Fitness, in terms of the soft tissues in the legs (tendons and ligaments) and muscular strength and tone, is an aspect of training that's often poorly understood or overlooked by many riders and even trainers.
Just like humans, horses aren't born naturally fit for an athletic endeavor. Yes, certain breeds are better built for certain sports, but developing their fitness must be an intrinsic part of any training regimen. It's actually more important today than ever before, as the majority of American horses don't have the space to live an active life on their own.
To do most of our sports, the horse's fitness needs to more akin to gymnasts, weightlifters or pole-vaulters than to marathon runners or soccer players. They need strong, supple muscles and tendons and ligaments more than they need extraordinary cardiovascular capacity. So this article is about building the overall strength of the average competitive horse, not about preparing a horse for a three-day event or endurance ride. It's about taking him to the gym or yoga—regularly—not working out on the track.
Train, Don't Strain. Human sports coaches or personal trainers know that the key to making progress while avoiding injuries is to make sure their charges are "training, not straining."
"Straining" is spending five days a week sitting at your desk and on Saturday playing an hour or two of tennis or 18 holes of golf, then spending Sunday wondering why your knees and shoulders are so sore. "Training" would be walking for 30 or 40 minutes three or four days a week, plus some weight training or yoga, to get fit to play that game of tennis or golf.
Good coaches plan regular workouts for their athletes, progressively doing more and more work. You don't run 5 miles in your first workout, not run for a week, and then go for another long run. Again, that's straining—and it's just about guaranteed to discourage the person from training, because, even if somehow they aren't injured, it will certainly be painful.
Your horse needs the same type of progressive work. If you haven't ridden him for three months and take him for a demanding two-hour trail ride or do a rigorous jump school, he's likely going to be sore, and he probably won't be too eager the next time you show up to ride him. He could also be suddenly lame, leaving you to wonder why your horse is "always hurt."
Here are some examples symptomatic of an unfit horse: He looks scrawny or weak, and you're exhausted after you ride from kicking him every step; he trips over nothing or strikes himself, especially late in a riding session (these could also result from poor hoof trimming or a medical problem); he won't take one canter lead, either on the flat or landing after a jump (this too could be a medical problem); he runs out of gas after 20 minutes of work or two classes at a show; he seems to lack scope over fences or has no bounce to his stride.
If you make your horse stronger and, thus, more able to do the work you're asking for, usually he'll become more willing to do the work (less kicking for you), because it isn't exhausting. Instead, fitness makes it fun and gives him a sense of accomplishment, which most horses like.
Don't Be Afraid. So often people are afraid of getting their horses fit. "It'll make him crazy," they insist. Our experience is that almost always what they become is more eager and more workmanlike—because you've given them the ability to do the job. But some riders become so used to riding a lazy bones that they become uncomfortable when their horse actually has some spark.