot error”, since the rider’s position and aids play such an important role in helping a horse to execute what they are being asked to perform.  Also, since your skill as a rider doesn’t seem to negatively impact your horse at shows or jumping, or other horses that you ride,  I would proceed with techniques for helping your mare become more responsive to your aids as well as strong enough to carry you when she doesn’t have adrenaline providing the energy necessary to sustain her canter.  I would lunge her on a 20 meter circle with side reins, initially to see if she has any trouble holding the canter without a rider.  If that is not a problem for her, I would do lots of trot canter transitions in both directions to make sure she is comfortable in the upward transitions, i.e.  that she doesn’t need to kick or buck into the canter.  If she is able to make the transitions easily on the lunge and maintain her canter, I would have someone lunge you on her to see if her response is appreciably different.  If she is unhappy with a rider, I would wonder if she physically uncomfortable with a rider on her back, and whether there is some source of pain you haven’t discovered.  But assuming that she is unhappy about going forward from your leg rather than uncomfortable in her back (some horses really mind the leg), I would then focus on your problem as a training issue.

First I would make sure your horse is responsive to your driving aids in an open area that is most like the situations she finds agreeable, riding long lines and big circles.  She should be willing to go forward from with quiet aids from your leg. I would start with transitions  that she is comfortable with, i.e. walk-trot, walk halt, etc. all the time reinforcing the idea that you want her to go forward from you leg with light pressure.  If she doesn’t respond, use a stronger pressure, if she still doesn’t move immediately forward, touch her with your whip behind your calf as you close your leg.  I like to use an “ask, tell, demand” sequence when teaching a horse to go forward.  Use enough aid to get an “answer”, and then soften the aid as a reward, and praise her verbally or with a pat.   Once the horse understands your expectations, test her understanding from walk to trot, and then trot to canter.  Any time she is slow in responding to the appropriate aid, go back to the ask, tell demand sequence until you get a positive response to light aids.  Also practice lengthening and shortening the stride within the trot until she clearly understands that more leg means go more forward.  Now it’s time to test your training by asking her to make a canter transition from the trot, and then go immediately more forward, more like a hand gallop in a little half seat so that she has no impediment to her forward thinking.  Repeat the same ask, tell, demand sequence the moment she begins to offer resistance in any form, i.e. slow down, pin her ears, offer to buck. Once she clearly shows the desire to go forward when you increase your leg pressure, you can begin to sit down, steady the canter rhythm, and begin to ride large circles.   Begin to lengthen and shorten the stride on the circle until she clearly goes forward from a light increase in leg pressure.  The goal is to be in the “ask” mode most of the time.  This will make her happier because your leg pressure will be softer, and you happier because you will be able to sustain your position without tensing your muscles. 

And last, practice trot canter transitions on the circle that are forward and relaxed.  At any point, if your horse starts to revert to the balky behavior, go back to the point in her training that she can do successfully, and reinforce your expectations that she move forward from your leg. Then work you way back through the sequence until you can reliably produce and keep a canter for as long as you want.

Leads and resistance at the canter in general sometimes are very complex problems that do not have simple answers. To start with, we will assume that your horse simply doesn’t know what is expected of him. If this is the case, work on the longe line or in a round pen would certainly be the logical place to start.  
 
Having a pole or very small jump that the horse must cross just as you ask him to canter will also frequently get the horse on the correct lead. Of course you will have to get him used to going over the obstacle at the walk (pole on the ground) and trot (for a small jump) first.
After the horse will pick up the canter smoothly and on the correct lead with some consistency, you can advance to cantering with a rider. The same techniques can help you with the leads—moving out on a circle (or leg-yielding out) and going over a pole or small jump during the transition.  

The above was written for the sound and sane horse that is just uneducated. You must realize that with a ten year old horse you are buying a lot of history. There are very possibly issues complicating your canter problem.

1. The horse does not understand the riders aids.
2. The rider does not understand what the horse recognises as  the canter aids.
3. The horse has never been taught the correct canter aids in the first place.
4. Some horses are actually trained to trot faster and faster until they canter because they have no choice - they will fall over if they don't and so they become scared of cantering because they don't like the sensation of thinking they will fall over .
5. The rider is unbalanced in the saddle and instead of cantering off an aid, the horse just trots faster and faster until it "breaks" into canter. This is of no use really because the canter has no impulsion and cannot be maintained. The horse's outline becomes too  long and unco-ordinated and the horse breaks back into trot.
6. The rider that leans forward into the transition,puts his weight onto the front end of the horse which puts the horse onto the forehand which unbalances the horse and the canter cannot be maintained.
7. The rider grips up with the legs, hangs onto the reins and the horse cannot canter because the rider says "go" with one aid and "come back" with the other and is therefore confused.

Now, I can't tell you which of these is correct in individual cases but I can tell you that it is always rider error that causes problems and it is not the horse being bloody-minded!!

