Britney Spears loves this unusual fitness trick - but experts don't recommend it.

Like many famous women, Britney Spears‘ weight has been constant headline fodder over the years.

After multiple ups and downs (unfortunately noted and analysed by anybody and everybody), it’s great to see the singer looking happy and healthy. After taking to the stage at the Billboard Awards looking fabulous in what was frankly a KICK-ASS sheer cut-out black bodysuit, it’s no surprise people are eager to know how she keeps fit.

Her trainer Tony Martinez (who also trains Janet Jackson and Pink) has shared her workout secrets, but there’s one activity in particular that has fitness experts seriously concerned. (Post continues after gallery).

In preparation for Spears’ ongoing Las Vegas residency, Martinez revealed he made the 33 year-old run backwards during the ‘intense’ 40 minute workouts they did three to four times a week.

“She loves to run, but I would make her go backwards,” he told the Daily Mail.

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“She’s like ‘I’m gonna go backwards?’ and I’d be like ‘Yeah let’s go on the treadmill and walk backwards.’ She’d say ‘I could lose my hamstrings and my glutes – what are you doing to me!?'”, he said.

While this may sound like just another quirky, relatively innocent fitness trick, trainer Mark Moon says to think twice before trying it during your next gym session.

"It isn't something I would advise doing, unless you had a specific purpose for needing to improve your backward step. Maybe Britney was looking to improve her backward stepping for some upcoming dance routine that is yet to be revealed?" he says.

As you've probably guessed, walking backwards on a treadmill could end badly.

"For a fit person with good balance and coordination, it wouldn't be extremely dangerous, however I wouldn't put it in the safe zone either. For the average person, walking backwards would put you off balance and increase your chance of falling, twisting an ankle, or worse still looking like a twat in front of the other gym bunnies," Moon says.

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Health and fitness expert of FitFantastic, Ali Cavill, believes the technique poses a number of risks.

"I wouldn't recommend it for a number of safety issues. Gym equipment should be used for recommended purposes which is to simulate running or jogging without going outside," she says.

"If you're walking backwards, you don't have the emergency stop button in front of you and it's much harder to monitor the speed and incline."

"There's also the danger of falling backwards. It's dangerous falling any way on the treadmill, but you'd fall into the screen and again, it's harder to stop if you did fall because you can't easily reach the emergency stop button," she warns.

"When you're walking backwards you're landing differently on your feet, and it's also harder to build up speed, which makes it harder to get your heart up to the working zone to actually make it effective."

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"Given she's a role model, I think it's a dangerous practise to promote. You can do whatever you want at home, but I wouldn't advise it at gyms and if I saw it at mine, I'd definitely ask the person to stop," she says.

If you're looking to make your treadmill workout more intense, Cavill recommends turning up the incline or speed to do sprints with interval training.

"This will give your workout a kickstart - short, sharp changes makes the exercise harder and keeps the body alert as it has to adapt," she says. (Post continues after gallery.)

Cavill says while Youtube might be filled with people doing gymnastics and acrobats on the treadmill and it's amazing to watch, it's not advisable in real life.

"A lot of those crazy practises are to do with fancy footwork and speed but even those are generally facing forwards!," she says.

That said, walking backwards (not on a treadmill!) does have its benefits.

"If I was to break it down from a technical point of view I could see two points that may have benefit to Britney's training regime. Firstly,  proprioception and balance for some of her dance choreography would be improved. Secondly the muscular focus would place more emphasis on the quadriceps (front) of the legs. Having said that, there are far better ways to focus on the quads," says Moon. (Post continues after video.)

"Sorry ladies, you're NOT going to get Britney's booty by walking backwards on the treadmill," he adds.

"Walking backwards and in different directions certainly does have benefits, as you're using different muscles and it's good for shaking up your workout, but it's dangerous on the treadmill," Cavill agrees.

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"If you were looking to incorporate it into your workout, then you can try doing what I do with my clients by jogging or walking backwards and including hills or speed, but you've got to be so careful on equipment."

As well as revealing Spears' prowess at tennis and dedicated attitude ("She loves all core workouts, she's a very tenacious women. She wants to get it done and get it done right," he says), trainer Martinez shared how he's changed the singer's workout as she's grown older.

"I don't make it as strenuous as when they were younger. So I basically do a little more rehab as well too — shoulders, the knees, hips, back, and a lot more stretching as well," he explains.

These adaptations are important, says Moon.

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"As we get older it's important to remember that maintaining lean muscle is your ultimate goal. It will keep your body fat percentage down, keep your metabolism up, and keep celluilite at bay," he says.

"Before you choose any exercises for your program, you need to answer three questions: 1. What do I want to achieve? 2. Is this exercise most effective to achieve desired results? 3. Do the results far outweigh the risks?"

"We also need to remember that as we get older, we have more wear and tear in our bodies which can make us more susceptible to injuries. Choose exercises that will give you the most reward for the least amount of effort," he says.

Cavill recommends doing more weight-based activities rather than cardio as it becomes harder to shift weight as you get older.

"You also have to change it up more and ensure you're doing adequate stretching and allowing recovery time," she says.

"It's better to do shorter and more regular exercise - it will be more effective."

What's your favourite way to work out?

Check out some of The Glow team's favourite body image quotes from celebs we love.

Do use the treadmill in your workout? How have you changed your workout as you've grown older?