"Running is a simple sport. All you need is a body, a piece of ground, and the ability to put one foot in front of the other. But between the body and the ground is a piece of equipment that is far from simple." From The Running Shoe Book by Peter Cavanagh

"There is something terribly wrong with this shoe," said the customer, handing over the well-worn structure of nylon and leather. "I know it's not me because I've never had this happen before."

The shoes in question, less than three months old, were heeled inward like a sailboat in the wind. Evidently, the woman was practically running on the side of her feet.

"You've got the wrong shoes," said the sales person. "That shoe is made for someone with a very stable foot and you are pronating severely."

"What does that mean?" asked the bewildered customer. "This is getting so complicated. It used to be so much easier."

Shopping for running shoes did used to be easier.

When Distance Running News, the forerunner of Runner's World, did their first survey of shoes in 1967, they reviewed 15 shoes. In 1980, they reviewed 178 different models; and now, even though the number has decreased somewhat, choices are confusing.

In an estimated $1 1/2-billion quality athletic shoe market, the question, "What is the best running shoe?" is not answered easily. It depends.

It depends on -- among other factors -- how many miles a week you run, how hard you are on your shoes and how much money you're willing to spend. With upper-crust running shoes going for $35 to $80 -- and lasting only 400-600 miles -- it's a major investment.

"When a customer comes into the store, we always ask him a number of questions," says Henry Barksdale, manager of Moss Brown in Georgetown. "If you don't consider your own running needs, you're liable to get something that a friend has recommended that may be good for the friend, but not for you."

"Many important properties of a running shoe cannot be assessed by simply looking at the shoe," says Peter Cavanagh, 35, professor of biomechanics at Penn State University. He conducts research on locomotion and footwear and is the author of The Running Shoe Book (Anderson World, Inc., 1980, $11.95). To make the selection of shoes somewhat less complicated, Cavanagh and other shoe experts say that you should:

1) Examine your old shoes. They have stories to tell. For example, if the inside midsole is compressed and the back of the shoes leads inward, you may pronate excessively. If you've worn a hole through the top of the shoe over the big toe, you need a shoe with a larger toe box. If your midsole cushioning is hard, it's time for new shoes. "The life of the running shoe is in the midsole," says Barksdale, 28, who came in 13th in the Boston Marathon this year and has seen plenty of "dead midsoles" -- he goes through a pair of shoes monthly.

2) Look at your feet, legs and body weight. Following the trend toward specialization, there are shoes designed for flat, rigid and flexible feet. A rigid high-arch foot is a problem, says Cavanagh. It usually needs good shock absorption, and a flexible flat foot generally "needs all the support it can get." Many experts recommend that a rigid foot, contrary to the majority of the population, probably needs a shoe that allows rearfoot movement. In addition, if you weigh over 160 pounds, you probably need a much firmer shoe.

3) Consider your injury history. If you have a constant record of shin splints, look for good shock absorbency. "If you're having knee problems," says Mark Dole, 23, manager of Running, Etc., in Alexandria, "it could be excessive pronation, in which case you need a shoe with a very rigid heel counter."

4) Consider your running style. Are you a forefoot or rearfoot striker, something you may be able to tell from the wear on your old shoes. For forefoot strikers, look for forefoot cushioning, an area that was until recently largely ignored "because of the mistaken idea," says Cavanagh, "that all distance runners land on their heels."

5) Consider your running habits. "If you're not having any problems and you keep your mileage low, you may be able to get by with a $30 pair of shoes," says Dole, who runs about 60 miles a week for the Washington Running Club. "But if you're running 50-100 miles a week, you need a more sophisticated shoe," says Barksdale. "You need all the protection you can get."

Armed with this information, you're ready to start trying on shoes. But still, how do you know what's right?

"Most people are used to wearing their shoes too tight," claims Dole. "They aren't used to a little heel slippage, which is natural, and they don't leave room for their toes to move. And a lot of people go for the color. You can tell them all you want about technicalities but when it comes to the bottom line, they pick the color they like."

Both Moss Brown and Running, Etc., generally recommend a half to a whole size larger in a running shoe than in a regular shoe. "Your feet expand when you're exercising, and your toes are pushed forward into the shoe," says Barksdale. "Our rule is that you should have a thumb's width from the end of your toe to the end of the shoe."

Barksdale also recommends that if you are having orthotics made, get the orthotics before buying shoes: The orthotic "takes up room in the shoe and may require a larger size."

Another consideration when fitting shoes, says Cavanagh, is that you may have different-size feet. If you are right-handed, your left foot will probably be larger. Many people just try on one shoe, the top one in the box, which is generally the right shoe.

And speaking of different-size feet, consider the time of the day you are shopping.

"The worst thing you could do," says Cavanagh, "would be to go to a shoe store at 8 a.m. and try the shoe on while sitting down. Under these conditions your foot will be at its smallest, shortest and narrowest."

Once you have made the big decision on your running shoes:

* Use running shoes for running only. They do not provide adequate lateral movement for aerobic dance or tennis, and may cause injury. Neither are the soles made to take the wear and tear of hiking.

* Avoid getting them wet as much as possible. Do not wear wet shoes; they may become permanently overstretched.

* To make the inside of the shoe last longer, always wear socks, especially in hot weather. The 150,000 sweat glands in your foot can quickly break down the lining.

* Maintain the outer soles, especially softer Vibram ones with some type of shoe "glue," applied regularly in a very thin layer on the worn parts. Two popular brands are Shoe Goo and Shoe Patch. Start application after the first 50 miles.

Finally -- and this comes from other than those with a vested interest -- when your shoes are dead, bury them, no matter how attached you are.

When Cavanagh ran an ad in the Penn State University paper offering to pay $3 for old running shoes, the response, he says, was "overwhelming" and "worrisome."

"Some of the shoes had been used far beyond the limits of safety. They had been worn until they were falling apart, until they had no outsoles, holes in the uppers, midsoles that were rock-hard and counters that were completely shredded."

Adds Cavanagh: "The economics of wearing shoes until they fall off your feet do not make good sense. The additional 500 miles carry with them the risk of getting injured."

"I see people," says Barksdale, "who spend $300-$400 in medical bills but hesitate to spend $50 for a good running shoe."