Why do we exercise? Is it to be fitter, stronger, faster . . . or just thinner? For women, the answer is always the latter.

From TV to radio programmes, we are nagged to join a gym, while magazines engage in soul-searching on why we can't keep to our promises of losing weight. In January in particular, it's hard to find anywhere that talks about women's exercise without a focus on weight and looks. Even fitness magazines are at it, with cover models who are not just healthily slim, but thin. Women's Fitness this month will help you to "Lose half a stone now!" and get a "Flat tummy fast!" Zest insists you "Get the body you want", which is of course: "A flat tum for life, drop 10 inches fast, be 7lb lighter."

And the gym provides no escape. My sporty sister recently complained: "At inductions they don't seem to understand it when I say I just want to be fitter and stronger – in fact at one induction the guy actually looked at me and told me where I could lose some weight, after I had told him I didn't need to." A friend, who cycles everywhere, plays Frisbee, runs a couple of times a week, plays squash and tennis and enjoys triathlons, was similarly dismayed that gyms seemed stumped by customers who actually enjoyed exercise for its own sake. "The guy at the gym kept asking whether I wanted to lose weight, or inches off my arms. He didn't seem to understand I joined because I thought it would be fun to take some classes with my friends. And when he told me about them he just explained how it would tone me – not whether it was fun." The message to women is clear; what matters is not your health or enjoyment, but your weighing scales.

But for most women who exercise regularly this is a side issue. Instead they rhapsodise about how exercise makes them happier and proud of what their bodies can acheive. One sporty friend tells me: "I feel more energetic, it gives me a clearer head. There is nothing like a lunchtime run for forgetting how annoying your colleagues have been all morning, and getting the day's deadlines in perspective. And I am a genuinely nicer person when I have cycled to work." My colleague Rachel Dixon, who regularly writes about fitness, agrees: "It's the buzz that you get from it that makes it addictive. No one is going to be committed enough to go just to lose weight."

The "get thin" tack hasn't even worked on the 80% of the UK women who are not doing enough exercise to maintain their health, according to Sue Tibballs from campaign group the Women's Sport and Fitness Foundation. "It feels like all the negative energy we put into trying to control our bodies with diets, corsetry and surgery is driven by not liking ourselves – we are really undoing ourselves at the moment by battling our bodies."

Yet when women exercise regularly she says, they become evangelical instead, because their focus has changed from what their body looks like to what they can do.

Men are typically expected to enjoy sport and exercise – from kickabouts to cycling, but for women the idea that exercise is a chore done to improve our looks is omnipresent. When online feminist magazine Jezebel posted an article on how exercise changed the way women thought about their bodies, it was inundated with responses. One poster wrote: "There is an indescribable feeling of power – like a secret I have with myself – knowing that under my clothes there is a strong body, one that can jump, punch, kick, lift, climb, swim, and run really fast."

Tibballs says such attitudes are far from rare even from women who also wouldn't mind losing some weight. "We ran a project called Sweat in the City with 2,000 women aged 15 to 24. They all joined to lose weight, but as they progressed, the desire to lose weight became far less important, even though they didn't lose significant amounts. They felt better and the size they were became less important.

"A friend of mine was training for the marathon and assumed she would lose weight, and it was very frustrating for her when she didn't. She went to see her GP, who said: 'How do you feel?' She said, of course: 'I feel fantastic.' And so the doctor asked: 'So what's the problem?'"