Next time you're searching for the perfect pair of trainers, take our handy cheat sheet along. Your feet will thank you from the bottom of their soles

From winklepickers to stilettos to six-inch platforms, the dictates of fashion have not always been kind to our feet. When it comes to buying trainers, too, it's easy to be seduced by gimmicks, gadgets and go-faster stripes.

The quest? To find a perfect match between your fragile feet and your chosen sport. Just as our physiques vary, so different activities make different demands. So, it's essential to buy shoes that are designed for a particular purpose. These can seem expensive compared to everyday footwear but, before you even walk into a shop, remember this: the price of a good pair of trainers is a fraction of the cost of a very average physiotherapist.

Lateral thinking
Let's start on the court. Tennis, squash and badminton involve frequent side-to-side motion, so shoes for racquet sports must provide lateral stability, support the ankles and have relatively firm, flat soles: a built-up heel would make them unstable when you're moving quickly. A quarter of all ankle ligament injuries are sports-related, and most ofthese are due to wearing ill-fitting or unsuitable shoes.

Straight walking
Running and walking, on the other hand, are generally done in a straight line, therefore lateral stability is less important. Key features here are shock absorption and devices to correct the idiosyncrasies of our running or walking styles. Devoted walkers should really buy a walking-specific shoe, if possible.

Unshockable
Aerobics involves both lateral and straight-line movement, so shoes need to be strong, stable, low-heeled and offer adequate shock absorption for high-impact work. Companies like Reebok have always designed specialist aerobics shoes for women, but men wanting to perfect their grapevines have less choice and should choose a cross-trainer.

Cross-trainers are all-purpose shoes for both sexes - supportive, stable but comparatively heavy, and they're suitable for the gym and for exercise-to-music classes. However, they're absolutely not designed for running. 'If you're going to run for more than 15 minutes at a time, a pair of specialist shoes is essential,' says Max Bower, assistant programme director at Central YMCA in London.

When time runs out . . .
How do you know when trainers are at the end of their useful life? After all, they may still look in reasonable condition, with little evidence of wear on the sole. However, the midsole - the important bit that contains shock-absorbing material - degrades every time the shoes are worn. Considering runners create a load of up to five times their own body weight each stride, and aerobics devotees up to eight times, it's easy to see why skimping on shoes is a false economy. Running shoes need replacing every 500 miles - which, for a 20-miles-a-week athlete, means twice a year. Cross-trainers used two or three times a week will last for around 100 hours of exercise or nine months. If you still need convincing, try one old trainer and one new trainer in the shop - you'll feel the difference in cushioning and support.

Get in line
Wear patterns on shoes do, however, have a use. They reveal the condition of your feet, how you move and how your body may be out of alignment. This is particularly important when buying running shoes, many of which carry correcting features. 'Asymmetry in wear reflects asymmetry in function,' says Yorkshire-based exercise consultant, Annette Burgess. 'One leg may be slightly longer than the other, or one foot may excessively pronate, where the heel hits the ground and rolls inwards, or it may supinate, where the heel rolls outwards.' Wear on the outer heel is normal, but if the inner heel is worn and the plastic heel cup at the back of the shoe is bent inward, then you pronate your feet when you run. Wear on the front of the sole indicates 'forefoot strike' - common in sprinters, but potentially damaging over longer distances. Wear on the middle of the sole may mean that you supinate. If you take your old shoes with you, specialist staff will be able to analyse your gait and recommend appropriate trainers.

Underneath the arches
The other key information to know when shopping is whether your feet have high, medium or low arches. Simply wet your feet and make footprints on a hard surface or a towel: a thin line between the outer edge of your forefoot and heel indicates high arches. If the complete outline of your foot is shown, you have low arches. A medium arch is somewhere in-between.

A high-arched foot tends to be inflexible, so you'll need a well-cushioned shoe with a narrower heel. Low arches mean your feet are too flexible and need the stability and support offered by a 'motion control' shoe. For those with normal, medium-arched feet, the balance between stability and shock absorption is down to personal preference.

Men and women are not the same - just look at their feet
The final point to note is the physical difference between women and men. Most shoes are still made to a man's 'last' - the shape of the foot on which the shoe is designed. Yet women tend to have narrower heels, while their wider hips cause the foot to strike the ground at a greater angle and thus pronate more. Women should buy shoes created on a women's last - most manufacturers now offer such a range, with Asics and New Balance providing the best selection.

Buying the perfect pair
Thus informed, here's how to purchase your ideal trainers:

• Buy at the end of the day, when gravity will have caused your feet toswell.
• Try them on with a pair of workout socks.
• Have both feet measured: if they're different, buy the pair that fitsthe larger foot.
• Ensure that there's a thumb-width of space between the end of yourlongest toe and the end of the toe box. Shoes with square toe-boxes areless likely to cramp your toes than those which taper in.
• If the shoes are ready-laced but in a style you don't like, remove thelaces and tie them as you prefer. Don't be rushed by the sales assistant.
• It may sound obvious, but even if you tend to buy the same brand and size shoe every time, try them on. If you're changing brands, then size 9 Nike, for example, may not be the same as size 9 Adidas.
• The footbeds in many shoes are inadequate. If you are heavy-footed or want additional protection, shock-absorbing insoles such as those made by Sorbothane are a good investment. These insoles are available at good sports shops and there's a range of products from heel pads to full insoles. The latter cost around £17. Take them along with you, lift out the footbeds provided and replace them with your insoles. You may need a size larger as a result.
• If there's space in the store, run, twist and jump around in your chosen shoes. Don't feel embarrassed. Are they still fitting snugly? Can you wiggle your toes? Are your heels held firmly in place?
• Finally, the best advice of all. If your current trainers are the most comfortable you've ever worn, don't be tempted to try something new. Simply buy another pair.

Where to buy
Sports shops usually stock a wide range of trainers but with little information about products on display - let alone staff who can provide specialist advice or help with fitting. Instead, we recommend the following smart shops:

1. Sweatshop, seven stores in London (including Selfridges and Harrods), eight around the country, plus mail order (020 8758 0044/www.sweatshop.co.uk): Most branches have the Adidas 'Footscan' system to ensure the perfect fit, and a treadmill to give shoes a test run. Voted sports retailer of the year by readers of 'Health & Fitness' magazine.

2. Bourne Sports, Church Street, Stoke on Trent (01782410411/www.bournesports.com): A good mail order service with regular special offers. Recommended if you know which shoe you want to buy.

3. Run and Become, 42 Palmer St, London SW1 (020 7222 1314): Lots of sympathetic advice here from experienced runners. This shop organises the regular Sri Chinmoy races in Battersea Park.

4. Runners Need, 34 Parkway, London NW1 (020 7209 3413/www.runnersneed.co.uk): Friendly staff know what they're talking about at this mecca for north London runners.

5. Birmingham Runners Shop,1506 Stratford Rd, Hall Green, Birmingham (0121745 6007/ www.birminghamrunner.com): This running specialist store has a vast range and great special offers.

6. On the Run, Unit C1, Whitwood Enterprise Park, Whitwood Lane, Whitwood,Castleford (01924 886666/www.ontherun.co.uk): This running specialist store has a fantastic website with news, reviews, a race diary and results.