The weather is heating up, and so is our desire to get into shape.

But before you jump to the gym, heed this warning: according to personal trainer Paul Baird "cross fit, Bikram yoga, and all these new types of fitness things that are popping up, do pose a risk of heart attack and cardiac arrest."

Sisters Susan Doggett and Emma Putaura learnt this the hard way, when Doggett's heart stopped while they were working out.

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Doggett was on a treadmill when her leg went out from underneath her, and she fell backwards and hit her head.

"There was a blockage in my blood vessel. So when I was doing exercise it was trying to push more blood to the heart, but it couldn't get through this vessel, so obviously it expanded and it couldn't push through anymore," Doggett said.

The gym didn't have a defibrillator at the time, so CPR was performed on the 28-year-old for fifteen minutes.

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There are no laws requiring gyms and personal training studios to provide defibrillator. Currently around one in two independent gyms have them, but Theron Vassiliou from St John's Ambulance Victoria says that's not enough.

"Sudden cardiac arrest can happen to anyone at any time," Vassiliou said.

"If you have access and are treated with a defib you have a 95 per cent chance of living through that event. Without a defib your chances are as low as five per cent."

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There is a recommendation of how hard you work yourself. The basic rule is your maximum heart beat should be 220 minus your age. For example, if you're 40, you shouldn't go over 180 beats per minute.

According to trainer Baird "if you do step over your maximum theoretical heart rate of your body, obviously if you've never done that before in your life, your heart's gonna have a bit of a shock."

Baird claims that extreme fads like Tough Mudder, Cross Fit and Bikram yoga can be minefields for certain people.

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"People that are more likely to have cardiac arrest or injury are overweight, or old, or haven't trained in a long time," Baird said.

"They need to know if they are at risk and just take it one step at a time, take it nice and slow, and just listen to their body."

Is your gym 100 per cent prepared for emergencies? Find out more at www.stjohnvic.com.au.

This reporter is on Twitter at @tinekae