The days of lazing about on a beach gaining weight during the offseason have been replaced by the likes of Cristiano Ronaldo, LeBron James and Lewis Hamilton posting shirtless workout clips to their millions of Instagram followers.
And they are not alone -- scores of English Premier League, NBA and NFL players, among others, have jumped on board.
It's certainly a testament to the athletes' commitment to fitness, but should a $100 million superstar working out away from a club's training headquarters be a concern to his team?
Sports strength coach Simon Brundish has worked with a number of Premier League clubs over a 21-year career, and is appalled by some of the videos he has seen of top soccer players working with trainers whose qualifications are a mystery.
"Bad strength training just leads to bad injuries," he told CNN. "If it looks bizarre to you, I promise you we find it very strange and a little bit insulting."
Brundish has compiled a list of notable Premier League players who have raised eyebrows with unusual Instagram workouts, and suggests the trend is a "fairly new phenomenon" which first took off in the US.
"Now European players are starting to follow, and I'm pretty sure it's mainly because of Instagram," he says. "It's cool to be seen jumping ropes on social media."
The most prominent American athlete to have picked up on the Insta-training trend is LeBron James, who has been posting workout clips
for the past few years to his nearly 34 million Instagram followers.
But James' offseason conditioning is monitored by trainer Mike Mancius, who is also employed by the Cleveland Cavaliers. Reporting back to the club is an essential part of his role.
Any personal trainer working outside of club duties "has to be great at communicating. That's the bottom line," Mancias stressed in an exclusive interview
"These teams, these owners, these GMs (general managers), they are the ones investing millions upon millions of dollars in these players' contracts, and so we want to make sure that they are in the right hands of someone with knowledgeable credentials," he said.
However, not every personal trainer believes in working closely with teams. Some prefer operating under the radar with high-profile athletes -- without the knowledge of their clubs
Meet German strength and conditioning coach Moritz Klatten, dubbed "Agent 00Fitness."
The Hamburg native specializes in training boxers, including recent WBA Light Middleweight champion Jack Culcay-Keth, and is the author of The Klatten Power Boxing System
Over the years Klatten has also discreetly trained a number of Bundesliga players in northern Germany.
The situation reached a head in November 2014 when Hamburg played Werder Bremen in the Norderby. Bild Sport reported that Klatten was hired by footballers on both clubs, with headlines referring to him as a "double agent" and "00Fitness."
"I normally never really have a strong relationship with the club; I only have a relationship with the player," the 36-year-old Klatten told CNN. "The players approach me secretly."
Klatten, who says he has trained as many as five members of Hamburg at a time, says players approach him because they are often dissatisfied with their club's strength conditioning.
"You would expect in football, because there is so much money, that the way they train is ideal and everything is perfect. But it's not," he says. "There is so much room for improvement.
"I would say that probably 95% of players will never reach their full potential, because they are not training right.
"There is too much emphasis on long-term endurance work and not enough on strength training and speed work. I have my own methods."
'Best possible care'
Klatten spent last summer working with Tolgay Arslan, a former Hamburg player who is now a two-time league winner with Turkish club Beşiktaş, and says he previously helped rehabilitate former Hamburg player Piotr Trochowski from injury.
Remarkably, Klatten says he has worked out with Hamburg players during the season, and even on the day before big matches -- a stark contrast to most independent trainers who are limited to summers or brief in-season breaks.
"On top of their own training, they train with me," he explains, adding that he is careful not to double up with exercises the team has already put them through.
"(If) I work with him once and he doesn't perform again because of the work, then I lose my job straight away."
When asked to comment on Klatten's relationship with Hamburg, a club spokesman said that no member of the current staff was around when his involvement with club players took place.
The new policies of head coach Markus Gisdol, who took over last year, appear to be more rigid.
"How and if players can work together with a personal trainer is always decided by the head coach," the club said in an email, clarifying that players are allowed to workout externally "only in close consultation" with the team's training and medical staff.
The club added that Hamburg players are "getting the best possible care from us, so nobody perceives this possibility" during this season.
'The unknown is the issue'
Naturally, Klatten is quick to downplay the threat of players getting injured or picking up bad habits on his watch.
"I see way less risk," he says. "When you train them individually, you can really adjust the training much better to their needs and to their actual injuries.
"If it's done professionally then I don't see any risk whatsoever."
In England, West Bromwich Albion's head of physical preparation Paul Caldbeck says his players working out without the team's knowledge is "not an epidemic, but it definitely happens at times."
"The unknown is the issue," he told CNN. "If we are not aware of it, then that is the problem."
Having a "mature discussion" without vilifying the player -- who is ultimately trying to preserve and maximize his body's earning potential -- while discovering what's lacking from the team is key, says Caldbeck.
"We have to understand that these guys are adults and we can't just be derogatory or unhappy with their decisions," he says. "We have to support and manage them accordingly."
The biggest issue for Caldbeck is the lack of adequate insurance taken out by the average personal trainer handling the bodies of multi-million dollar players.
"These guys aren't covered like club staff so it's probably going to blow up in somebody's face at some point," he says, explaining that his staff are personally insured for both the loss of a player's earnings in case of injury, as well as any legal expenses that can come up as a result.
Klatten, incidentally, says his insurance covers up to $20 million in case of accident or injury to his clients.
'Both sides to the argument'
Even in Australia, the debate is alive between football clubs and personal trainers.
The Socceroos' head of sports science, Dr. Craig Duncan, says he has seen players on the Australian national team receive "poor training methods" from their clubs which "can be detrimental to their earning power."
"I can see both sides of the argument," he says. "If you don't have service at the club, why wouldn't you get advice from outside sources?
"Happy players always perform better. People at clubs should just offer wonderful service and the rest will take care of itself."