here you go!
i agree with the above, and will repeat what I tend to say for all similar situations like this. Trot, trot, and more trot. And then when you think you are done, trot some more. Better canter comes from better trot, not from working more at the canter. Trot builds muscles and balance while cantering burns calories, so by focusing more on the trot, you will actually be setting your horse up for a better canter.
Limit canter to just a few strides max per ride, and focus on trot sets. A great exercise is to change diagonal every few strides (on the straight) and see if the horse stays balanced and even. If they lose rein contact, pop their head up, or something like that when you change diagonal, it indicates a weakness. When your horse can tolerate your change in diagonal every 3-4 strides with no change in their way of going, that's a pretty good indication that you have a balanced horse that is moving through their topline. Be sure to have a soft, correct contact on the bit because if the horse is leaning, than this exercise is useless. But done correctly, this is a great indication of fitness and conditioning and when your horse can move up to regular canter work, as well as starting OF.
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i think she looks like she is well on her way, and agree with the other reply.
For muscling, trot trot and more trot. And when you think you are done, trot some more. Build up in increments so your horse has time to condition (start with just 10 - 15 min depending on how fit she is now) and then added 2-4 minutes per workout every other to every third workout. And stop cantering. For a few reasons.
1) canter burns more calories than it builds muscle, while trotting builds more muscle while burning less calories when compared to the canter
2) horses develop a better canter by doing better trot, not by cantering more, so by taking a break from cantering, when you come back to it, you will be amazed with the improvement
3) lots of bending and changing direction and working on a light rein contact will do wonders for also helping her balance and relax and again help her canter in the long run
4) all that trot work will build amazing topline and muscling
Pictures to prove it:
Sky, aged 4, just off the track going through "letdown"
Sky, age 7, after a few years of remuscling (notice neck still very thick/upside down):
Sky, age 11, schooling at a show:
Sky, age 14 (this year) widest he has ever been - at a hunter pace/horse park:
Read more: muscling?