Feel the burn: The hardcore world of extreme outdoor gyms

If you go down to the woods today...prepare to sweat, burn and have your stomach turned. The latest fitness fad is 'extreme outdoor exercise', and it's not for wimps, says Jamie Merrill.

It's a cold winter afternoon and I'm ankle-deep in mud and struggling to lift a felled tree trunk on to my shoulders. I'm not a tough guy who lifts weights in the gym and the last time I took my dust-covered road bike for a spin around the park was nearly three months ago, but my coach doesn't seem to care. His name is Michael Cohen and he's soon got me down on my hand and knees in the thick, black mud of Ruislip Forest in west London.

The wet, dark stuff is oozing through my fingerless gloves and seeping into my shoes as I attempt what Cohen calls the crocodile stalk. It's a sort of front crawl that relies onf me pulling press-ups at different angles to push myself along. And the recent cold snap means I'm doing it in a near-to-freezing muddy mess. Cohen doesn't look cold, though. He's too busy concentrating on getting me as muddy as possible and inflicting as much pain on me as he can. It's not long before I'm drenched in sweat and dry-heaving.

Cohen, who is 45 but apparently at the peak of fitness, leads me in an hour-and-a-half Wild Forest Gym session, a training programme he offers that follows his 'functional natural movement' approach of improving five core elements of endurance, strength, speed, flexibility and coordination for the mind and body. To the uninitiated, it's a form of muddy torture – and I'm in agony.

In another life, Cohen was a complementary therapist while also dabbling "in property", but last year set up Wild Forest Gym to offer personal coaching to athletes and fitness freaks who compete in endurance and obstacle races with testosterone-pumped names like Tough Mudder, Tough Guy and the Spartan Race. These are extreme endurance races with terrifying obstacles – often set up by former special forces soldiers – and they are rapidly becoming big business. They're not for wimps, and neither, as I'm discovering, is Cohen's training regime.

Perhaps trying to be kind, Cohen starts me off with what he describes as "a gentle warm-up jog" past the ancient oaks of Ruislip Woods, down muddy tracks and across deep bogs.

"If I had to choose a second home, it would be the woods," he says, as I struggle to keep pace. "So many of us are locked away all day in offices behind ours computer screens." That's me, I realise, as I wish I'd been out on my bike a little more in preparation. "Why, then, would we choose to exercise in the sterile and clinical environment of a gym with more screens and air conditioning? Outside you can let yourself go wild, get back to nature and just mess about in the mud. That's why events like Tough Mudder have become so popular."

This link to nature is important to Cohen and each of the exercises he takes me through has a distinctly mucky dimension. From arm-bending pull-ups off dangling tree branches and leaping over logs, to back-breaking crawls and bench-pressing hefty logs, they all get me dirty and tax muscles I only vaguely remember having. This is where his functional natural movement comes into play.

"In the gym, you might repeat a lift countless times in an identical fashion with the aim of increasing the weight or number of repetitions, but that's a far too blinkered view of exercise. In the real world nothing we do is like that," explains Cohen. "In the real world, our physical challenges aren't uniform and part of what I aim to do is help you anticipate the unknown and unknowable elements of sports and day-to-day life, because in the forest you never know how much grip you'll get and when a hidden tree root will bring you down."

In practice, this means Cohen has me lifting a hefty log to my shoulder before dropping it down and picking it up lengthways, then longways. It takes far more energy than a simple lift and my muscles are soon burning. As we switch to a reverse crab crawl Cohen explains that not all of what he does is about strength. "Stamina is important," he says. "As is my focus on balance and dexterity."

I see his point but after a "warm-up jog" and an hour of lifting, jumping and crawling, I've already dry-heaved twice and am starting to trip over myself on the runs between exercises. Cohen's solution is unorthodox: "Imagine you are breathing oxygen into all of the areas of the body where there is pain. Imagine it flowing into your arms and legs where there's pain." I'm not sure there is any science behind this approach, but Cohen seems convinced and it at least takes my mind off the burning pain in my calves and biceps.

