(CNN) England's rugby sevens players have a secret weapon, but to find it they have to dig deep.

It lies at the end of a grueling fitness regime which involves recreating the state of utter exhaustion the body experiences in the "death zone," a term used by mountain climbers to describe extreme altitudes above 8,000 meters where the air is so thin the human body rapidly deteriorates

This is when oxygen levels in the blood are lowered drastically; breathing is a struggle, thinking becomes fuzzy, and muscle fatigue is accelerated.

Pushing their bodies to the limit, while still being able to make tactical decisions, is the key.

Former Ireland captain Brian O'Driscoll -- one of the greatest players to play the game's 15-a-side code -- described these training sessions as taking yourself to a "dark place."

The thinking is that training and practicing at a much higher intensity makes competitive matches comparatively easier.

"You can think right, OK, I'll be working at 110 percent so that when you're working at 95 percent you can make good decisions and you can make the right choices and stuff like that," England forward Phil Burgess tells CNN.

"Our coaches work together to create those sessions that allow us to train at that level so that when we get on the field of play we can perform and make those good decisions and make those good plays."

The challenge, then, is as much mental as it is physical.

"The thing that gets you through it is your head," Burgess continues. "Your body can give up, but your mind will overcome that and you can run yourself into the ground. That's the interesting thing that sevens specifically has taught me -- you're fitter than you think you are. You can push yourself with your head as opposed to just your body."

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'He seemed to have two hearts'

Sevens has long been seen as a home for rugby's fastest sprinters, and while it is true that searing pace is one of the game's most prized assets, just as important is the stamina required to run fast over and over again.

This is where the yo-yo test comes in, even the mention of which is enough to make England's sevens players shudder.

"When you find out you're going to do the yo-yo it's like, oh, gosh. There's always that sinking feeling in your stomach," winger Dan Norton tells CNN. "When it gets dark, it gets pretty dark."

Some plastic cones and a tape recording are all that are required to summon this world of pain. Participants have to run two 20-meter shuttles in time with a beep, with a 10-second rest between efforts.

As the levels progress, the speed increases until the legs and lungs can take no more.

A civilian with a decent level of fitness might manage to get to about level 15, but England's players are expected to get over 19. Some of them even breeze past that.

"I think my best has been around 21 or 21.2," says Norton, rugby sevens' all-time record try-scorer. "Somewhere around the 21 mark anyway.

"The best I've seen is probably like a 22.2, 22.5 ... One guy, John Brake, he used to play sevens and seemed to have two hearts. He could get round about that level. So yeah, there are some fit guys out there."

It's a test of speed, endurance and how quickly your body recovers between sprints. Players will end up pushing themselves to an aerobic limit they would rarely achieve during a match, which reaps rewards.

"It's kind of a similar state you're in when you're playing the whole time," explains Norton. "There's always that cloud of fatigue and your decision making is impaired, your vision's impaired, and you're feeling a bit dizzy.

"But that kind of makes the whole thing so much more enjoyable -- knowing that you can still produce the goods at those levels as well."

Dan Norton shows his speed at last year's Paris Sevens.

Building strength

Speed and endurance, of course, is just one aspect of a sevens player's fitness program. After the yo-yo test comes carrying and wrestling exercises. These can be done with weights, but the England Sevens coaches often have players use each other to perform the drills, replicating as closely as possible the tackle situation in games.

The challenge here is not just to have the strength to carry your teammate, but also to adopt the correct body position. Get it slightly wrong and the lift becomes substantially harder.

It's tackling practice as much as it's strength building.

No weights are required for the next part of the session, either -- a nine-minute ab routine involving crunches, holds, and leg lifts -- before the session moves to the gym to end in the same way it began -- with another dreaded "maximal" test.

Developed by the British cycling team as a way to measure the power a rider produces, Wattbikes are a unique form of torture. Just one kilometer is the standard test for an England Sevens players.

The tactic, says Burgess, is to go all out from the start and try to hang on until the bitter end. Staring at the digital display through a haze of fatigue and willing the numbers towards the kilometer mark is the definition of suffering.

England players are expected to get close to -- or under -- a minute.

"It hasn't got easier," says Norton, reflecting on the numerous fitness sessions he has undergone over the course of his 10-year sevens career. "The issue with when you get fitter is that you can just endure more.

"You probably get experience of how to kind of beat the test a bit more, understand the nuances where you can kind of push the envelope a bit. You do and you don't enjoy it, it's the nature of the beast.

"Training on your own is pretty bleak because you haven't got anybody pushing you, no one's really watching, and you can potentially skive a few reps, whereas here, everybody's watching you. You're all competing and there's a good level of banter. You don't want to be that one person who's missing out on something."

England Sevens' Jamie Barden carries teammate Norton under the instruction of Phil Burgess.

It was five weeks ago that the England Sevens team last took to the field in Singapore for a tournament, so recently there has been plenty of opportunities to push their bodies to the max -- in the gym and on the training field.

The players will be hoping that their brutal fitness regime -- and all the camaraderie that comes with it -- will pay dividends when they run out on home soil at the London Sevens this weekend.