Great dressage riders and trainers may have different training philosophies and different strategies when it comes to their horses, but all of them have one thing in common, and that is a high degree of personal physical fitness. An effective rider has a great deal of core strength in particular, that strength which holds us in balance over the horse. I always think of the rider's seat as going from mid thigh to up to the rib cage, and this area of the rider must be strong enough to hold the rider from coming off balance laterally and longitudinally. A strong core that really helps the rider balance also allows the correct muscles to relax to the degree necessary to sit with the horse's movement. This in turn helps the rider control all the aids, and use them in a clear and precise way.
Ok, it's pretty clear how important that strong core is, how are we to go about getting it? If you are a professional and ride 5 horses or more a day, I really think that is good enough to get a super strong core. I always feel that when I ride 5 horses I have had a very good workout, any less than that and I need to do more to help me stay fit enough to ride well. I do know some top riders who ride 8 horses a day and then go to the gym - while I think that is great, I figure 5 horses and some stretching and I'm good to go...
I know it's a often a struggle for many amateurs to ride even one horse a day. Life is a balancing act, and there are often more important things in life than riding dressage. However, whenever and however it can fit in, additional fitness is required if you are interested in really improving your ability as a dressage rider. Additional physical training emphasizing strength, balance and suppleness can only help your riding.
My favorite thing to do is dance, dance, dance. Grab a partner and learn to ballroom dance. Many aspects of dance are similar to dressage - there is tempo, rhythm, balance, frame, contact and suppleness. And it's really fun too - you don't even realize the workout you are getting!
One of the best new sports out there for core strength, balance and fitness is paddle boarding - I read somewhere that it burns about 800 calories an hour - and most of those calories are burned maintaining balance with your core. If you live anywhere near the water I highly recommend it!
Other great sports are most of the martial arts (Thai Chi is supposed to be wonderful) and any type of yoga. Pilates is a guaranteed bet to improve your symmetry and muscle control. If you can run, use a treadmill, or elliptical machine for 30 minutes a few times a week it can make a huge difference, and it helps even more if your posture and technique is kept in mind while working out.
For those of you with very limited time, use some of your down time to work out. Watch TV and do the "Plank" (the pilates move where you hold your body straight as an arrow on your elbows and toes). Any other good abdominal exercises, especially those for your oblique abdominals are great. Have small weights near the couch, and use them for arms, shoulders, chest and legs. Lunges and squats can be easily done from in front of the TV.
I'm sure that there are many other great sports you can do that will enhance your riding, I think the most important thing is not so much the sport or what you do, but just that you do it...