Mention the word “bodybuilding” to most people and it conjures up a certain image: big men in small underpants, a comic world of posing, protein shakes and pumping iron. And in Pudsey Civic Hall, where the International Bodybuilding & Fitness Association (IBFA)’s Mr England championships are being held, that stereotypical view is out in force. First, Wakefield’s Steve Johnson, a former Mr Britain and a name on the bodybuilding scene, steps on stage; a man mountain in small Speedos, he flexes his muscles in the spotlight. From there, the flesh parade continues, all popping veins and rippling muscles, treading a fine line between deathly seriousness and cartoonish parody, as the junior category, followed by the over-40s, the over-50s and the men’s first-timers, all take to the stage in slivers of Lycra and fake tans the colour of Victorian sideboards.

The competitors, judges and the 200-strong audience, most of whom are bodybuilding obsessives, take it all very seriously. Then the next group of men step on stage: they have the same deep tans, muscles big enough and clothing small enough to make the average person wilt with embarrassment. But there’s something different about these four men that causes the applause to resound a little more loudly. This is the disability class of the Mr England competition, the first time this section of the contest has been held.

Standing at the far left of the stage is Mark Smith, a 30-year-old former Grenadier Guard who has muscles as prominent as the bright white smile he flashes when he contorts his body into a pose that showcases his biceps. Look below the waist and you’ll see one well-defined leg. On the other side, stretching from underneath his green trunks, a glistening metal leg is affixed near the hip.

Smith was a career soldier, having joined the army aged 18. Six months’ basic training at Catterick in North Yorkshire was followed by six months in London, completing ceremonial duties. As a 19-year-old based in the West End, Smith had his fun, guarding the royal palaces and participating in trooping the colour.

Smith has his final layer of fake tan applied. Photograph: Abbie Trayler-Smith/The Guardian

His first tour was to Bosnia, in September 2004, and he spent Christmas away from home. He didn’t mind much: “It was all new to me,” Smith says. “I was enjoying it, and the independence.” Two years later he was in Baghdad and Basra, the Falklands and Kenya. In 2009, he went to Afghanistan to serve on the frontline. After that, he was sent to Canada for pre-deployment training.

In 2011, he was acting as a safety staff member for a live-fire exercise with the Yorkshire Regiment in Canada. He was standing behind the wooden wall of a building where soldiers were practising clearing out a village of enemy fighters, when seven bullets hit his leg and shoulder. Smith had been in the wrong place at the wrong time. One bullet passed through an artery.

He was rushed to intensive care in a Calgary hospital, where he was resuscitated six times in a week, underwent a number of operations and had his right leg amputated above the knee. Stabilised, he was flown back to the UK, where his weight dropped to 60kg (9st 6lb) as he underwent more than 20 operations at the Queen Elizabeth hospital in Birmingham, watched over by his wife, Natalie, a schoolteacher, and his eldest son, Ellis, then just six months old.

It was during Smith’s 10-week stay in hospital that reality kicked in. “I was overly optimistic and naive when I had to have my leg amputated,” he says. At that time, he had just one goal in his head: to stay in the army, get back to where he was before, then climb up the promotional ladder, and even go back on tour. “I’d joined from school, so the army was almost all I’d really known,” Smith says. Now, he realised, things had changed. “Once my rehabilitation was done, that was my time in the army done.”

Smith had spent a decade in the military and had the drive and competitive spirit that come with a career spent dodging bullets. Physical activity had been a major part of his life, and suddenly he was faced with a life that looked entirely sedentary.

Smith with his eldest son during his recuperation. Photograph: courtesy Mark Smith

According to Ian Waller, operations director of Blesma, a charity for limbless veterans, Smith’s was a common experience. “To have done well in the military, soldiers will have had to have been competitive,” Waller says. “They’re fit young men and women, used to running, swimming, playing sport. That’s what you do in your downtime in the military. They want to continue with their life.”

