Rory McIlroy wearing a Whoop band during last year’s US Open at Pebble Beach © USGA/Chris Keane

New Year's resolutions come in all shapes and sizes but for golf's global elite, performing at the peak of their powers has more to do with heart than you might imagine.

Whether they love to pump iron, stretch, squat or perform explosive lifts, everyone from Rory McIlroy, Tiger Woods and Pádraig Harrington to the humble hacker at your club is focussed on finding tools that allow them to track their heart rates and maximise performance.

Sportsmen and women need to know when to go the extra mile and when to rest so they can make the right decisions under pressure and perform.

The fitness marketplace is now bursting with gadgets that measure your heartbeat and tell you how your body is coping and ordinary golfers — the "weekend warriors" as Harrington likes to call them — are now looking to the tools that make the greats great.

The next time you see McIlroy, Justin Thomas, Lee Westwood, Tiger Woods or Jordan Spieth on TV, check their wrists and you may well see them wearing a Whoop band.

It's a device that when used with the accompanying phone app, logs your movements 24 hours a day, monitors your average heart rate, your resting heart rate, your heart rate variability (HRV) and the hours you sleep.

It's designed to measure your activity and come up with what it calls 'day strain' calculation, using heart rate variability and resting heart rate to calculate your "recovery" score and tell you how many hours sleep you might need in order to perform the next day.

McIlroy wore it in most events last year and Harrington, who has tried every golfing gadget known to man, was wearing one at the Turkish Airlines Open to find out what all the fuss was about.

"Whoop seems to have stolen the glory at the moment in terms of marketing," said Harrington, who once owned not one but two heart monitors but now prefers to check his body's readiness to perform by having his blood checked by Galway-based Orreco, who use machine learning and data analytics to optimise performance.

"I used Firstbeat for a quite a while and they were streets ahead in the quality of what they are providing. It is at a different price point and not for the weekend warrior. If I am going to go back to this, I am going back to something that actually measures your heart rate variability rather than using an algorithm.

"If I was tired, I would just override my own system and go and practice. I needed something that was going to tell me that I am going to break down if I do it. So for me, there is nothing more detailed and scientific than Orreco, which looks at the biomarkers in your blood. They are streets ahead.

"Most professional athletes and teams that are worth their salt are using their system, and it is absolutely the way forward. For the weekend warrior, I am sure they will bring something out eventually. But Orreco is the top of the pile when it comes to figuring out how you actually are rather than how you think they are."

Jude O’Reilly with Henrik Stenson at the Masters

Fine-tuning an elite golfer these days has as much to do with their swing coach as it does with the ability to get themselves into a fit state mentally to perform well.

Sleep hygiene is now a major part of a golfer's routine and products like Whoop, or techniques such as HeartMath, are as much a part of their bag of tricks as their clubs or the ubiquitous Trackman.

"Guys are paying a lot of attention to this area and I have helped quite a few guys improve their HRV numbers, simply by doing some breathing exercises," explained Sligo man Jude O'Reilly (50), a qualified HeartMath coach, who caddied for the likes of Christy O'Connor Jnr, Darren Clarke, Masahiro Kuramoto, Shigeki Maruyama and Henrik Stenson during a 12-year career.

"A big thing with golf is the state that you are in. The better state you are in, the more chance you have of performing at your best level. I always felt a big part of my job was to help a player be in as good a state as possible, whether that is happy and joking or serious and focussed. Everyone is different but it was about being in control of that and being in the best possible state.

"If you don't sleep well, it is more likely that emotions are going to affect you. And if you have less access to your emotions, you have less access to all of your brain for a start. If blood is pooling to the centre of the body, it reduces your fine motor skills and your touch at the extremities.

"So when it comes down to putting and chipping, that's where you are going to lose our most, as well as decision making. Being on a better state allows you to have better control over decision making, fine motor skills, and that is enough for a golfer.

"So often, when a player is in 'a good mood', there is a much better chance that they are going to play well.

"If you are led by emotions, you are in a more primitive state. So you see it in varying degrees in other sports. Brian O'Driscoll, for example, was one of the most coherent players there was at the time.

"He was able to see things and it was almost as if thing were happening at a different speed for him, because he had the ability to see things and decipher what was happening and rather than reacting, he was responding.

"Reacting comes from an emotional state. Responding is where you are able to choose how you respond. Your respond-ability, is your ability to respond."

O'Reilly recalls how Ian Woosnam introduced to HeartMath, which is designed to allow athletes to self-regulate their emotions and behaviours to reduce stress, to the 2006 Ryder Cup team.

