We take our clients in and out of three zones: Zone 1 is 65 percent to 75 percent of your maximum heart rate, and Zone 2 is 80 percent to 85 percent of your maximum heart rate. Zone 3 is 85 percent to 90 percent of your maximum heart rate, and you usually don't want that to comprise more than 10 percent of your total training volume for the week because it's so intense. But ultimately the goal is to get everyone to that point where they could exercise at that level of intensity.

You should be able to sustain the upper level of Zone 1 for 30 minutes before trying to do some interval training. If you're able to do half an hour to 45 minutes in zones 1 and 2, you're a good candidate to incorporate some Zone 3 work in small intervals, within a workout that utilizes zones 1 and 2.

The work-to-rest ratio can change. You can go up into Zone 2 for three minutes, then down to Zone 1 for three minutes. Then you can work a 2-1 ratio of work and rest. A good rule of thumb is - and this is where the heart-rate monitor comes in handy - that when you drop down a zone, you should see a 20-beat-per-minute drop in your heart rate within one minute. That's a good indicator that you're in the recovery zone.

Kelly Stemp

General manager and personal trainer at Vert Fitness in Santa Monica, Calif.:

If you're doing cardio, try a 2-1 ratio. So if you're on the elliptical trainer, do a 30-second sprint with a minute at an easy pace. But you can also spread it out, especially if you're just starting. So if you're doing a 30-minute workout, you could sprint five times for 15 to 30 seconds. Then you can add in one more interval the next week, then the next week go back down to five, then back up to six. Don't keep upping it all the time because it will make you more prone to injury, and you'll get discouraged, because no one can keep that up.

When you pick up the pace, you engage your core more. Also, you're doing something your body's not used to, and when you force it to adapt, it will get more efficient. ... Doing intervals shocks your system - if you don't do them, then you plateau.

If you're going at the same pace for months, eventually your heart rate is going to be lower doing the same amount of work (because it adapts). The heart is a muscle, and intervals make the heart stronger, so it can pump more blood per heartbeat. You can't make that happen by doing the same thing you've always done.

Interval training basics

The basics of interval training are tough to figure out without doing some math first to figure out one's maximum heart rate.

A general formula for calculating that number is 220 minus age. To work in a lower aerobic zone, in which the body burns primarily fat for fuel, calculate 65 percent of the maximum heart rate. To work in a higher anaerobic zone, calculate 85 percent of the maximum heart rate. The anaerobic zone, in which the body burns primarily carbohydrates for fuel, is the high-intensity portion of the interval (some trainers may put it at about 80 percent).

For those who like precise numbers, some gyms and training facilities offer more technically advanced testing, but that can be costly. Heart rates can be taken manually at the wrist, and some cardio machines have built-in monitors (although accuracy varies). Heart-rate monitors provide the best readings.