Do your knees crack when you bend them? Do they ache occasionally, especially after a particularly strenuous exercise session or a day of skiing on hardpack? Are you a woman, or knock-kneed or a hockey goalie or a carpet layer? You may have chondromalacia.

Chondromalacia is a condition caused by abnormal wear on the underside of the kneecap brought on by improper alignment of cartilage. The rough cartilage on the back of the kneecap irritates the ligaments below, which makes them swell, which hurts whne you bend your knee.

The bad news is that the condition is chronic. The good news is that it does not have to keep you off skis. With some adjustments in your preseason conditioning exercising, and maybe some adjustments in your boots, you can ski better this season than you did last season.

The kneecap has a glass-smooth undersurface that is designed to glide easily over the end of the femur, or thigh bone. If you put a lot of pressure on your knees, or if you tend to stand with your weight over your arches, you can throw the kneecap out of line, and the thigh bone can scratch that smooth surface, which leads to the irritation of the ligament.

Dr. Richard Steadman, chairman of the U.S. Ski Team Alpine Medical Group, is an orthopedist who practices in South Lake Tahoe, Calif. He said in a telephone interview that he sees a lot of chondromalacia in junior racers. Pre-teen knees are not yet mature enough to handle the pressure of a lot of hard edging on hard-packed slalom courses, he said.

Steadman has found very little chondromalacia among U.S. Ski Team members, however. For one thing, he said, they do all the right exercises. For another, they all wear orthotics: specially molded arch supports in their ski boots. And finally, Steadman said, ski racers with chondromalacia usually wash out long before they make the national ski team.

Whether or not you are planning to join the U.S. Ski Team, you can adjust your preseason conditioning program to help your chondromalacia while you are strengthening yourself.

Bicycle riding. Steadman recommends bicycle riding for chondromalacia sufferers and for everyone else. You get the best workout if the seat is high enough so that your leg is fully extended at the bottom of the pedal stroke. He suggest that chondromalacia sufferers use only low gear to ease strain on the kneecap.

Swimming. P.M. Palumbo, a Washington Orthopedist and sportsmedicine specialist, recommends swimming as an all-around exercise. He cautions chondromalacia sufferers to stick to a crawl and to be sure to do the flutter kick with knees straight.

Hamstring stretches, straight leg raises and knee extensions.

Other exercises are fine as long as they do not require you to put pressure on your knee. You can do bentleg situps, all straight leg exercises, and any exercise that builds the upper body. Keep away from jogging, however. Both doctors agreed that the constant pounding on a hard surface is extremely aggravating to the knees.

You might also want to look at pronation devices. Your foot pronates when you put weight on it and flattens the arch. Some people pronate more than others. Each time you flatten your arch, you shift your weight to the inside of your foot. This throws the kneecap out of alignment; it is meant to ride on the thigh bone when, the weight is on the outside of the foot.

Pronation devices are simply arch supports. They hold your arch up so that your weight cannot shift to the inside of your foot. That keeps you kneecap in place. It also helps you edge. When you drive your knee into the hill the movement drives itself to the ski faster if your foot doesn't waste time flattening out before the movement is transferred to the ski.

Pronation devices range from universal inserts to custom-made insoles designed by technicians who work from a mold of your foot. Steadman prefers the latter, because the mold is made while your foot is unweighted and your arch is highest. Check with your favorite ski shop to find out what is available and what will fit your foot.

If all else fails, you might want to consult an orthopedist and use a brace. Palumbo has invented his own brace to keep the kneecap in place.

Finally, there are surgical cures. But most doctors will not consider this until you have exhausted all other possibilities. So start with your exercises. They might improve your skiing.