How did Karl Henry, Operation Transformation's resident fitness expert, spend his Christmas break? Lots of surfing and paddle boarding", he told RTÉ Lifestyle, after weeks of busy work in the lead up to Christmas and a new series of the show looming in the new year.

Operation Transformation is back on our screens and inspiring viewers across the country, as well as spreading the word on how to measure health in the long run.

The ever-active personal trainer and podcast host practices what he preaches, but wasted no time in getting stuck into the show, which brought with it an innovative way of testing its five fearless leaders.

"You're always trying to bring it in line with science, with what's gone on generally on a global perspective. This year we've done that with DCU, with the health related fitness testing that we're doing."

These tests are five practical and accessible exercises that are available on the Operation Transformation site and which viewers can do alongside the leaders, measuring their progress as they go along.

"We're seeing already the impact of the training programme on the testing because we retested the leaders after five weeks and like we've seen or after four weeks and we've seen really big progress", Henry said.

"We have 14 indicators of health, more than we've ever had. And it's just lovely to talk to people about that, about hydration, about muscle, about fat, about all about visceral fat, about all the different indicators of health. And the show gives you the chance to be able to do that."

These exercises are designed to be tested over time, Henry added. "We know that if you score a low, you're at a higher risk of trips or falls or hazards, you're going to age quicker, your bone density is going to reduce quicker, you're going to lose muscle mass quicker, you're going to age quicker."

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Rather than elaborate HIIT routines or demanding long runs, Henry says these tests and routines are intentionally accessible. "It has to be practical", he said. "A lot of people miss that when they watch the show, if they're criticising it, they miss the fact that it has to be practical. We don't use equipment, it has to be free, it doesn't have to be done in the gym."

Operation Transformation is arguably more about lifestyle habits, whether it's cutting out smoking or getting into the habit of walking somewhere instead of driving. This came to the fore in a recent episode where leader Stephanie was asked to hand over her fitness tracker as she was becoming too fixated on progress.

"Trackers do help, right? And they do make it easier to measure, so but it is important that if you become over-reliant, that's not healthy either", Henry said. "It's about balance. What gets measured gets changed, so you do have to measure yourself, but if you're over-aligned, just like Stephanie was, it's time to ditch the tracker and just trust your body a little bit better too."

Although many of us turn to trackers to aid our progress and meet our goals, Henry says that "any obsession is unhealthy", and that can apply to smart devices. "Knowing how many steps you're doing in a day, that's very beneficial. Knowing what your heart rate is, that's beneficial. But over-checking, it's an overreliance on something. From a long-term perspective, it tends not to lead to health."

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One area that Henry encourages more people to be aware of is rest, and particularly good quality sleep. "Rest and sleep particularly are the foundation of health", he says. "When you exercise you need to recover so that the benefits kick in, so rest will do that."

Understanding and listening to your body when exercising is massively important, he says, and requires "knowing the difference between exercise and movement". "Movement is lower intensity and exercise is higher intensity and having adequate rest and recovery."

So how do you know if you're over-training or your body is in need of a break? Henry shared his warning signs.

"If a session becomes unusually hard if you're getting sore, an unusual amount after a session. If you go out for a run and you're just not feeling it, turn back and go home and rest and put your feet up on the couch. An increase in resting heart rate can be a sign of overtraining, lack of sleep, changes in appetite, all signs of overtraining. And you just pull back and recover a little bit more and let the body rest."

Watch Operation Transformation on Wednesdays at 9:30pm on RTÉ One.