IT is Janaury and dark and the majority are swaddled in a little extra layer of fat, given festivities just past.

Fitness classes are filled with the newly-resolved, ditto diet clubs and the vegetable aisles of supermarkets.

It's the ideal time to launch a campaign to encourage exercise, said the cynic in me when I heard of Sport England's new campaign, This Girl Can. "But can women?" was my first, churlish, thought.

Until I saw the campaign advert. It opens with a woman in a bikini striding purposefully towards a swimming pool and pausing only long enough to run a finger under each side of the bottoms and snap the swimsuit elastic off her bum.

That smack serves as the downbeat to Missy Elliot's Get Ur Freak On, the soundtrack to 90 seconds of women of all ages and shapes, colours and sizes, dancing, boxing, running, swimming, sweating, cycling and kicking footballs. Blue eyeliner smudges under exhausted eyes, sweat-spiked fringes whip back and forth, and bellies, boobs and thighs wibble unrestrained. "Sweating like a pig, feeling like a fox" and "I'm slow, but I'm lapping everyone on the couch," run the straplines. The women are hot and not bothered. It is quite spectacular.

Watching the advert it occurs that I don't think I've ever seen cellulite on television before. I definitely have not seen it in such an unabashed, unashamed, rippling way. Unless you take part in sport you probably don't know what other women's bodies look like, beyond the sanitised selection, an almost alien species, packaged and presented in women's magazines. Another positive for taking part.

Sport England found that two million fewer women than men are regularly taking part in sport or exercise, despite 75% of those aged 14 to 40 saying they would like to do more.

We are not doing particularly well when it comes to encouraging women into sport: the UK ranks third in Europe for numbers of men playing sport yet only 19th for women.

Sport England has set about to try to change this. Focus groups, research and talking to women revealed that they were stopped from taking part for fear of judgement: judgement about their ability, their looks, their figures, their fitness levels.

The women in the advert, who have been chosen to be sport ambassadors, speak of how difficult it was for them to begin exercising, given their size, their confidence levels and how they feel other people will feel about them.

These are all things most women can sympathise with. It's easy to become locked in a cycle of inactivity where the fact you are inactive becomes justification itself for the inactivity. Last year I just didn't have time to go to my regular class and the weight crept on. Soon the weight, and the fact I was out of shape, became my excuse for not going back. I had to somehow get fit before going to back to the class that had been keeping me fit.

The logic is nonsense, but the thought of appearing in a leotard in a room full of lithe, limber women is an act of bravery too far.

And I hate that it is so but women can be their own worst enemies. The fear of judgement is quite real but is judgement the reality? The cliche is that all-women exercise classes are cliquey. It's hard to go unless you have a chum to go with. I have been to classes where no one speaks to you, the in-class groups are established and it's tought being the new girl.

Maybe part of This Girl Can could be an onus on women to encourage other women to get and stay involved. After all, sport is as much about teamwork and solidarity as it is physical fitness. It reduces stress, gives us a break from constant responsibilities and cements friendships.

The Chief Medical Officer has described inactivity as Scotland's fourth biggest killer, a direct cause of 2,500 deaths a year at a cost of £660 million to the public purse - the positive response to This Girl Can is something Sport Scotland should keep a close watch on.