Cohen's coaching seems as much emotional as it is physical, and he draws heavily on his complementary therapy background, but the science increasingly suggests he is right about the benefit of exercising outdoors. Last year, a study of older adults found that those who exercised outside did so for longer and more often than those working-out indoors. Other small studies have found that people have lower blood levels of cortisol, a stress-inducing hormone, after exercising outside compared to inside.

Dr Jacqueline Kerr, a professor at the University of California in San Diego, who led the study into outdoor exercise in the elderly, is a big advocate of exercising outside where possible: "There are multiple fields of science that suggest there are benefits from being outside. For example, there is a long history off evidence that suggests you'll recover more quickly in hospital if you have a view of nature. There's also evidence that if children don't play outside they are not as able to use the creative sides of their brains as much."

Perhaps it's no surprise that events like Warrior Dash, Savage Race, Total Warrior and Tough Guy are booming then, but none has matched the marketing magic and scale of Tough Mudder.

Tough Mudder was set up by Will Dean – a British graduate of Harvard Business School and a former Foreign and Commonwealth counter-terrorism officer in the Middle East and Asia – in America in 2010 and quickly spread to the UK and the rest of Europe. In its first year there were three events, but by last year there were 35 events with 460,000 people taking part, 19,000 of them in the UK.

The course is 12 miles long with sadistic obstacles that include ice baths, fire runs, live electrical wires, tunnel crawls and barbed wire. In a piece of marketing genius, they have used names such as Arctic Enema, Ball Shrinker and Electric Eel, and the company, of course, sells images of them on branded T-shirts.

Unsurprisingly, injuries are common, and one event in Wisconsin in 2011 saw 26 participants hospitalised with injuries including a broken femur, a broken neck, multiple dislocations and heat stroke. This explains the four-page legal waiver all participants must sign. The annual Tough Guy event in Staffordshire, which started in 1987 and is seen by many as the inspiration for Tough Mudder, is perhaps even tougher. It's billed as the Safest Most Dangerous Event in the World, but over the years hundreds of people have been treated for hypothermia, and there have been two fatalities.

Why, then, do people want to take part? Sarah Harvey, vice president for Tough Mudder's UK operations, echoes Cohen's thoughts: "People take on Tough Mudder because they are keen to break away from the humdrum of their day-to-day existence and experience something which gets them back to nature, while introducing an element of danger. For me it has as much to do with the concept of being a wild human as it has about getting out of an exercise rut."

From his background, Cohen isn't necessarily the sort of person you'd expect to get involved with such a gruelling event – he competed in Tough Mudder, Tough Guy and the Spartan Race and is also a rock climber and barefoot runner. From the age of 14, when he was diagnosed with major spinal problems, he was, he says, nothing more than "a weak pile of bones, unable to even lift his children" – until he turned his life around in his mid-thirties. He claims this transformation came from practising a form of bio-energy treatment to rewire his body's electrical circuitry.

He describes it as a form of acupuncture without needles. Out in the woods, and struggling to keep pace as he combines all his activities in a final activity exercise, this seems to me a pretty unlikely explanation, but he's the one dangling from a tree branch by his legs while I'm in a heap on the floor. I'm not sure I can argue with him – I'm simply not tough enough.

Visit wildforestgym.wordpress.com

THE TOUGHEST RACES

TOUGH GUY – UK

FOUNDED: 1985

Twenty-five fearsome obstacles over an undulating eight-mile course of mud and freezing water in the Staffordshire countryside. Up to 5,000 competitors take part in what its founder describes as the "spiritual home of mud and obstacle running" that battles "the lethargy that's condemning generations to the nanny state".

SPARTAN RACE – US, UK, CANADA, MEXICO

FOUNDED: 2010

The Spartan Race prides itself on offering pain in all shapes and sizes with everything from three-mile sprints (agony) to the 26-mile Spartan Ultra Beast (practically suicidal), which includes spear-throwing and all the usual fun and games with ice, mud and razor-sharp wire. Unsurprisingly, it has a 10 per cent completion rate.

TOUGH MUDDER – US, UK, AUSTRALIA, CANADA, GERMANY

FOUNDED: 2010

A 12-mile obstacle course featuring mud, ice, baths, barbed wire and electric shocks, Tough Mudder donates to veterans' charities including Help for Heroes in the UK. If you make it to the finish you are rewarded with free beer and rock music.