How athletes train themselves to defy the body’s limits

Brain power can help athletes push beyond the limits of the human body. But is it cheating — or just smart?

THIS month brothers Bryan and Taylor Fletcher will race for America in the Olympic Nordic combined, a hybrid sport with short bursts of ski jumping and long bouts of cross-country skiing. Like all elite athletes, the Fletcher brothers are angling for a legal edge over the competition — and they say they’ve found it in a new “brain stimulating” gadget.

The device is called Halo Sport, a $700, half-pound headset that fits over the ears and “electrifies” areas in the motor cortex of the brain, associated with coordinating movement. The device, combined with training, can accelerate “improvements in power, endurance, and skill,” according to Halo Sport’s website.

Bryan Fletcher, a childhood leukemia survivor who won the Nordic combined Olympic trials, is a believer. “We were able to repeat movements with precision more often with Halo Sport,” he said in a video filmed for the company. His brother added that the headsets not only improved their training, but also increased their ability to memorize ski courses. He says the US Nordic combined team uses the gadget in their practice sessions.

It may sound too good to be true — and to many members of the sports research community it is. The scientific jury is still out on the legitimacy of the company’s claims, and some critics have slammed it on ethical grounds, calling it “brain doping.”

Still, it’s just one more way that athletes are trying to get a leg up on the competition. In a game of inches, or fractions of inches, every little bit counts.

OUR push for that extra little bit — stretching ourselves to the absolute limit — is the focus of the new book “Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance” (William Morrow) by sports science journalist and runner Alex Hutchinson.

The story started two decades ago when Hutchinson, then a college runner at McGill University in Can- ada, tried in vain to break four minutes for a 1,500 meter sprint. He had “hit a wall, running similar times again and again,” Hutchinson tells The Post.

Eventually he managed to break through that wall, running a personal best of 3:44, which qualified him for the Olympic trials, though a fractured sacrum would ultimately force him to give up his Olympic dreams. But questions still plagued him: How far could he have gone? “If I knew that I had run as fast as my body was capable of,” he thought, “I’d be able to walk away from the sport with no regrets.”

Hutchinson initially set out to write a running guide. Nine years later — after studying super athletes like Usain Bolt and LeBron James and reading stories of deep sea divers and shipwreck survivors — Hutchinson has gone even further, exploring how the power of the human mind and our beliefs can help us achieve the “impossible.”

It should be a simple equation. The body comes with self-imposed limitations: You need a certain amount of oxygen to stay alive; your body’s temperature can only rise so much; your heart can only beat so fast. But it’s far more nuanced than a numbers game.

“The will to endure can’t be reliably tied to any single physiological state,” Hutchinson says.

With notable exceptions — the person who can hold his breath underwater for over 11 minutes, the 300-mile ultra-marathon runner — few people are able to approach their breaking point thanks to limits imposed by their brains. We have “anticipatory regulations” that shut down our body when things get too intense. One of these regulations is called the “pacing instinct” — your brain not only slows down your body when pedaling in a hot room, for example, it also recruits fewer muscles to aid in your movements.

In other words, the true god of endurance is not our body but our mind. “Limits that feel concrete and physical are often created by the brain and are negotiable as a result,” Hutchinson says.

For example, if you ask a group of bikers to take a mentally exhausting exam before riding a stationary bike they’ll pedal on average 15 percent