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"For the first time in the America's Cup, racing boats will get past (opponents on the water) because they have fitter people -- that's quite nice," Percy told CNN.

Oracle Team USA skipper Jimmy Spithill added: "We've never seen anything like it in sailing. It's like sprinting up a hill for 25 minutes.

"Some days you look at the heart rate data and it looks like a few guys are having heart attacks."

'Endurance base'

Prior to the last installment of the prestigious race in 2013, sailing changed drastically.

Tall aerodynamic wing sails that harness wind power better, and foils mounted under the hull that can lift the boat out of the water, both helped to dramatically enhance speed when conditions were right.

Subsequently, effort from the sailors was needed in more intensive bursts, with huge 110 kg grinders -- manually-operated winches that help power the boat -- the norm.

Sailing has become "a completely different sport," says the Land Rover BAR team's head of strength and conditioning Ben Williams.

"I broke down the old America's Cups and the work-to-rest ratio was 1:6. In the last Cup in San Francisco it was 6:1," he told CNN.

"Because that's more of a constant output, the guys need more of an endurance base."

The new AC50 boats have only six sailors on board, compared to 11 in the last edition, with the helmsman and wing trimmer tied up with steering, foil control, sail-setting and strategy, leaving four crew in the engine room. Even then, the aft grinder often doubles as tactician.

Cup rules limit the crew's total weight to 525 kg, an average of 87.5 kg per sailor. Where possible, the duo in the stern will be lighter, giving the grinders more potential in terms of muscle power. The designers' job is to build the most efficient systems they can to harness that energy.

"The fact is we're heavily undermanned," Spithill says. "There is always too much to do and not enough guys to do it."

'Watt farming'

On top of a busy sailing schedule, Land Rover BAR's training regime involves 12-15 hours a week in the gym, combined with yoga sessions to enhance mobility and prevent injury.

It also focuses on nutrition, and blood analysis for vitamin and mineral monitoring, tailored to each individual.

The British team combines upper-body weight training with four cardio sessions a week so the crew can produce power over long periods of time, similar to a cycling time trial.

High-intensity interval training -- using a grinder, rowing machines, ski ergos or bikes -- is designed to improve the sailors' performance when they are nearing their VO2 maximum -- the highest rate of oxygen consumption.

"The reality is, your best sailor mentally and tactically may not be your best sailor when his heart is beating at 186 beats per minute and he's head down sweating all over the pedestal," adds Williams, who describes himself as a "watt farmer."

'Smart decisions'

The ability to think clearly while performing at the extremes of physical capability is the reason Spithill has sought the help of US Navy Seals as his team bids to win a third successive America's Cup.

Sessions have focused on taking sailors to exhaustion in the gym and then asking them to make decisions or complete tasks, such as puzzles, both individually and as a team.

In one exercise -- the ice-bucket memory game -- sailors were immersed in ice baths while exhausted teammates had to solve puzzles to release them. If they got it wrong, they had to do a 50-meter dash and then try again.

The Seals also put them in a hostage simulation, armed with paint guns, where the emphasis was on team work, leadership and planning.

The Oracle team has recently worked with professional freedivers, learning techniques to hold their breath for up to four minutes under water -- both for safety and to develop the ability to remain calm and not panic in stressful situations.

"Everyone on the boat has to have a strong mental game," said Spithill, who skippered the US syndicate to that remarkable come-from-behind 9-8 win against challenger Team New Zealand in San Francisco in 2013.

"Guys have to make split-second decisions, and usually when you make mistakes it's when you're tired or stressed. That's exactly what these boats do to you, and the consequences if you make a mistake are like an F1 driver going into a wall.

"It just comes down to making smart decisions when you're exhausted or under stress. That's what special forces guys have got to be good at. The consequences for them are pretty big.

"We've taken out big-wave surfers, X-Games stars, NFL and MLB guys, and every single one of them has been shocked and said they couldn't believe the environment we have to work in."

Arm vs. leg power?

For five of the six teams in this year's America's Cup, all this power generation has come from traditional arm-operated grinders, meaning most of the training has been focused on the upper body.

But Emirates Team New Zealand sent shockwaves through the competition when it unveiled its new boat with cycling-style winch pedestals.

The Kiwis have kept their innovative design under wraps for more than a year, alongside covert cycle-specific training with Olympic bronze medal-winning cyclist Simon van Velthooven.

The theory is the bigger leg muscles will be able to produce more power, for longer. That, in turn, means more energy available for maneuvers, and the option to choose faster foils which need more adjusting because of their instability.

"Traditionally it has always been about upper-body grinding, here we're completely flipping everything on its head," physical trainer Hubert Woroniecki said in an Emirates Team New Zealand video last month.

"There's a substantial increase in power output using the legs. It's been about a year of hard work, we've gone through a lot of base work and now we've hit a few phases of intensive and the guys are making big jumps."

Olympic gold medalist rower Joseph Sullivan described the set-piece test rides, using Watt bikes, as "brutal." His fellow grinder Sam Bell, in the same team video, opted for "disgusting."

"It's actually quite similar to a rowing race, you just hit a physical limit where your body is telling you to stop, but you just have to stay there, you just have to keep going," Bell said.

'Addictive'

Spithill and Softbank Team Japan's former New Zealand skipper Dean Barker are among those to have dismissed the pedal technology, arguing the increased wind resistance and the inefficiency in dismounting a bike to run across to the other side of the boat during a turn outweighs the power advantages.

Amid all this arm and leg bulking up, Russell Coutts, a former America's Cup winner and now chief executive of the organizing body, also believes sailing expertise shouldn't be ignored.

"I really don't think this will be a deciding factor, overall I think there are other factors which will be much more important, such as the way they control the wing sail or the way they position boats on the racecourse," Coutts told Yachting World.

The proof will out on Bermuda's Great Sound in June, but either way the modern America's Cup has taken sailing fitness into a new realm.

"It's incredibly addictive," said Spithill. "Once you start operating at that sort of level where you're right at the edge -- man, it's a lot of fun as well."