One of the occasional hazards of Gary Roberts’ day job involves the friendly competition that arises in the weight room.

Roberts, the former Maple Leafs forward who has built a fitness empire as personal trainer to NHL stars like Connor McDavid and Steven Stamkos, often works out alongside the world-class athletes he trains. While Roberts is still a physical marvel at age 50, his charges are mostly 20-somethings in their uber-explosive primes. Late in the summer then, Roberts partook in a workout pushing weighted sleds alongside his young proteges and tore a calf muscle.

Since then he’s had to endure about three weeks’ on crutches, hours in various therapies and, perhaps most insufferable of all, a told-you-so zinger from his most reluctant client, one Philip Joseph Kessel Jr.

“I get hurt and Phil says to me, ‘See? That’s what happens when you train too hard,’ ” Roberts said with a laugh in a recent phone interview.

As NHL training camps opened this week, Kessel hadn’t played a meaningful shift since he racked up 22 points in 24 playoff games to help the Pittsburgh Penguins win the Stanley Cup. Still, he ranked among the most discussed skaters on the planet, specifically thanks to the infamously barbed tweet he let fly in the moments after the U.S.’s elimination from the World Cup of Hockey at the hands of Canada on Tuesday. It was a social-media firebomb that suggested Kessel was more than a little resentful he wasn’t named to the U.S. roster despite leading the U.S. in scoring at the Sochi Olympics. And on Thursday, the chatter around it continued.

U.S. coach John Tortorella dismissed Kessel’s tweet as “self-serving.” U.S. general manager Dean Lombardi, meanwhile, said he had no regrets about not having Kessel in his lineup.

Said ex-Maple Leafs teammate Nazem Kadri: “You guys in the media never saw it, but Phil’s a funny guy. And I thought he should have made that (U.S.) team, for what it’s worth.”

Roberts, for his part, said it was good to see Kessel stung by his failure to be included in the American mix. Kessel, he said, reminds him of old Calgary Flames road-trip roommate Brett Hull, the Hall of Fame goal-scoring whiz, and not only because both Hull and Kessel are possessed of rare skills.

“They’re also both kind of relaxed — almost too relaxed for some guys, some teammates,” Roberts said.

Too relaxed for some training partners?

“Honestly, Phil is a fun-loving guy. He likes to keep it loose. My job is to keep him on task,” Roberts said. “Is training a challenge for him? Yeah, it’s a challenge.”

To say it’s a challenge doesn’t mean Kessel isn’t absurdly good at it. When it comes to moving barbells and sleds and other heavy implements over short distances, Roberts said Kessel is a gifted marvel.

“Oh, buddy — his power and strength. I would put him up against my No. 1 guy, which is Stamkos,” Roberts said. “We did sled pulls one day. (Kessel) was in a group with McDavid and Stamkos. The first five sled pulls he beat everybody because he has so much power over 40 yards. But then the last five sled pulls, he was last.”

So, Kessel’s power is peerless. And his endurance?

“I say to him, ‘Phil, you’ve just got to put a little more time on the aerobic side,’ ” Roberts said. “That said, the fear is if you do too much of that, then you lose your power. It’s tough to be a really powerful guy and have a lot of endurance. It’s a tough balance. But my point is, if you want to play the rest of your contract (and Kessel has six seasons remaining on a deal that pays him about $8 million (U.S.) annually), then you’ve got to change some things in order to do that.”

Indeed, Kessel turns 29 next month. Roberts figures most players begin to lose their natural speed “around 30 or 32.” But, as Roberts preaches to his players, the inevitable decline can be delayed by diligence, and by adapting the “high-performance lifestyle” of clean diet and smart training Roberts prescribes.

That Kessel realized the value of off-season sweat says something. Roberts said Kessel first approached him about training in the summer of 2015, before the July 1 deal that sent him from Toronto to Pittsburgh.

Last year Kessel put in seven weeks in Roberts’ gym, a little less than the eight or nine weeks Roberts recommends for optimal performance. And this summer, because Kessel spent most of July recuperating from a hand injury, No. 81’s training window was only open for about six weeks, Roberts said, “so he’s going to really have to pick up the slack here in training camp.”

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Roberts, who also played for the Penguins, still has ties to that organization; he spends about five days a month in Pittsburgh, where he heads a training facility named after Mario Lemieux. But now, with his NHL clients back to work, Roberts has the team of therapists and chiropractors that spend the summer tending to his pros tending to him.

“I said, ‘Okay, all my players are gone. Now you guys have got to get me healthy,’ ” Roberts said.

“I’m definitely on the mend. I’m about three weeks away from being back to full strength, hopefully. And I’m getting ready for my hockey season, I’m still playing a little beer-league hockey.”

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