Waller’s team have seen a huge increase in the numbers of soldiers needing support since the wars in Iraq and Afghanistan. “We’ve had 340 new injured veterans [since 2001], and support 3,600 in all,” he says. The increasing potency of weapons, including improvised explosive devices, coupled with medical advances, means 97 of that 340 are double amputees; a further 19 have lost three limbs.

Most of the veterans Waller works with go into cycling, rowing or swimming, but although Smith tried all these, it was bodybuilding that stuck. “I tried disabled sports,” he says, “and they weren’t quite filling that void. Bodybuilding was the one thing.” Smith has a theory about this: army life gave him routine and discipline – particularly in the Grenadier Guards. “Coming from quite a disciplined regiment, where everything’s about your turnout, your appearance, there’s a lot of routine, you’re physically fit. Bodybuilding is quite a similar lifestyle.”

There was another reason he took up the sport: the family photographs taken during his recovery, when he was gaunt and pale. “I wanted to get as far away as possible from looking as ill as I did when I was in hospital,” he says. So he started training – hours each day, straining and making small improvements. Over the course of a year, he trained day and night, and ended up with a physique that could hold its own in the hypercompetitive ranks of bodybuilding. Training is intense, with 5am starts for 16 weeks before a competition, but Natalie has been an important source of support. “She just likes the fact that I’ve found something I’m passionate about now.”

There have been moments of doubt. Backstage at his first competition, in November 2014, Smith says, “I was thinking, ‘Do I really want to do this? Am I prepared to stand up in front of all these people?’” Everyone’s an armchair expert, he says, and their comments aren’t always discreetly made: they stand in the crowd, nitpicking over competitors’ muscle mass, picking up on the slightest imperfections. Still, he went through with it, and the reception was invigorating. He stepped off stage asking himself one question: when was the next one?

Since then, Smith has taken part in nine events, winning five separate competitions. He is the only veteran competing in the disability category at the Mr England championship.

Smith relaxes before the show, draining excess water from his leg. Photograph: Abbie Trayler-Smith/The Guardian

The main stumbling block to his success so far? The strict diet. It’s not just counting calories: the food required to get a bodybuilder’s body is infuriatingly bland, all turkey and brown rice. It makes competitors crave something sweet, tasty and salty. Smith dreams of chocolate and peanut butter now, and will wolf down around £40 of Domino’s pizza when he gets home after a competition. But as he sits silently in a dressing room with a dozen other bodybuilders, many lying on the floor with their legs up on chairs in an attempt to drain the water that gathers in their calves, he’s running a fork through a Tupperware box full of dry chicken and couscous.

Sitting next to him is Josh Goodfellow, a rangy, outgoing 23-year-old who is busy engaging the others in conversation. Goodfellow has cerebral palsy, and is not walking well, in large part due to the torn hip flexor he sustained while competing in a Tough Mudder competition six weeks earlier. He’s been unable to work out since, and has lost nearly 9kg (19lb) in five weeks as a result. But he’s happy to be here, and pleased that today someone will be crowned Mr Disability England. “Fifteen months ago, this wasn’t a sport; it didn’t exist,” he says. He and Smith have taken it upon themselves to call British bodybuilding promoters – there are at least 10 in this factionalised world – and ask them to consider setting aside a section of each of their events for disabled bodybuilders. Some said yes, some said no. From no competition, there was one. Then last year, there were 20 events with disabled representation.

“We’re looking at it,” says Martyn Yates Brown, UK president of the IBFA. Barrel-chested and wearing a branded tie, he says there are challenges in admitting disabled athletes. “How do you compare and how do you compete? How do you make the criteria fair? The rules have to be written up: where do you put amputees, where do you put cystic fibrosis sufferers?” There are practical considerations, too: an extra category requires more time set aside during competitions. But it’s worthwhile, Yates Brown says. “We want to encourage people to compete. There’s nothing like getting people up on stage, getting that reception.”

