At the Zebra Gym in Moscow, the off-season training home of Maple Leafs defenceman Nikita Zaitsev, there is little tolerance for slacking.

A typical 90-minute CrossFit training session led by local CrossFit champion Artem Nehoroshih often leaves professional hockey players in sweaty heaps, Zaitsev included.

“It is so intense, it’s crazy,” Zaitsev was recounting in a recent interview. “My heart rate is always 175, 180 sometimes.”

That’s an oxygen-debt-inducing180 beats per minute thanks to a circuit workout that can include everything from high-repetition barbell squats to high-intensity turns on a stationary bike. But even in the most painful moments of a summertime that included four or five such workouts a week, Zaitsev stood out as a client who “never complained,” said Nehoroshih.

“Other hockey players we train — often they send us a text saying, ‘Oh, sorry, I cannot attempt the training session today because everything is in pain and I’m not feeling well,’ ” Nehoroshih said this week, speaking over the phone from Moscow through an interpreter. “But Nikita didn’t miss a single training session, even though I know the training was very hard on him.”

Said Zaitsev with a smile: “I love CrossFit.”

CrossFit, the branded fitness regimen to which Zaitsev is an adherent, is one of the most successful fitness businesses of the past 20 years — and one of the most controversial. Its mix of elements such as Olympic weightlifting and high-intensity internal training has created a network of thousands of affiliated gyms around the world, not to mention a professional sporting competition known as the CrossFit Games.

And certainly there are those, like Zaitsev, who see CrossFit workouts as an ideal off-season addition to any pro athlete’s schedule. A few years back, when the Maple Leafs held training camp in Collingwood, Ont., they stopped by the CrossFit gym owned by ex-NHLer Scott Thornton, a first-round pick of the franchise in 1989.

“I think it’s effective for any sport. The whole premise of CrossFit is that you’re training all the fitness domains, so you’re not specializing in one,” said Thornton, who added he sold his CrossFit gym in 2015 due to time commitments. “Endurance, stamina, power, strength, speed, agility, balance, coordination, flexibility, accuracy — you have all these 10 fitness domains, and you try to improve in all those areas. And if the programming’s done right, you should see some improvements in all those areas. So I’m becoming a better athlete by working on all these.”

But CrossFit isn’t universally embraced around the sporting world.

“It has its haters,” Thornton acknowledged.

It has been criticized in some corners for exposing its trainees to a high risk of injury. Even CrossFit’s creator, Greg Glassman, has acknowledged the inherent risk of repeatedly hoisting heavy weights to the brink of exhaustion.

“It can kill you,” Glassman told the New York Times in 2005. “I’ve always been completely honest about that.”

It certainly isn’t difficult to find performance coaches with NHL client lists who’ll tell you CrossFit isn’t a good fit for pro players. In a recent interview Clance Laylor, the Toronto-based trainer who counts P.K. Subban and Joel Ward among his charges, was quick to praise CrossFit as a popular phenomenon that has inspired legions of regular folk to embrace fitness. But as for its efficacy as the bedrock of an NHL off-season, Laylor was less complimentary.

“In CrossFit, the bottom line is they’re trying to make you fit for everything,” Laylor said.

But by training “everything,” it’s difficult to hone in on essential things.

“In hockey, speed is king. You cannot build speed in a fatigued state. Let me repeat: You cannot build speed in a fatigued state,” Laylor said. “You do not do Olympic weight-lifting movements in a fatigued state. You’re inevitability going to hurt yourself.”

Laylor said building speed requires a more targeted approach. It’s complicated and science-based, but suffice it to say it involves lifting heavy weights for fewer repetitions at maximum speed with a focus on impeccable technique. Laylor said the approach he advocates isn’t always popular with hockey players who grow up believing a good workout involves “getting bagged” — pushing to exhaustion.

This is an issue that’s also been observed by Scot Prohaska, the southern California trainer whose clients include Leafs goaltender Frederik Andersen. In an interview last year, Prohaska has called it “an epidemic right now in sports: Athletes doing training that just releases endorphins and makes them feel good. Athletes who are addicted to getting tired. If the goal is elite performance, it’s just counter-productive.”

Laylor said there’s a time and place to push an athlete to exhaustion — with NHLers like Subban, Laylor uses a six-week phase immediately preceding training camp to ramp up the cardio. Even then, the heavy breathing is generally left for the conclusion of strength- and speed-building workouts.

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Which is not to say his clients don’t ask for it sooner.

“That endorphin rush (that comes with CrossFit-style exertion) is so powerful,” Laylor said. “But you cannot build true strength in a fatigued state. You cannot build true explosiveness, true power, in a fatigued state.”

You can, however, get hurt.

“You should never get hurt in the gym. That’s our motto — do no harm,” said Daniel Noble, a Vaughan-based trainer whose summertime group includes Leafs forward Mitch Marner. “CrossFit is always on that border of being high risk. I think that’s part of the allure. Because I think some people like the risk.”

Zaitsev, for his part, says he likes the competition that’s inherent in every workout at the Zebra Gym. One of his frequent co-trainees is Andrey Pedan, currently a member of the Pittsburgh Penguins’ AHL affiliate. Zaitsev is listed at six-foot-two, 195 pounds; Pedan at six-foot-five and 213.

“He’s a huge guy. But I always compete with him. I like to do the same weights with him,” Zaitsev said of Pedan. “I don’t like to just lift weights, go home. That’s not interesting. Now (with CrossFit) you compete, you go as fast as you can, do your laps, and it’s a very interesting workout.”

Zaitsev said he is unconcerned with the risks of his chosen regimen. He pointed out that he’s been a professional hockey player for a decade now. During that time he’s trained with a bevy of experts, among them Gary Roberts, the former Maple Leafs forward who has become one of the more sought-after off-season fitness gurus in the sport. So he’s sure he knows what he’s doing.

“If you do it right, it’s not dangerous,” Zaitsev said of CrossFit.

It’s difficult to argue with Zaitsev’s results. Since he arrived in Toronto in 2016 he has spent more time on the ice than any other Maple Leaf. His only notable absence from the lineup came in last year’s playoffs, when he missed the opening two games with a concussion. At the Zebra Gym in Moscow, Nehoroshih might attribute Zaitsev’s durability during the NHL season to his uncompromising work ethic during the off-season.

“I use the elements of what you know as CrossFit training, but I adapt it to the needs of the clients. We understand we have to train hockey players in a way so they don’t get injured in the process. And Nikita understands that,” said Nehoroshih. “Even though the workouts I give (Zaitsev) were very difficult and stressful he always wanted more. Sometimes I would say, ‘Okay, let’s stop here and we’ll continue next training session.’ And he would say, ‘No, let’s do it now.’ ”

Which is not to say, even given Zaitsev’s enthusiasm for those summertime sweatfests, that he is ready to take on the champions at the CrossFit Games.

“I’m not ready yet to compete in that. It’s like, impossible. I’m playing hockey right now. I can’t do that. I will die,” Zaitsev said, smiling.

“These CrossFit guys are crazy.”