With spring, horsemen's thoughts turn to the Kentucky Derby and dreams of Triple Crown winners. Realistically, most people won't get close to a big time Thoroughbred racehorse, but there are plenty of other racing opportunities - many of which have owner/trainers. These include breed-specific  racing such as Arabian, Quarter Horse, and Appaloosa, as well as smaller race circuits at county fairs.
How do you start conditioning and training your back pasture race prospect? First, look at your horse objectively. Evaluate structure and gait while keeping in mind the stresses that moving at speed will generate.
Also, look at your horse's attitude. The sweet colt who never pushes the other horses out of the way, who is content to follow in last from the field, may not have the drive to be in front that a top racehorse, even at the smaller tracks, needs.
Look at your horse objectively to evaluate structure and gait while keeping in mind the stresses that moving at speed will generate. New window. 
Running, as opposed to trotting or pacing (as Standardbred racers do), causes some different stresses on the horses. It is harder for a horse to accelerate from a standstill out of a gate versus a "rolling start". Standardbreds have a thorough warmup before their races, while most running racers merely get a walk to the post and perhaps a very short jog. Runners put more stress on their joints and bones and compete much less frequently.
There are multiple areas to consider as you condition your horse for racing. You need to build up the respiratory system, so your horse has the best possible oxygen exchange. Aerobic conditioning promotes the use of energy through oxygen dependent pathways.
Aerobic conditioning and your horse
Aerobic conditioning will help delay the time before your horse has to rely on anaerobic sources of energy and shorten the recovery time from a workout or race. It is a fine line, almost more of an art than a science, to have a horse in the ideal condition without causing any physical distress or breakdowns from over-training.
Long, slow distance work is the basis for aerobic conditioning. This is also the ideal way to start the conditioning of a young horse or to bring a horse back into working condition after a layoff. Remember that a young horse will not have the bone development or oxygen capacity of a mature horse and must be trained accordingly.
Consider multiple areas as you condition your horse for racing and build up the respiratory system, so your horse has the best possible oxygen exchange. New window. 
For aerobic work, it is important to keep track of your horse's heart rate. A heart monitor would be ideal, but you can go low tech by feeling for your horse's pulse via the facial artery under the lower jaw. Count the beats for 15 seconds and then multiply by four to give you the approximate heart rate in beats per minute.
Most horses have a pulse of about 40 just hanging out. Walking and trotting can raise the heart rate to 80 to 140. Most aerobic conditioning is done with the horse's heart rate at 150 or less.
This first conditioning program will generally consist of walking, trotting, and cantering the horse over gradually increasing distances and for longer times. Start with the slowest speeds for a warm-up, then work up to more speed. After a long workout, the horse will need a "cool down" period of slower work as well.
Depending on his age and condition when starting, a horse may need anywhere from five weeks to nine or ten months of this slow, steady work. This will also improve bone strength and slowly build up muscles.
Your horse's diet and weight
Be sure to evaluate the horse's physical condition before you start. An overweight horse will need to shed some pounds during his workouts, while a thin horse may need increased feed - especially in the area of fats.
Fats should always be added gradually to prevent dietary upsets. On the days your horse has long, slow work, he will be able to use fat for his aerobic energy. That saves his glycogen stores in his muscles for the anaerobic workouts. A proper diet  can increase the amount of time and work it takes to cause fatigue in your horse.
The diet should also have balanced vitamins and minerals, as well as provide access to plenty of fresh, clean water.
For high-powered work such as racing, a horse will have to rely on anaerobic energy as well as aerobic energy. Generally, at a heart rate of around 150 beats, horses will switch over to anaerobic energy consumption.
Sprinting or "breezing" is the type of work that improves a horse's anaerobic capacity. This type of speed work will also help strengthen and remodel bones. Just galloping, without occasional pushes for speed, has been shown to simply mimic a horse at pasture.
There are two ways to work on speed conditioning. You can increase the speed for a short distance and then gradually add distance or you can work the horse up to his full distance and then gradually push for speed.
A horse should not be pushed to top speed at all his workouts - in fact, most conditioning should be done at 70 to 80 percent of the maximum speed expected. Equally important, the horse should not be pushed to sprint his full expected race distance all at once.
