The coordinated actions of the cardiovascular and respiratory systems result in the transport of oxygen and energy products
(glucose, fatty acids) to the muscle fibers, where they are used for aerobic energy production, and the removal of waste products
(lactate, carbon dioxide, water) from them. During exercise, oxygen delivery is improved by increases in the volume of air
breathed, the amount of blood pumped by the heart, and the oxygen carrying capacity of the blood, together with a selective
redistribution of the blood flow from the viscera to the working muscles. Many of these exercise-related adaptations are initiated
by epinephrine release from the adrenal glands.
Cardiac Response to Exercise
VO2 Max: The athletic capacity of horses is attributable to a large number of physiologic and anatomic adaptations that allow
an extremely high maximal rate of O2 consumption (VO2 max). VO2 max is the maximal volume of oxygen the horse's body can use
per minute. VO2 max of a horse can be 200 ml/kg/min vs. 85 ml/kg/min in an athletic man. Various adaptations including large
lung size, high cardiac output and stroke volume, high hemoglobin concentration and the capacity for splenic contraction increases
the horse's oxygen carrying ability of blood by 50%. Training in horses usually increases VO2 max by 10-25%. In addition,
horses have a very efficient thermoregulatory system which allows heat generated during exercise to be lost by evaporation
from skin surface through the production of large volumes of sweat, evaporation from respiratory tract and from convective
loss from skin and mucous membranes. Furthermore horses have developed an efficiency of anatomy and gait, whereby muscle mass
makes up a large proportion (45-55%) of bodyweight and muscular work is halved by energy storage in elastic structures. The
structure of muscle itself has various adaptations including a high mitochondrial content for aerobic energy production and
large stores of energy substrates
Heart Rate Monitoring Methods:
Heart rate measuring technology is rapidly progressing in horses and is being used more and more by trainers in their training
programs. Several methods have been available for many years.
1. Feel the digital or mandibular pulse. A simple, low cost method, however, it is difficult to consistently count heart rates
above resting values, and it is impractical during exercise.
2. Stethoscope – more consistent than the finger pulse method. However, it is difficult to accurately determine the beats
per minute at heart rates > 100 bpm and is impractical to use during exercise.
3. Electronic monitoring - Most reliable method during and while recovering from exercise. More recently they have become
Heart Rate: The normal resting heart rate of a mature horse is between 30 to 40 beats per minute. This rate is difficult to
obtain in some situations, as certain horses become excited by external stimuli, which elevates the resting heart rate. Although
resting heart rate in humans can decrease dramatically as a result of physical conditioning, the resting heart rate of horses
does not appear to change appreciably with fitness. Athletic horses often have low resting HRs. This indicates large stroke
volume capacity. During exercise the rise in HR is the major contributor to the increase in cardiac output and it is responsible
for 53% of the increase in oxygen consumption. At exercise onset, HR increases rapidly from approximately 30 beats/min to
approximately 110 beats/min via parasympathetic withdrawal, with the consequence that at low running speeds heart rate may
elicit an early over shoot. At faster speeds further elevations are achieved less rapidly and are driven by the sympathetic
nervous system and circulating catecholamines.
The anaerobic threshold in horses is around 150 to 170 beats per minute. Heart rates below this threshold indicate that a
large percentage of exercise is being performed aerobically. When exercise intensity or duration increase, the requirements
of the cardiovascular system increase, which in turn results in an elevated heart rate. Heart rates above the anaerobic threshold
characterize rates of metabolism that exceed the abilities of the oxygen dependant pathways supplying energy. Heart rates
of 170 beats per minute or greater characterize a large percentage of metabolism occurring anaerobically or without oxygen.
Anaerobic metabolism is supported primarily with glucose and glycogen as the fuel source. The threshold heart rate, like resting
heart rate, does not change dramatically with physical conditioning in horses. It is likely that the threshold HR for anaerobic
exercise may be influenced by genetics.
The maximal HR (HRmax) of a horse is in the range of 210-280 beats/min, which represents a 7 fold increase over resting values. Each horse has
its own HRmax, which is reached at a particular exercise intensity. Once a horse reaches its HRmax a further increase in speed is still possible, but it does not elevate the HR any more. A horse with a high HRmax is favored as an athlete because it can perform more work at a specific heart rate (ie a horse with a lower HRmax works relatively harder at a given HR). Conditioning does not alter a horse's resting HR or the HRmax. After conditioning the horse reaches its HR max at a higher workload, and travels faster/works harder at a given HR. In
humans HRmax decreases with age and an age related decline in HRmax has recently been described in horses such that as a horse ages it performs the same workload at a higher HR than in its youth.
Maximal heart rates should not be used as a major part of conditioning programs; rather, they should be monitored as a danger
zone suggesting that fatigue may occur quickly. The speed or velocity a horse can achieve or sustain at a submaximal heart
rate of 140, 170, 200 beats/min (i.e. V140, V170, V200) provides information about stroke volume and cardiovascular capacity and can be used as an indicator of fitness and racing
potential i.e. as a horse gets fitter the velocity at which it travels at a heart rate of 170bpm should increase.