Over the past 25 years the use of near infrared spectroscopy in exercise and sports science has increased exponentially. The majority of these studies have used this noninvasive technique to provide information related to tissue metabolism during acute exercise. Recently a NIRS system was used to measure skeletal muscle metabolism in endurance athletes and these devices now have the ability to be wirelessly attached to the body making the application of NIRS to the stage where it can be reliably applied to exercise training and sport in humans to design training programs for athletes, both professional and recreational.
The advantage that an NIRS system gives is that the technology is a single sensor which noninvasively, simultaneously and continuously determines and reports values for muscle oxygen saturation (SmO2) and blood hematocrit (Hct) during exercise. SmO2 is a measure of the adequacy of tissue perfusion, i.e., whether or not there is enough oxygen to meet metabolic demand. Hct is the volume percent of red cells in the blood and is directly related to the oxygen carrying capacity of the blood. These two variables alone are vitally important for not only assessing athletic potential, but designing a training program that adequately meets the requirements of the muscle.
So how does this apply in racehorses?
Racehorses improve oxygen delivery during running by mobilizing blood from their spleen (they dump up to a third of their total blood volume through their splenic reserve), which is high in hematocrit (more oxygen carrying red cells) and by increasing their heart rate and stroke volume to pump more blood per minute. SmO2 is a direct measurement of the level of oxygenated blood in the muscle, after the muscle has used the required oxygen to power the cells. Thus a continuous and active measure of SmO2 will indicate if what the muscle requires to do the work requested, is outstripping oxygen supply in a work, or more specifically by how much this is occurring. Horses adapt to training incrementally and a work that is too much for the horse at any one time can lead to injury while a work that is handled too easily doesn't elicit the required metabolic adaption.
The Hct measurement in a horse would be expected to directly track the increase in Hct that results from the combination of spleen contraction and decreased plasma volume that occurs during running. As Hct is the volume percent of red cells in the blood and is directly related to the oxygen carrying capacity of the blood, a continuous Hct reading during exercise may be a very strong assessment for the natural athletic potential of the horse.
As of today there is no commercially available NIRS system for racehorses but as ~60% of the racehorse is muscle, any device that can non-invasively measure how the muscle is performing is going to have significant applications in the selection and training of racehorses in the future.