Andre Fergus has just come back from his holidays and has a smile on his face. "How was St Lucia?" a young woman asks him as he begins my tour of the gleaming new Accolade gym in Southampton. "All right, Andre?" a man calls out from inside the steel skeleton of a weights machine. Then, as Fergus is explaining the fire drill, a passing bodybuilder comes up to shake his hand. "Hi Andre, thanks for that tip," says the man. A note of bashfulness creeps into the Fergus smile. Southampton has clearly missed him.

And it soon becomes clear why. Coming from a family of sportsmen and women, Fergus's enthusiasm for his work is infectious and unstoppable. "My uncle played semi-professional football", he says. "Another one of them was a basketball coach, my auntie was a long-distance runner, so it was kind of a natural thing that I would go into sports."

Even as a schoolboy growing up in London, Fergus could usually be found in the gym, working out or practising martial arts. "It's in my blood," he says. "And it was a way to get me off the streets as well, because I was surrounded by a lot of bad influences as a youngster." Instead of picking up antisocial habits, therefore, he was always learning good ones and he could never resist passing them on to others.

"I'm getting paid to do something I would do for free anyway," he admits as we finally sit down together in front of a large, rain-streaked window. "In the gym I walk around saying, 'Excuse me, can I show you this?' 'Can I show you that?' People say that I give away too much information, but I think that personal training is for people who love fitness and want to help people."

As a teenager, Fergus worked part-time in a bodybuilding gym, cleaning toilets and mixing protein shakes in exchange for free membership. Eventually he was taught to do basic induction sessions, and all seemed to be going well. Yet despite appearing destined for a career in health and fitness, he remained reluctant to go full-time.

"People kept on saying to me, You should do this as a job, you're good at it", he remembers. "But about 12 year's ago personal training was really non-existent in this country unless you were at the most elite level, training movie stars or pop celebrities. It was almost unheard of to have a personal trainer in a normal gym."

As a result, though he continued training as a sideline, Fergus chose a more conventional career when he left school. "I did normal jobs,"" he says, "admin-based jobs. But I didn't find them fulfilling. I like being outdoors and having the ability to move around. I got sacked from a few jobs because I wasn't putting in 100%." Finally, he realised he had had enough. "I came to a turning point in around 2003 when I thought, I'm sick of working for other people part-time," he says. "So I did my YMCA training course, and I attended Southampton University and did a sports studies degree. I just thought, 'I'm going to do it myself'. And I've never looked back from there."

Nor, it seems, has he ever stopped taking exams - from sports conditioning and postural correction to time management and small-business skills. "I work harder now that I'm working for myself than I ever did when I worked for anyone else", he says. "And at the start I tried to do everything. I spent hours reading food diaries, which wasn't good time management, but I didn't realise." These days, his knowledge is at such a level that he can even supplement his income by giving lectures and motivational seminars.

"How you doing buddy? Good holiday?" asks a young man passing our table on the way home from his workout. "Yeah, not too bad," comes the reply. Yet, for all his extensive qualifications (as a massage therapist, he must know more about anatomy than most GPs), Fergus is adamant that a personal trainer must be very careful not to exceed their skills. "If I tell you something's wrong with your hip, and I've got it wrong, and start pulling things and moving things, then I can do more harm than good," he explains alarmingly, as I do indeed have a slight hip strain, that I thought invisible, which he spotted the moment I walked into the room.

Despite this - though such remarkable feats of diagnosis cannot help - it is the irksome perception of many people that personal trainers must be superhuman. Eyebrows are raised whenever Fergus bites into a chocolate bar. Business, he knows, would suffer if he spent one week looking even fractionally out of shape. "And the biggest thing I always hear - I wish I had a pound for every time!" he splutters, trying to get the words out, "It's ... But you're a trainer, you shouldn't be tired!" He sends his eyes to the ceiling, and waves hello to another client through the window. "And I'm like, But I've just done a 12-hour day for the last three days!"

Besides being mildly irritating, this assumed perfection of the personal trainer also makes their job more difficult because if we, the tubby masses, think only an Olympic athlete could complete our exercises then we're hardly likely to bother with them ourselves. The solution, Fergus believes, is straightforward: to flaunt his deficiencies. "You have to join in with people now and again and let them see that they might beat you at something," he says. "If you beat me when we run down the road now and again you're going to be much more motivated." I'd absolutely love it, I agree, even if I knew he was putting it on. "Well I try not to put it on." He looks momentarily wounded. "I can't be good at everything."

Even so, the things he must be good at are pretty numerous. The psychology of his clients, for instance, is almost as central to his work as their beating hearts and dodgy hips. "To do this job, you have to understand more than just the training," he says. "You have to understand some key elements of counselling. I've done a counselling certificate and a few basic things along the way to help me listen to people. By nature I'm a happy, jumpy person, so I talk a lot, and because of that you have to learn to listen to what people don't say as much as what they do."

To my surprise, Fergus says he even uses elements of neuro-linguistic programming, a form of motivational psychotherapy, to help his clients think positively. "You can really mess peoples heads up if you sell them a false idea of what they think they're going to do," he explains. "And then they don't get it, and they feel like a failure yet again. "By the same token," he says "he also does everything possible to avoid limiting peoples potential. "My job is to realise that potential, if I think its realistic," he says, and help them fulfil it. If it's not realistic, I have to also be honest and say, "OK, you can't be Mr Universe, but you can be Mr Britain." Mr Guardian, I assure him, would do.

Either way, whether his work with a client succeeds or fails, it can be emotionally charged. "People hug me, people swear at me, people cry", he says, "People say that I made them cry. They make me cry. I get people telling me that I've changed their life. I get people telling me that I'm horrible. But they still keep coming back. Personal training really does mean 'personal.' I care about people, and I want my clients to know that I care about them. One of my clients, Liam, invited me to his wedding. That's the biggest bonus for me, when people treat me like a member of their own family."

Interestingly, through everything he has learned over the years, Fergus now finds himself at the age of 34 with quite different priorities for his own fitness than those he cherished as a teenager. "When I first started training I used to do what all the guys did," he says. "I used to work on the arms and chest real machismo training. But then as I've educated myself I've realised that things like pilates and yoga are just as beneficial if not more beneficial than just doing one style."

These days", he says, the goal is just to keep himself enjoying sport long into his old age, like the men of 79 he still sees playing squash in the gym. "The vanity, you can't keep that going forever," he sighs. "But then," and he leans forward, about to confide something important, "the bodybuilding was never really something I was interested in. I've always felt a bit too self-conscious about getting on a little pair of pants. Here comes that bashful smile again.

Curriculum vitae

Pay Outside London, between £35,000-£45,000 a year, or £35-£40 per individual session. "Above that you tend to be more specialist. I do lectures, so I can make more."

Hours "As a freelancer they' re of my own making. But if you want to make the money you have to put the hours in. This gym, for instance, opens at 6.45am and finishes at 10pm, so they can be long."

Work-life balance "When you're full-on with work you don't have a social life, especially at first. But I've had clients who've invited me out on their boats for the weekend."

Best thing "A client wrote me a letter saying I'd helped them to change their life. When people tell me I've made a difference it lifts my heart."

Worst thing "A lot of trainers don't want to share their secrets, so it can be hard to find people to talk to who know what you're going through."