Inside our shoes are the most abused parts of the human anatomy: our feet. They absorb the initial impact of running, and pass it upward to the ankles, legs, knees, hips, back, neck and head. Most people have weak feet, which when pounded on the earth thousands of times a week cause a wide range of injuries.

The arches in our feet commonly bring pain and discomfort. Often, flat feet or feet with high arches both require support. Flatfootedness, however, does not necessarily mean that the arch is flat; it can be a condition in which weak ankles let the foot roll medially, giving the appearance that the arch is flat. Exercises to strengthen the muscles of the foot - picking up marbles with your toes, rolling a soda bottle under your foot, standing on a towel with your toes over the edge and then picking up the towel with your toes - all can help.

The Achilles' tendon connects the powerful lower leg muscles to the heel. An injury to this tendon is painful and long-lasting. Some major causes of Achilles' tendonitis:

1. Running tightens the calf muscles, and runners who can't bend their feet upward (dorsiflex them) to the ideal 10 degrees beyond the right angle to put strain on the Achilles' tendon.

2. Many runners neglect warmups and stretching exercises. Tight, tense muscles and tendons are more susceptible to injury.

3. Many runners, especially beginners, run on the balls of their feet, which puts a strain on the Achilles' tendon.

4. Bursts of speed by a runner who has not built up to it will strain the tendon.

5. Running shoes with low heels, or shoes that don't readily flex under the ball of the foot are inadequate.

There are other causes that may precipitate an attack of tendonitis. Any sudden change in footwear (thick heels to thin heels), running surfaces (soft to hard) or training pattern (flat terrain to hill work, endurance to speed, increased mileage) may trigger tendon strain. If you overtrain a fatigued body, or run on worn-down heels, or stretch with fast or jerky motions, or force a stretch beyond the first point of discomfort, you may harm your Achilles' tendon.

You should stretch the Achilles' tendon more than once a day. It should always be done before and after running. I often lean against the wall while waiting for an elevator, lower my heels off the curb while for a traffic light to change. I may look a little weird, but who notices?

The The purchase of a good pair of running shoes is the one critical investment a runner must make.

Dr. Harry Hlavac, a sport podiatrist, advises that if a person has recurrent problems, certain types of shoes may help and other types may aggravate them.

"For instance," he says, "relief from Achilles' tendonitis requires a flexible shoe with cushioning of the bottom of the heel and good elevation. Ankle sprains and instability need a shoe that will provide support and balance at heel contact. Arch problems and flexible feet require shoes that have a good shank area (under the arch) and a conforming arch support."

The foot fiexes about 30 to 35 degrees as you push off your toes. The shoe should flex, too. If the shoe isn't flexible under the ball of the foot, the front and back of your leg take up extra stress, which can cause injuries to the Achilles' tendon, calf muscle and shin. A balance must be achieved between cushioning and flexibility.

Check the ball - the widest part - of the shoe. Dr. Richard Schuster, another sports podiatrists, says that any the shoe that's difficult to bend with your hand is probably too stiff. He sometimes solves this problem by slicing across the widest point of the sole of the shoe under the ball with a hacksaw. Three cuts, spaced an inch apart and deep enough to cut through the first layer of rubber, may make the shoe more flexible. Or you can choose a shoe that fits and is flexible in the first place.

The sole of the shoe in the arch area should lie flush to the ground. In some shoes the arch is cut away to save weight or produce cheaper shoes. This bridge buckles under the severe stress of your footstrike, and can cause heel and arch injuries. Avoid running shoes with such a design.

The following "shoe stuffings" are often inserted into running shoes:HEEL LIFTS - Used to soothe shin splints, Achilles' tendons and calf tightness. A lift of 1/4" to 1/2" allows the runner to hit farther back on the heel, thus decreasing the pull on the Achilles' tendon. Makeshift lifts may be constructed from surgical felt, powder puffs or similar material. Try first one, then up to three or four, under the heel to find the best level for you. The lift should be tapered to the middle of the arch so the foot doesn't sag in front of the lift.

HEEL CUPS - Used to stablize and cushion the heel and to prevent or contend with injuries such as heel spurs and bruises, callouses, shin splints and ankle sprains.

ARCH SUPPORTS - The ones that come with running shoes are usually inadequate. Many runner rip them off right away. You can buy cheap supports, but why treat your feet so poorly? The best mass-produced support is Dr. Scholl's No. 610 or Athletic A Arch Support. They are not always available in your local stores, but can be ordered.

INSOLES - Good ones cushion the feet and help prevent blisters. Foam rubber and other insoles don't hold up under the pounding and the exposure to sweat, rain and snow. The "Spenco" insole is the best on the market.

When you try on new running shoes, bring whatever you may put inside them - you might take a half-size large than usual. be sure you don't raise your heel up so far that you lose support in the back of the shoe for your Achilles' tendon, or crowd the toes against the top of the shoe.

Runner's World magazine annually prints a special running-shoe issue that analyzes all the top running shoes in great detail. This issue is valuable to you as a runner. There are hundreds of quality running shoes available, and you need a manual to tell them apart.

Anything of quality costs money, and lasts longer. You must plan to invest $20 to $40 for a good pair of running shoes. It's your only major investment; cheaper shoes will cause aches and pains, and perhaps doctor's bills; and tennis shoes can lead to stress on your body, your pocketbook and mind.