Distances

The very earliest human footraces were usually over extremely long distances. It is recorded that a race of 237 kilometers (km) (147 miles) took place in Rome during the Roman Empire (Noakes, 1992). Recognized distances for modern footraces range from sprints of 100 meters (m) to ultramarathon races of 1000 km or more, lasting many days. The top speeds achieved by human sprinters exceed 36 kilometers per hour (km/h; 22 miles/h), whereas speeds of around 16 km/h (10 miles/h) are more common in marathon races of up to 100 km and of 6 to 8 km/h (4 to 5 miles/h) in races of 1000 km or more.

Distances

The earliest horse races were run over distances of 6 miles. However, the distances of modern track races for Thoroughbreds vary from 1000 m for “sprinters” to the longer 3000- to 7000-m races for “stayers.” Endurance races of 80 to 160 km (50 to 100 miles) and longer are also held over all types of terrain. Quarterhorses race over 400-m tracks, attaining top speeds of up to 70 km/h (44 miles/h).

Distances

The length of greyhound races varies from 250 m sprints to “long-distance races” of 600 to 1000 m. Dogs can reach speeds of up to 60 km/h (37 miles/h) during races of up to 500 m.

Oxygen consumption

In both humans and horses, measurement of oxygen consumption allows calculation of energy expenditure at any specific workload or running speed and gives an idea of an individual horse’s efficiency or “economy of movement.” Maximal oxygen consumption (O_2max) is the maximal amount of oxygen used by the athlete during maximal exercise to exhaustion. O_2max is sometimes referred to as the peak aerobic power.

In humans, there is a trend for the best athletes to have the highest O_2max values. Elite human athletes have O_2max values ranging between 69 and 85 milliliters oxygen per kilogram per minute (mL O₂/kg/min), whereas Thoroughbred racehorses have O_2max values twice as high, about 160 to 200 mL O₂/kg/min (Noakes, 1992; Rose et al., 1988).

Cardiac output

Cardiac output (CO) is the volume of blood pumped by the heart each minute and increases during exercise as a result of an increase in both heart rate and stroke volume. During exercise, both these responses occur to a greater extent in horses than in humans.

Resting heart rate (HR) values are in the low twenties in fit horses, whereas values of 40 to 60 are more usual in athletic humans under the same conditions. During exercise, maximal HR values of between 240 and 250 beats per minute (beats/min) have been recorded in racehorses, whereas maximal values in the range of 180 to 200 beats/min are more common in athletic humans (Evans and Rose, 1988; Noakes, 1992).

Thus, the Thoroughbred racehorse has the ability to increase its HR almost 10-fold from rest to maximal exercise, whereas in humans this range is of the order of three- to fourfold. This difference contributes in large measure to the greater O_2max of the Thoroughbred racehorse compared with the elite human athlete.

The greyhound has a maximal heart rate of about 300 beats/min, only a threefold increase from resting values (Snow, 1985). O_2max in this species is in excess of 100 mL O₂/kg/min, although this has proven difficult to measure.

The camel, in contrast, has the lowest O_2max value (51 mL/kg/min) of the four common athletic species, the horse, the greyhound, the camel, and the human (Evans et al., 1992) (Table 2-1). The racing camel can increase its HR fourfold from a resting rate of about 33 beats/min to about 150 beats/min during maximal exercise.

Stroke index

The resting stroke index (SI, divided by body weight) for horses is between 1.3 and 2.3 mL/kg, increasing to 2.5 to 2.7 mL/kg during maximal exercise (Physick-Sheard, 1985), similar to the human resting value of 1.1 to 1.4 mL/kg, which increases to around 1.5 mL/kg during maximal exercise (Ganong, 1985). Although the stroke index increases in both humans and horses, the increase is, at most, of the order of one- to twofold. Hence it is the much larger (fourfold to 10-fold) increase in HR that is the main contributor to the increase in CO during exercise in both racehorses and human athletes.

Blood oxygen content

The hematocrit is the percentage of the total blood volume occupied by red blood cells (RBCs). The human athlete maintains this hematocrit value at between 40% and 50% (Wilmore and Costill, 1988). During exercise, in humans, the hematocrit tends to rise slightly as a result of a fall in the amount of fluid, the plasma volume, in which the RBCs circulate. The total number of RBCs contained in that volume may increase only slightly during exercise in humans.