Few players are as adept at controlling their emotions as Tiger Woods — another Whoop band user — whose Zen-like calm was remarkable before and during the final round the Masters last year.

"He looked like he had a real poise and calmness about him," Paul McGinley remarked in commentary on Sky Sports when analysing Woods' 15th major win. "As emotional as I am sure it was for him on the back nine, he was gaining on the lead and saw the guys in front of him make mistakes.

"He never got out of his comfort zone. He made sure his heart rate was down, he talked a lot about that, and stayed in control of what he was doing. Slowly he reeled those guys in and when he had a chance to put the foot down — on 13, 15 and 16 — he did.”

Breathing techniques, meditation and being in a fit "state" to perform are now part and parcel of the modern game.

Like Harrington, Justin Rose has taken steps to find the best technology available to monitor his heart-rate and fitness to train using an app developed by his performance coach Justin Buckthorp.

Justin Rose during the third round at the 2019 U.S. Open at Pebble Beach Golf Links © USGA/JD Cuban

“I don’t use Whoop but I track my HRV every morning using an app developed by Justin Buckthorp, who has developed his own software and programme through his business, 360 Heath and Performance," Rose said in Turkey last year.

"I tend to spike on the high side when it comes to my HRV. I don't tend to crash but overcompensate. So it tells me where to go with my training.

"It's sometimes easy to go to the gym with willpower and gut through it but to what cost? If you are making smart decisions daily it's all about being in a good zone. But if your physiology is not in a good spot and you go and train heavy, you could be worse the next day, not better for it. Even though mentally you have done something that's better for you, you can make a dent in the wrong direction.

"Knowledge is power but the important thing to know is that you can have a bad score and a great day. If you have a bad week, there might be something underlying - stress or diet or whatever but on any given day, there's a saying I like to use - Psychology can win in the short term, but physiology will win in the long term.”

Science can help golfers find that happy place and that feeling of being in "the zone”.

Shane Lowry found it gradually last year, culminating in his Open winning performance at Royal Portrush after a long learning period alongside his coach, Neil Manchip.

With Manchip's help, Lowry learned to recognise his bad habits in terms of attitude and body language, discovering what worked and what didn't, especially under pressure.

Lowry's happy family life and the comfort level brought about by his change of caddie, allowed him to play the game the way he's always played it best - with rhythm, panache and a smile.

The same can be said of Woods.

"There can be a lot of things in play with a player like Tiger," said O'Reilly, who spent time with Woods' caddie Joe LaCava during last year's ZOZO Championship in Japan

Tiger Woods walks off the third green during the third round at the 2019 U.S. Open at Pebble Beach. © USGA/Michael Reaves

"The bigger picture and perspective are all factors and after talking with Joe about it, it was clear that The Open at Carnoustie in 2018 was massive for Tiger. Carnoustie was a big boost because it triggered what was for him, what golf was about. It was about using your imagination, seeing the shots, using the little banks and the contours, the subtleties of getting totally into the course and the game.

"I don't think he would have won the Masters had he not had that injection of love for the game again. The course were he won the Zozo in Japan, also suited him. It wasn't a course where you could stand up and rip it. He had to plot his way around, place the ball in certain places and use his golfing mind to get around it as effectively as possible.

"If you see golfers get excited like about a course, those of the golfers to watch out for that week. Zach Johnson was like that after his first round at St Andrews in 2015. In his interview with the BBC, he spoke about feeling like a kid in a candy shop."

When it comes to the heart and golf, professionals like O'Reilly try to help golfers get to know. their bodies so they are ready to allow their minds to take advantage.

"It is about allowing those things to happen," he said. "It is about planting seeds in fertile soil. Sometimes the soil is not ready. So it is about being ready to allow that seed to take.

"You have to create the environment to allow things to happen. That's where state and sleep come into things so much more. Having more sleep doesn't mean everything is going to fall into place. But it's interesting that a businessmen like JP McManus is religious about the sleep he gets and the time he goes to bed."

Champions Tour star Scott McCarron (54) has won 11 times since he turned 50 and become religious about using Whoop having learned much from the likes of Woods about how to manage his body during their fishing trips to Ireland earlier in his career.

"There are a couple of things why he was so successful," McCarron said. "One, he knew he was the best player out there. And he knew that you knew he was the best and he intimidated a lot of people.

"He was able to rise to the big moments because he stayed calm in those times. He meditated. He practiced all these techniques. When we went to Ireland, he was studying to become a master diver and there was a lot of breath work for that and for free diving. And because of that, he was holding his breath for up to three minutes or more. So he was able to have those high highs and then have the ability to come down and have that calm.”