Rory McIlroy was weak.

He was winning some golf tournaments, but ultimately McIlroy realized he needed to upgrade his fitness to stay competitive.

When he started hitting the gym, he struggled.

"I couldn’t stand on one leg for more than 10 seconds and I couldn’t hold a plank for more than 30 seconds," McIlroy says.

That was at the end of 2010. Since then, McIlroy has transformed his body. In doing so, he has also sharpened his mental outlook.

"I’ve always been naturally quite a confident person especially when it comes to my chosen arena, which is that golf course," he says. "That’s where I’m most comfortable, but getting into the gym and building strength has been great -- my posture was terrible when I started, and just having better posture, standing up straight with your shoulders back and your chest out, that gives off the air of confidence.

"That’s one thing the gym has done for me -- you carry yourself better, you feel good about yourself. Mentally it’s a great thing for you as well, as training releases a lot of good endorphins. Training has definitely become a necessity and basically an everyday activity for me – I need to sweat at least once a day to make myself feel good."

McIlroy's enthusiasm for fitness -- and success with four major championships since committing to workouts -- folds in nicely with his Nike sponsorship. Nike Training is producing a new documentary series, which launches Tuesday, to illustrate the unique ways that some of its top athletes train and what motivates them to keep challenging themselves.

With The Masters coming up in less than two weeks, McIlroy was the company's choice to lead off the series. Additionally Nike is hoping to connect with McIlroy's fans by making his path to fitness relatable. His trainer, Dr. Steve McGregor, gave Nike some insight into his approach that weekend warriors and recreational golfers can apply:

  • The one key principle to keep in-mind is range. If individuals have range in their limbs, then they have stability. Without that range and stability, any high-end strength and power work will not only be more difficult to pursue, but it can be less efficient, no matter how hard one works.
  • In basic training, avoid moving too quickly to high-end strength training and focusing on individual body parts; don’t train body parts, train function.

McIlroy's story is also an organic way for Nike to do some cross-promotional marketing for its fitness apparel. One of the booming segments in fashion is athletic gear. In early March, Dick's Sporting Goods made headlines by adding music star Carrie Underwood’s Calia line to its store inventory by reducing the shelf space it had given to Adidas and Reebok. Nike is a leader in this category but competition is prevalent from these brands, plus others such as Under Armour and Lululemon

When McIlroy appears in the documentary about his regimen, most of the time he is clad in Nike workout gear -- shirt, shorts and shoes -- not golfing attire. McIlroy regularly works out five to six times weeks, even during a tournament. Most average people have better chance of working out five to six times a week, even if the activity is something as simple as running around the block, than golfing, so there is also practical element to Nike's series.

Check out more fitness stories on ThePostGame.