"Persevere!" the college students yelled from a microbus as my bicycle and I crept up past timberline in Colorado's Rocky Mountain National Park.

This was six years after one of the nation's leading knee specialists had advised me to quit riding my bicycle. Long rides on several occasions had inflamed my right knee so badly that I could not bend it. I learned I had chondro-malacia, a chronic knee condition that strikes fear into the hearts of cyclists, joggers and dancers. Generally, chondro-malacia does not go away.

But after two bikeless years, I developed the overwhelming urge to get back in the saddle. So I disobeyed doctors' orders and tried my knees out on the towpath -- the flattest place I could think of to ride. I found that with gentle cycling, the further I rode, the better my knees felt. Since then I've biked 18,000 miles.

People with many kinds of handicaps can bicycle, and cycling can sometimes help their problems. Rebecca Shankle, co-owner of the Parrot, a restaurant in Frederick, walks with a cane and avoids climbing stairs. Her knees creak and grind, the result of having been hit by a car while cycling to work, an accident that severed her right knee cap and broke her left leg next to the knee. But she can bicycle 30 miles.

My mother, Tufts University professor Mathilda Holzman, has a different problem. Multiple sclerosis has gradually crippled her over the last 25 years. She gave up tennis, skiing and air mattress surfing years ago, and now she can barely walk. But she rides a British racing tricycle as far as nine miles.

For the partially paralyzed, tricycling can offer a new form of mobility. And for those of us with chronically sore knees or hips, bicycling may offer relief and even induce some healing.

My experience has inspired several of my friends. One is a dancer whose career was threatened by sore knees until she started taking long bicycle rides. Another is an all-around athlete who tore tendons and ligaments in a soccer game. Since his cast came off, he finds that bicycling has helped his knee regain strength and stability.

To work through these kinds of knee problems, it is important to understand how the knee works. In general, use strengthens it, and disuse leads to atrophy.

Consider the cartilages that slide across each other during flexing. No blood flows through cartilage. Instead, nourishment and waste products flow in and out with the joint's lubricating fluid, much the same way water might diffuse in and out of a sponge. Sitting around does nothing to help the joint beyond allowing a painful inflammation to subside. On occasions when I have overworked a knee, a day or two of rest suffices, followed by gentler cycling than usual.

But rest is no cure, says Dr. John Bland, a rheumatologist at the University of Vermont Medical School. He recommends "remaining as physically active as possible within the constraints of pain," adding that this "may actually stimulate regeneration of cartilage."

I am quite certain that bicycling helped heal my cartilages. I began feeling improvements when I got my distance over 10 miles, and since have found that long tours in places like Colorado do wonders to rejuvenate my knees, provided I don't overwork them.

You must fully accept your condition before cycling; otherwise you will not be able to read the signals from your knees that tell you how to treat them. After my knees first went bad, but before the doctor ordered me off the bicycle, I rode 40 miles one day with no ill effect. I deluded myself into thinking I was cured. I began pushing hard, ignoring the pains that developed, until one day they stopped me dead and I had to hitchhike home.

Once you accept your condition you can learn to interpret pain. Twinges at an 90-degree flex of the knee tell me to stop, but I can usually work through other kinds of pain by gentle cycling.

Learning to cope with bad knees is an individual matter. Rebecca Shankle's knees hurt much of the time, and she rides through much more pain than I would. You have to figure out for yourself how far you can go, how hard you can push, and how to read your knees. A doctor, especially one who specializes in sports medicine, might be able to help you in a general way, but you are the only one who can judge your body's capabilities.

Good equipment is vital to improving your knees. My recovery vehicle was my $600, 23-pound Bottecchia. My $200, 29-pound Peugeot could also have done the job, but I wouldn't have even tried it on a three-speed or a department store 10-speed.

Riding should be fun, not frustrating, because attitude and state of mind have a profound effect on joint problems. I first noticed this during the two years I didn't cycle. Back then, the pains in my left knee kept me from walking more than a mile or two; yet during vacations I found I could walk as far as I wanted.

My own euphoria during those first forays down the towpath prompted me to see if cycling could help my mother.

I arranged for a 15-speed, 34-pound racing tricycle to be custom built by the Thompson Bicycle Co. of Huddersfield, England. It made the outdoors newly accessible.

"I love countryside, but you don't get any sense of being outdoors in a car," she says. "The tricycle gets me to places I couldn't go on foot. I can even climb long hills because the very low gears make them feel level."

My mother is not sure whether the tricycle has therapeutic value for her illness. But in a 1982 article in Bicycling magazine, former Olympic star Jimmy Heuga credited bicycle riding with rescuing his strength and coordination from the ravages of multiple sclerosis.

When asked about Heuga's story, Dr. Nancy Lellelid, a neurologist in Everett, Wash., and an MS victim, told Bicycling, "I know that exercise helps, because it did with me. And I am equally convinced that aerobic training has a place in the rehabilitation of MS patients. Unfortunately, there has always been the feeling by a lot of physicians that MS patients should not exercise, that instead, they should rest so they wouldn't get worse."