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One person about to experience that reception for the first time is Peter Copsey, a mountain of a man with a bushy beard, dark tan and prominent veins, who is stretching out with Smith and Goodfellow in an empty changing room. A former powerlifter, Copsey has been working out for decades – he’s 51 and has four children – but this is his first competition. He had never told anyone at the gym about his disability, spina bifida, until he read about Smith and Goodfellow on social media. Then something in him changed. “It’s taken all these years of hiding to realise that I had nothing to hide,” he says. “And that’s because I saw Josh and Mark.”

Josh Goodfellow (left) and Smith prepare for the competition. Photograph: Abbie Trayler-Smith/The Guardian

He started training, asked his wife to apply the requisite layers of fake tan, and entered the contest. “Now’s a good time for me to say, ‘Look, this is me. Here we are,’” Copsey says, after completing a series of push-ups. “And it’s nice that there are guys like this who I can talk to freely about my disability. It’s not the same for able-bodied bodybuilders. Don’t get me wrong – they go through the same amount of pain and prep – but with us… there’s added bits.”

The two men who inspired him are now swigging booze from bottles (every competitor is given a goody bag that contains a miniature bottle of Irish whiskey: it helps dehydrate the body, bringing out muscle definition, but it also gives much-needed dutch courage to men about to step onstage in bathing suits in front of a few hundred people).

Cardio is difficult for Smith. He can’t run around the roads near his home, so he walks before his children wake every weekday morning. (His second son, Ethan, was born in 2012.) Yet he can’t do two lots of cardio a day, a standard routine for bodybuilders: the sweat such exercise causes aggravates an exit wound near his groin. One summer, after overdoing the exercise, he ended up with a hole in his body through which you could see the tendons.

A man pokes his head around the changing room door as Smith, Goodfellow and Copsey pump up their muscles in the final moments before going on stage, holding their noses inches above the ground in press-up position. Lined up with a fourth competitor, Matthew Leake, the members of the disability category make their way on stage. There, they run through a series of mandatory poses, called out by a monotone voice. Straining, gurning and visibly sweating, the men turn their backs to the audience. Tattooed across Smith’s back is the closing couplet of William Ernest Henley’s poem Invictus: “I am the master of my fate, I am the captain of my soul.”

Bodybuilding involves months of dieting, weeks of preparation, hours of sitting around before a competition, and mere minutes of posing. The judges confer – and their decision is surprising. The winner today isn’t Josh Goodfellow, and it isn’t Mark Smith: Peter Copsey is Mr Disability England, at his first competition. Backstage, the losing competitors are gallant, shaking Copsey’s hand and chatting as they pack up their bags, praising him for the muscle definition he has honed over decades of work.

The competitors pose after the competition (from left): winner Peter Copsey, Matthew Leake, Mark Smith and Josh Goodfellow. Photograph: Abbie Trayler-Smith/The Guardian

After Smith has glugged down a bottle of water and towelled himself down, he moves into the audience and spends the next half-hour talking to a former member of the Royal Engineers and a double leg amputee with whom he struck up a friendship at Headley Court, the Surrey hospital for the rehabilitation of injured veterans. The baby-faced double amputee is toying with the idea of competing next year, and is one of several ex-servicemen with whom Smith is in touch. Some send him photographs and videos of their work-outs, updating him on their progress towards peak physical fitness.

Smith is keen to see his two children, to let them play with the heavy Adonis trophy he got for coming second, and to tell them about the competition. But as he walks to his VW Touareg, ready for the long drive home to Milton Keynes, he pauses to sum up his day.

“To have inspired Peter not to hide his disability, to say proudly, ‘I’m a disabled bodybuilder’, that’s an achievement in itself. Obviously I’m disappointed, but to see Peter here today is just as much of a success. Hopefully more people will come out like that.”

For Smith, the goal is twofold: first, to establish disabled bodybuilding as a professional category, rather than an amateur one. That would in turn fulfil his second goal: to make a new career out of bodybuilding. “Within five years, I think we’ll be a professional category, and then you’re looking at an income,” he says, squinting into the sun.

There’s a hard road ahead, full of long drives, physical exertion and discipline. But compared with the journey that Mark Smith has already made since 2011, it doesn’t seem insurmountable.