IT starts with the Braveheart Charge; a demented dash up a muddy incline towards the first of a series of obstacles that will test both the physical and mental fortitude of the men and women brave, or perhaps stupid, enough to take them on.

Awaiting them on World's Toughest Mudder course are the Arctic Enema, Devil's Beard and Dirty Ballerina, then the Electric Eel, Fire Walker and Kiss of Mud. It might sound hyperbolic and, frankly, a little silly but consider this: each competitor must sign a death waiver before being allowed to take part.

Looking at Katy Emslie, it is difficult to imagine the lithe 23-year-old thriving in such surroundings, yet tomorrow she will stand on the start line in New Jersey having earned her place in the most rigorous examination of human spirit anywhere in the world.

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Last year, only 90% of the entrants completed the 24-hour event, with broken limbs and hypothermia accounting for scores of those who took part. Indeed, even the winner of the female category suffered frostbite in two toes, having had to break the ice on the surface of a lake before plunging in to its icy depths. "Yeah, but that was in December and it's only November at the moment," protests the Banchory medical student, the only British woman in this year's field, which will also contain training partner and work colleague David Weir-McCall.

That attitude is, in part, informed by her success in the Scottish Tough Mudder in July but also by the motivation behind her entry. Having lost step-father James Duguid – who was just 42 – to cancer four years ago, Emslie was eager to raise money for cancer research but felt that a marathon was beyond her. Instead, she searched for something more extreme. Inspiration came while procrastinating during exam preparations when she stumbled upon video clips of Tough Mudder.

Within minutes she had registered and a few weeks later, she was among the fastest 5% of competitors around a single circuit of the 12-mile, 22-obstacle course in Dumfriesshire. "I was nervous but once I started I just zoned out and kept running. My mum, Anne, was volunteering and after I crossed the finish line she came over and handed me my T-shirt and it was only then I realised it was over. But that was nothing compared to this, though . . ."

Emslie's not wrong. The lap might be slightly shorter but will be more heavily pock-marked, with competitors expected to complete the course as many times as they can between 10am tomorrow and the same time on Sunday. A pit area will be set aside where they can pitch a tent in order to sleep, eat and warm up as they deem necessary, but medical checks will take place after each lap to ensure they are fit to continue. The victor is whoever finishes the circuit the most times, but the ethos behind the event is that it is a personal challenge rather than a race; helping your fellow "Mudder" succeed is as important as doing so yourself.

Founded by Will Dean, a former counter-terrorism agent for the Westminster Government who found himself bored by the sporting challenges on offer while studying for an MBA at Harvard, it has attracted half a million participants from around the world in the past two years, all energised by the prospect of exposing themselves to mud, fire, ice and electricity.

There are not enough forms in the entire Health and Safety Executive to start assessing the risks for, as well as the usual monkey bars and cargo nets, Tough Mudder boasts some of the most jaw-dropping obstacles ever conceived. How about clambering over bales of hay with pitchforks protruding from them? Or running through a trench of blazing, kerosene-soaked straw while choking on thick smoke? Or maybe crawling commando-style under barbed wire set eight inches from the ground? Then there are the live wires, some carrying as much as 10,000 volts, dangling inches above a shallow pool of icy water and the hundreds of sparking cables hanging like a beaded curtain across a muddy path.

"Yeah, it's just the usual stuff really," Emslie says, giggling. "They don't actually release any details about the course so I'll see it for the first time on the start line. I've looked at last year's one and that looked pretty bad but I've done the training and I reckon if you approach it properly and make sure you're not stupid about it, then it should be okay. At least I hope so, anyway."

Emslie can also rely on the memory of her step-father to keep her pushing though the pain during the dark moments. "After what my family has been through over the past few years, I believe life is for living," she explains. "I try to seize every day and I don't believe in long lies; 6:30am is a late start for me now. You don't get long in this life so you've got to make the most of it and take whatever opportunities, like this one, come your way."