In contrast, the horse has the unique ability, specifically during exercise, to release a large number of RBCs from the spleen. As a result, the hematocrit of the horse can increase from around 32% to 46% to 60% to 70% during maximal exercise (Snow and Vogel, 1987). This ability dramatically increases the oxygen-carrying capacity of the horse’s blood during exercise. Greyhounds have high resting hematocrit levels of about 54%; these increase to around 64% during maximal exercise. It is not known whether this is caused by the release of RBCs from the splenic reserve or results from a decrease in plasma volume causing an increase in the concentration of red blood cells (Snow et al., 1988). The hematocrit of the racing camel increases from 33% at rest to 36% at maximal exercise (Evans et al., 1992). Of the four athletic species, the camel, therefore, has the lowest concentration of RBCs. This, together with a relatively low maximum HR, might explain the relatively low O_2max of this species.

Some human athletes have attempted to mimic this physiologic response by using an illegal technique known as “blood doping.” In this procedure, RBCs are withdrawn from the athlete and frozen. At some point, usually hours before competition, the stored RBCs are reinjected, thereby increasing the oxygen-carrying capacity of the blood and potentially aiding performance (Buick, 1980). This procedure appears to have limited effects in the horse, possibly because of the innate capacity of the horse to increase the packed cell volume (PCV) in response to exercise. Another way to increase this effect is by using high altitude in training programs with the current recommendation of live high, train low for altitude training in people. However, in both humans as well as horses, the effects are relatively small and not consistent (de Paula and Niebauer, 2010; Wickler and Anderson, 2000).

The maximum arteriovenous oxygen difference, or (a-v)DO₂, measured in the horse during maximal exercise is only very slightly greater than values measured in elite human athletes under similar conditions. Thus, a larger (a-v)DO₂ accounts for only about 23% of the greater O_2max of the racehorse compared with the elite human athlete; the much greater CO and oxygen-carrying capacity of the blood accounts for the other 77% (Physick-Sheard, 1985).

O_2max as a predictor of athletic ability

Although O_2max is generally considered the best predictor of athletic potential, there is evidence to dispute this belief (Noakes, 1988). For example, although elite human athletes do have high O_2max values, so, too, do many less conditioned athletes. Athletes with similar athletic abilities may have quite different O_2max values (Noakes, 1992).

The same relationship is found in racehorses, in which there is no significant difference between the O_2max values of Standardbred (165–180 mL/kg/min) and Thoroughbred, (164–200 mL/kg/min) horses. Clearly, if O_2max was the sole predictor of athletic ability, the value should be much higher in Thoroughbred than in Standardbred horses. Hence factors other than O_2max must be important in determining the superior athletic ability of the Thoroughbred racehorse.

Two important factors are locomotive efficiency, which is the oxygen cost per kilogram per kilometer traveled, and the percentage of O_2max that can be sustained during prolonged exercise (Coetzer et al., 1993; Hammond et al., 1984; Noakes, 1992). Subjects able to run at a higher percentage of their O_2max for longer periods exhibit superior fatigue resistance. For example, the O_2max of the Thoroughbred racehorse is about three times greater than that of the racing camel, which is of similar mass (Rose et al., 1992) (see Table 2-1). Yet the camel can exercise at a very high intensity (100% O_2max) for a much longer period than can the horse (18 min versus 3–5 min). Thus, the performance of the camel in races lasting more than 10 to 15 minutes is likely to be more similar to that of the racehorse because of the former’s ability to exercise at a much higher percentage of O_2max for much longer despite a substantially lower O_2max. Hence, in the assessment of an athlete’s potential, it is necessary to consider not only the O_2max but also the percentage of O_2max that can be sustained during prolonged exercise.

An important factor in determining success in human runners and cyclists is economy of locomotion, or a lower than average oxygen cost at any running or cycling speed (Coyle et al., 1991; Noakes, 1992). This has yet to be evaluated in racehorses and the other athletic species. However, when comparing the oxygen cost of exercise in camels and horses, it is clear that the oxygen cost per kilometer traveled in camels is much less than in the horse, indicating superior economy of locomotion.

Thus, the greater capacity for oxygen transport in the racehorse is the result of a larger capacity to increase CO, with heart rate being the main contributor; the greater oxygen-carrying capacity of the blood during exercise; and finally, a small increase in the capacity to extract oxygen in the active muscles, measured as a greater (a-v)DO₂ (Evans and Rose, 1988).