We often sense that we've hit a plateau. Maybe your heart rate doesn't seem to go up like it used to with the same activity. Perhaps you aren't lowering your time around Green Lake, or increasing your distance during an hour of swimming laps. Or you can't add any more weight to your strength training. Even if you don't monitor your workouts with such numbers, you might feel you're not getting as much out of your exercise as you did when you started.

Naturally, there's a good reason for all that - the exercise is working. It's called the adaptation syndrome. You've stressed the body by starting some exercise you haven't been doing regularly. The body reacts by adapting, so it's better able to handle that stress next time. Then you do a little more, and progress that way. Lifting weights per se doesn't make muscle: letting the fatigued muscle recover and rebuild itself even stronger is what increases strength.

Progress tapers off, though, when the stress is the same each time. That's where altering your workouts comes in.

"You beat stress through change," says Rick Huegli, head coach for strength and conditioning for the University of Washington. We rarely continue to improve while doing just one activity. "We're given this five-day work week, right? They'd work us seven days if they could. But we need a break." (Maybe he should talk to the folks at Microsoft.)

When to change is a matter of debate, ranging from after three weeks to after 10 or 12. Those differences might matter to elite athletes, but for most of us the larger question is probably what to change, and how.

Many people try to get beyond a plateau by doing more exercise, or doing it more intensely. Mistake. Some plateaus are caused by such overtraining. "If they've worked way too hard, they deaden the nervous system," Huegli says. "The only way to get the nervous system to recover is to take some time away."

Rest, recovery and variety seem to be the main keys to avoiding plateaus. Huegli considers those three - with nutrition considered part of recovery - to be a full 50 percent of the formula when planning workouts. (Exercise is the other 50 percent.)

"They're crucial to making progress, or staying interested," he says.

Easy days - when you exercise lightly or not at all - should be built into any fitness routine, along with light and hard weeks.

For variety you could alternate aerobic activities, for example walk twice and bicycle twice each week instead of walking four times a week. Or add strength training: walk once, bicycle once and lift weights twice a week.

You can also break the year into specific portions. Whether you're swimming, working out at the club, training for a competition or only for fitness, you can give your program different phases, like the Husky football players do: offseason (building foundations and volume of work), spring practice (intensity shifts to the field), summer (intensive weight-lifting), preseason (skills) and in season (maintenance). Most UW training segments are 10 weeks, simply because the academic quarter runs 10 weeks; each phase contains smaller cycles. You can also vary volume and intensity: If your best lift is, say 100 pounds, a lighter day might include lifting just 50, or using 70 percent effort instead of 90 percent.

Some people, of course, may not care if they're on a plateau. Progress - physiological progress, that is - isn't the only possible goal of exercise. An unwavering routine still can release tension, provide a break from the kids or the office, maintain a fitness level or offer just plain ol' fun.

In fact, Huegli didn't use his own workouts to illustrate avoiding plateaus. "I'm a bad example," he said, "because I'm content to run the same course every day.

"And I eat the same thing for lunch every day."

Molly Martin is assistant editor of Pacific.

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NOTEBOOK Fitness news you can use

Comfort Levels

Which aerobic workout burns more calories: high-impact, low-impact or step? No significant difference, if you go by a recent study at Case Western Reserve School of Medicine. "People modify an activity to work at a comfortable level regardless of the regimen," says exercise physiologist Kathleen Little.

A Nose for The Toes

Left foot size 8 1/2 and right foot size 10? Folks with different-size feet can get mismatched pairs via The National Odd Shoe Exchange (NOSE), 7102 N. 35th Ave., Suite 2, Phoenix, AZ 85051; 1-602-841-6691.

Healthy Calls

Cooking Light magazine, in alliance with the registered dieticians at University of Alabama at Birmingham, offers a hot line for questions on food and nutrition: 1-800-231-3438, from 6 a.m. to 2:30 p.m. our time, Monday through Friday.

Your Ideal Personal Trainer

If you were granted the services of a personal trainer, what would that person do? Send thoughts on your ideal personal trainer to On Fitness, Pacific Magazine, Seattle Times, P.O. Box 70, Seattle, WA 98101, or e-mail mmar-new@seatimes.com