“When trainers use this type of hypoxic (lower-oxygen) training program properly, horses can increase exercise performance with the same training stress, such as training speed, duration, frequency, etc.,” said Kazutaka Mukai, DVM, PhD, of the Sport Science Division in the Equine Research Institute at the Japan Racing Association, in Tochigi.

“I think this would be beneficial not only for performance but also for health,” Mukai said.

Such benefits might result because higher altitudes force horses (and humans) to challenge their cardiovascular system to work with less oxygen, said Mukai.

“Horses use glycogen and fat as fuel mainly via aerobic (oxygen-based) pathways during low-intensity exercise (walk, trot, and slow canter), but the situation is different during high-intensity exercise such as canter and gallop,” Mukai said. “Horses mainly utilize glycogen via both aerobic and anaerobic (glycolytic, or oxygenless) pathways during galloping and rely more on glycolytic pathways at the onset of the exercise.”

If horses’ bodies can adjust to working with less available oxygen, Mukai theorized, perhaps they would perform better once they returned to a “normal oxygen” (normoxia) situation, such as sea level.

However, he also observed in previous experiments that some horses working in hypoxia appeared to lose body weight and have less shiny coats. Although individual response to the effects of hypoxia vary, Mukai said he hoped to find a middle ground that would apply to most Thoroughbreds.

Testing Untrained Thoroughbreds on Treadmills With Oxygen Masks

Mukai and his fellow researchers took seven yet-untrained Thoroughbreds (mares and geldings averaging 8 years old) and trained them to work on a treadmill while wearing oxygen masks. Then they worked them three days a week for four weeks in three oxygen concentration conditions delivered via air masks: normal (21% oxygen concentration), mildly hypoxic (18%), and moderately hypoxic (16%). All seven horses completed the four-week training program in each of the three conditions, in random order. During training sessions, each horse galloped on the treadmill for two minutes to reach 95% of his maximum ability to use oxygen.

The researchers found that when they tested the horses’ point of exhaustion (with humane means) in what’s known as an incremental exercise test (IET), the horses had longer run times after training under hypoxic conditions, Mukai said. In all three training groups, the horses showed similar increases in aerobic capacity according to cardiac output and their ability to use oxygen. However, the team noted a significant increase in speed, eliciting maximal oxygen consumption after horses trained for four weeks at 16% oxygen concentration—moderate hypoxia.

They also noted improvements at mild hypoxia (18%), although the results weren’t statistically significant, he said.

Blood analyses revealed that at both mild and moderate hypoxia, horses reached their saturation of oxygen in the blood at much lower levels then when they exercised at regular oxygen levels (21%), Mukai said. In other words, their bodies had to kick into anaerobic energy use more than they would normally. For many horses, this results in more efficient body conditioning, he said.

“Arterial oxygen saturation during exercise was very low even in mild hypoxia, and that would be never seen in normoxia,” Mukai said. “That can induce greater training adaptation. So, I think it is reasonable that even mild hypoxic training improved performance.”

Improved Performance Even Two Weeks After Returning to Normal Oxygen

The researchers noted that performance improvements continued for at least two weeks after horses returned to working at normal oxygen rates (21%)—similar to bringing horses back to sea level for a couple of weeks.

The key to this success might be in the intensity levels, Mukai added. Specifically, the best results might come from short-term but high-intensity training in hypoxia.

Despite these findings, the researchers said more research is needed before they can make practical recommendations for training regimes.