Many trainers stick to sprinting twice a week or so, doing multiple short sprints on those days with rest periods and cool downs in between. This is often designated as "interval training". The horse needs to be evaluated frequently as to heart rate (which will rise to 200 or 250 beats per minute when galloping), respiratory distress, and any sign of muscle or bone soreness.
A horse may build up to sprinting the full distance in total but will be doing it in three or four sets of shorter sprints instead of all at once.
The exact ratio or speed work to rest to long, slow distance work will need to be customized to your horse and the type of race he will be running. Arabian races are generally about 5/8 of a mile to 1 3/4 miles. Appaloosa races tend to range from one to eight furlongs (one furlong is 1/8 of a mile), with 350 yards and four furlongs being popular distances.
Thoroughbred races vary widely and Quarter Horses tend to specialize in 1/4 of a mile. The race distance may vary with the track size available.
Tracking your horse's progress
Tracking recovery rates can be very helpful in judging the progress of fitness. Taking a horse's heart rate right after exercise and then rechecking it at five minute intervals will tell you how fast a horse is getting back to resting rate. Charting this over a series of weeks will help a trainer to evaluate the improving condition of the horse.
Recovery times should go down as training progresses. You can also track the recovery rate between individual sprints to judge when to stop work on interval training days.
Taking a horse's heart rate right after exercise and then rechecking it at five minute intervals will tell you how fast a horse is getting back to resting rate. New window. 
Along with tracking physical condition and improvements in speed, it is important to do "hands on" evaluations of your horse each day, both before and after workouts. Run your hands down all four legs, feeling for any ouchy spots or heat. Also, run your hands down the back, using some pressure to locate any sore areas.
Passive stretches before and after workouts may help. Massage can also aid areas with tight muscles.
If your horse works on grass or wood chips at home, you may need to take some time to work him on the surface he will race on. Dirt tracks tend to be tougher on a horse's bones and joints than grass or wood chips. Working on dirt will strengthen bones.
Even after sprint training has started, a racehorse benefits from continuing the basic long, slow distance work. This helps to maintain muscle and skeletal strength. Most horses also enjoy and do better with scheduled "free days" when they are simply turned out and no official exercise work is done.
Basic fitness remains in a well-conditioned horse for about six to eight weeks. The cardiovascular fitness tends to remain longer than the skeletal fitness.
You can train and condition your own racehorse - whatever breed you choose. Remember that monitoring heart rate and following recovery rates can give you objective measures to judge the fitness of your horse. Too much work will stress muscles, ligaments, and joints beyond ideal working condition. Always work in downtime for your horse, preferably out at pasture, for both his physical and mental health.
Just for you
Editors at EquiMed have created a couple of useful tools to kick start your conditioning program. A sample set of Conditioning Routines  that you can download and print are supplemented with a Conditioning Worksheet . These are starting points that must be modified to fit you and your horse's particular needs, but serve as a good starting point.
A great selection of current articles authored by some of the equine industry's top authors is available in EquiMed's Fitness and Conditioning health center .
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Not every horse is suited to every discipline. It is important to understand how to evaluate a horse for a specific activity, be it barrel racing, horse racing, cart pulling etc. This book  covers horse physiology and how it relates to performance. If you are looking to buy a show prospet, learn what you should be looking for in terms of physiology.
Already have a great horse in training? If you want to learn the practical methods that have been developed to get your horse into a peak state of performance, this book  may be of interest. For you academics that want the theory behind it all, we recommend Dr. Claytons book, Conditioning Sport Horses  will give you deeper insights into the science of conditioning and fitness.
Got the horse. Got the knowledge, now get the tools. Heart rate monitoring is the best way to monitor the fitness and conditioning of your horse. The Polar Equine Inzone Heart Rate Monitor  from Polar relies on wireless technology to transmit the heart rate information from a sensor placed between the horse and cinch/girth to a wrist-watch device. Monitor pulse rate, recovery period and more with this essential conditioning tool.
About the author
Deb M. Eldredge, DVM is a Cornell graduate and horse lover from early childhood. She was active in 4-H and Pony Club, riding mostly huntseat but also Western. She has competed in various horse show venues as well as competitive trail rides and small three day events. At Cornell she was a member of the Women's Polo team.
Dr. Eldredge is a national award winning writer from both the Cat Writers Association and the Dog Writers Association of America. She lives in upstate NY on a small farm with 3 elderly horses, 1 miniature horse and 2 donkeys as well as various other animals.
Visit Deb Eldredge's Google+ Page