Other canter problems can arise because people lean into the canter instead of sitting down on the back side which means that their seat, which should drive the horse, can't do it's job because it's hovering six inches out of the saddle. Some times, people lean forward and pull back on the reins and this obstructs the forward motion. No horse will want to move forward if he is going to get a smack in the teeth for his pains.
If you have a canter problem, be very , very analytical about what is happening in every stage of the process (or ask a friend to watch) and then try and break down the problem into bite sized bits to improve it.

(My canter problem is that MGB gets long and unbalanced sometimes and that's entirely due to the fact that I should tap her with my whip to keep her going instead of driving her with my legs which takes my legs off her side and unbalances both of us. I just don't like that horrible lurch that I sometimes get that feels uncontrolled if I tap her behind  when were motoring but I know it's the only way to solve the problem in the long term.)

I will re-iterate again that what happens in the walk, reflects in the trot and the canter so if you can improve the walk and trot, the canter also improves . Walk -trot-walk transitions help the horse to balance itself and why not try cantering from a good walk instead of going through all the trot rigmarole ?
MGB can do it so younger, fitter more beautiful horses (ARE there any more beautiful than MGB !) most certainly can learn and it makes for a less traumatic transition.

The canter transition I use is ; Left Canter, from trot, half halt on the outside (right rein) and re-balance with seat, ask for bend to the left with the rein (inside) put the right leg flat  and back behind the girth and using the inside (left) leg, turn the toe outwards and tap with the heel lightly. Let the hands move forwards, almost lean back into the movement and push with the seat and the stomach muscles forward.

First, plan time to condition your mare. I would recommend a lot of walking, ideally outside on trails or in fields. Incorporate some hills, as well. This type of conditioning-long, slow distance work-offers a number of advantages because your horse will have to push herself along with her hindquarters. She won't have the advantage of allowing the trot or canter momentum to carry her. Since nature programs animals to find the most efficient locomotion, your mare will soon begin to track straight; there is no advantage in hill work to favoring half the engine.

So long as you pay strict attention to maintaining a marching gait, you'll strengthen your mare with minimal risk of injury. This will also help her to relax because as she develops the capability of stepping more and more under herself, she will be stretching her topline. I'd suggest 45 minutes to an hour of walking at least four times a week.

To help your mare find her balance at the canter, begin with work on the longe line or long reins. By eliminating the factor of your weight, you'll make her move much easier, both in terms of strength and balance. Spend a lot of time working with your mare at the walk and trot before introducing the canter work. Specifically, work on walk-trot-walk transitions until your mare can perform them willingly and easily. When she's working well at the walk and trot, and performing transitions with ease, introduce small segments of canter. Be satisfied with several strides initially; don't expect her to be able to canter around and around. In fact, she'll benefit most from practicing transitions, so focus on doing many trot-canter-trot transitions.

Be patient. It may take weeks or even months of consistent conditioning and ground work before your mare can perform trot-canter-trot transitions smoothly and maintain the canter on the longe line or long reins.

When you work your mare under saddle, the first thing you should evaluate is your own position. Have someone help you to sit straight. You want to feel equal pressure with both seat bones in the saddle. If you are sitting crooked, you can cause your mare to travel crooked. Keep in mind that much of your mare's difficulty stems from her balance being shifted toward her front end. Therefore, it's especially important that you sit back in the saddle. Although it may be tempting to throw your weight forward when you feel her begin to lose her balance, that just makes things worse by putting your weight on her forehand.
Focus your attention on the walk and trot initially, and learn to control her hindquarters to keep them straight. Usually, when tackling this kind of problem, it is best to focus on putting her shoulders in front of the haunches, not vice versa. To do this, keep a steady contact with your outside rein and use your inside rein as a leading rein to draw her shoulders to the track of her haunches. You may end up going slightly sideways for a while until your mare decides that it is simpler to stay straight.

Riding a shoulder fore on a 20-meter circle is an excellent exercise to help you straighten your mare. If you have difficulty riding a round circle, mark one off by mowing a circle in a field or use cones or buckets for markers. Then, at the walk, situate your horse so that her inside hoof is just to the inside of the circle. Once you can travel around the circle steadily at the walk, do the same exercise at trot. Just be careful not to expect too much too fast. Be satisfied with two or three steps of shoulder-fore when you begin.

When you can consistently straighten your mare at the walk and trot, introduce the canter under saddle. Sit straight and keep your upper body back. Strive for three to five steps initially. At this point, don't let your mare break; you control the canter. After two weeks of a few canter strides at a time, hold the canter for two or three additional strides. If she remains steady, you can add more strides.

Throughout this period, do this exercise as a practice of trot-canter-trot transitions, gradually adding more and more steps of canter as your mare becomes better able to maintain her balance. If you focus too much on the canter, both you and she are likely to get frustrated.

Whenever she tries to swing her hindquarters inward during transitions from trot to canter, bring her shoulders in front of her hindquarters or use your shoulder-fore position before asking for the transition. By practicing transitions with your mare's hindquarters remaining straight, you'll ensure that she develops the strength required to maintain her balance at the canter.