Proper training of the equine athlete can produce results in more than just muscle mass: it can and should create physiological changes in the lungs, spleen and heart.
Proper athletic conditioning can increase the actual size of the heart, which is a factor in cardiac output. Cardiac output is a combination of heart rate and stroke volume. The more blood that pumps through the heart, the more oxygen arrives at the muscles.
A horse that has been trained properly will have healthy lungs, which can take in more oxygen. The oxygen is then carried by the blood and distributed to the muscles. Called maximum oxygen uptake, this process provides power for a longer time. If all other equine systems are in order, the horse’s performance level is directly related to maximal oxygen uptake, which can increase by 35 times between rest and intensive exercise.