Use a heart rate monitor to determine the intensity of exercise. Maximum heart rates in horses range from about 210 to 230 beats per minute. To improve aerobic fitness, work your horse in the early training period (the first 8 weeks) at heart rates between 120 to 140 beats/min, and later (8 to 16 weeks), increase the work to meet higher heart rates (140 to 180 beats/min), including sprints that temporarily (30 seconds) increase the rate over 180 beats/min.
For more information, read How to Exercise a Cutting Horse
or How to Exercise a Pleasure Horse