The horse becomes fatigued and eventually some system will fail if work is continued. Aerobic systems, which are dependent primarily on the cardio-vascular system, can be conditioned rather quickly. Working the horse at a heart rate between 130 and 150 beats per minute for 20 minutes per day, five days per week will result in excellent aerobic fitness in four to six weeks. Anaerobic fitness may take an additional 45 to 60 days for a total of approximately about 90 days to build a base level of fitness. Of course for very strenuous events like racing, additional time will be required to get the horse completely fit.
Heart rate is an excellent indication of the status of the horse.
There are three primary factors that drive heart rate and an understanding of those is important in assessing the condition of a horse. Those factors are adrenaline, oxygen debt and pain. The heart rate of a horse at any given time will be the additive effect of those factors. Because of that, heart rate can be used to monitor the horse's response to work and a reliable and repeatable assessment of his fitness can be obtained. Assuming that adrenaline and pain are not factors, the heart rate in response to a given workload provides information on the amount of oxygen debt that a horse has accumulated.