Many Americans say they don't have time to cram exercise into an already hectic day. Now, one of the newest trends in fitness helps solve the time dilemma by combining strength and aerobic training into one efficient, 30-minute workout.

Called aerobic circuits, these popular new workouts are a fresh twist on an old concept. "For years, coaches have set up a series of exercises for athletes that gives them a tremendous workout in a fairly short time," says exercise physiologist Daniel Kosich, a technical consultant to Jane Fonda's Workout in Beverly Hills. Traditional circuit training involves moving quickly from one strength-training machine to another, keeping your heart rate up and achieving cardiovascular benefits while building muscle.

Add aerobic activity stations in between each strength machine, and you've got the new aerobic circuit. You might start with 30 seconds on an exercise bike, move to 30 seconds of sit-ups, jog in place for 30 seconds, move to a leg extension machine for 30 seconds, then step up and down on a small bench for 30 seconds. Proponents say the result is a balanced, whole-body workout that builds muscle and aerobic endurance efficiently while burning fat.

One example is the Universal Co.'s Aerobic Super Circuit -- 24 stations that take 16 minutes to complete. Universal recommends doing the circuit twice, for a total workout time of 32 minutes. Some gyms set up their own sequence of machines and use flashing lights or recorded voices to tell exercisers when to move to the next station. Instructors at other clubs change music, blow whistles or clap their hands -- a game of "musical machines" -- to signal time to move.

At Fonda's Workout, circuit classes have grown from two per week in 1988 to 20 per week now, notes Kosich. Some people want to combat age-related muscle loss while boosting their cardiovascular systems, and others want the fat-burning whammy that comes from pairing aerobics with strength-training. "My studies show that people who do strength training and aerobic exercise, rather than just aerobic exercise, lost more fat and gained more muscle although their diets were the same," says exercise physiologist Wayne Westcott, strength training consultant to the YMCA of the USA. "The aerobic-exercise-only group lost an average of 3.2 pounds. The group that did strength training, too, lost an average of 10."

But don't expect to get as great an aerobic benefit with circuit training as you would by spending the same amount of time doing something solely aerobic, such as jogging, walking, cycling or swimming, notes Neil Gordon, director of exercise physiology at the Institute for Aerobics Research in Dallas.

A traditional, strength-training circuit produces a 5 percent increase in aerobic capacity, compared to a 10 to 20 percent increase with just aerobics. While aerobic circuits provide a stronger cardiovascular benefit -- up to a 12 percent increase, say some studies -- Gordon advises twice weekly circuit training supplemented with aerobics for maximum cardiovascular fitness.

But for the busy person looking for maximum fitness in minimum time, "circuits can be an attractive option," says Los Angeles personal trainer Douglas Brooks. Membership in a fancy gym isn't essential, says Brooks. A typical home circuit could entail one minute of rope jumping, 30 seconds of sit-ups, one minute of running in place, 30 seconds of push-ups, one minute of stair stepping and 30 seconds of arm curls with dumbbells, and so on for 30 minutes.

Brooks recommends visiting a reputable gym or hiring a personal trainer for a few sessions to get started. People over 40 or those with risk factors for heart disease should consult a physician before beginning any vigorous exercise program.

Experts offer these pointers for circuit training:

Warm up for three to five minutes with easy aerobic activity; cool down for five minutes after the final circuit, then stretch.

Perform 10 or 15 strengthening exercises that target all the major muscle groups.

Alternate upper and lower body exercises with aerobics.

Keep strength intervals to less than one minute

Perform the aerobic circuits three times per week.

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