Working on Fitness/Exercise suggestions (equine)

As I work to move up the levels in dressage, it occurs to me just how hard this sport is. I mean really, really, hard. I have such unbelievable break throughs and then such enormously long plateaus. After the last horse show in El Paso, I decided I needed to spend the summer building up B's back muscles. This is a matter of time and patience, plus a lot of cross training. I've been reading Jane Savoie's More Cross Training book, and it's giving lots of ideas for exercises. I'm also looking at Ingrid and Rainer Klimke's exercises on cavalettis and jumps, just to get ideas on different exercises to do with B. This article, by Ron Meredith of Meredith Manor, offered some interesting insights into moving up the levels and fitness. I like how Ron writes, and I have a couple of friends who went to Meredith Manor and respect him greatly. When I'm searching topics I often find myself at his website.

Anyway, what I've figured out over time is that I've got a lot of work to do on B's musculature. So I'm practicing a little bit of interval training, just as I did when I was getting horses fit for eventing. I'll work in a stretchier, connected frame for awhile, and then ask for a more collected trot for just a half a circle, then a stretch frame again. In this way I'm hoping to build his endurance (and mine--he's not very light sometimes) and lengthen the amount of time he's in self-carriage. Do I sound like a book? Maybe too much reading and not enough doing?

Anyone out there have some good horse fitness exercises they'd like to share?