I'm sure you have a handful of song lyrics that we could call power producers. Lyrics that create such a surge of emotion that when you hear them sung, you feel supercharged. Suddenly, you can tackle that troubling task, feel hope when you had none, work out a whole lot harder.

Words, you see, can do more than relay instruction or transmit thought. Words can lift us up. Or let us down.

We've all listened to one of those power producers, passed the earphones to a friend, and witnessed what we could call the power outage. Those words that do so much for you do nothing for your friend.

That's because most words mean more than their dictionary definitions, and those additional meanings are inexorably linked to personal experience. In other words, words are relative. Like "hard."

A hard workout for you may not be a hard workout for me.

It's important to establish that hard is a relative term or today's topic, interval training, may turn off many readers.

Michael O'Shea, the writer of "GetFit," a column that runs most weeks in the Parade Magazine found in many Sunday newspapers, explains interval training as nothing more than "adding bursts of more intense exercise to your regular routine."

"Intensity" is a relative term, too, but to create an example, let's say you normally run three miles outside three times a week at a nine-minute-per-mile-training pace. After a half mile or mile at this pace to get loose, you now run faster for a time. That constitutes one interval.

You then run slower than typical to recover, returning to training pace when you can. You then do a second interval.

Whether you do one, three, or five more intervals is determined by a number of variables, including your fitness level, your training goals, and how fast and how far you're running the intervals.

But "fast" and "far," like "hard" and "intense," are also relative terms, which means one routine does not work for all that the most effective interval workouts are the ones created for you by you.

If you struggle to run a half mile, for instance, but would like to be able to finish a 5k race next spring, do interval workouts. Your training pace is fast walking, your recovery pace is slower walking, and your "hard" intervals are the 400, 600, or 800 yards of running you can do.

If you've been running 5k races for fun for a few years, but now want to run them faster, do interval workouts. Decide just how fast you want your next 5k time to be, and then run your "hard" intervals about 20 percent faster than that.

The faster pace albeit temporarily teaches the body to go faster. After a few sessions, reduce the recovery times between intervals. As race day nears, you make the "recoveries," again, a relative term faster.

As long as you give your body proper rest between interval sessions no more than two in any week and only do an abbreviated one in the week prior to the race you should set a PR in the next 5k.

But even if you don't, your use of interval training provides other benefits.

The vacillation between fast and slow in an interval workout, for instance, places greater demands on your heart. And just like your other muscles, ask more of your heart and it gets stronger and more efficient.

Even longtime exercisers who never used interval training will find that their resting heart rate has gone down, a great indicator of improved heart strength and greater blood flow efficiency.

Another benefit to interval training is that it burns more fat.

While working at lesser intensities burns a higher percentage of fat, the energy requirement for interval training is so much greater that more total fat gets burned even though your primary source of fuel is stored carbohydrates.

In fact, the calorie expenditure during interval training is so much greater that you should burn more fat and total calories in 45 minutes of interval running than 60 minutes of running at a slow, steady pace.

Additionally, interval exercise increases the after-burn. In the same way that your brain just can't shut down and immediately allow sleep after a stressful situation, the metabolism can't immediately shut down after exercise. The after-burn is that accelerated rate until the metabolism returns to normal.

While all exercise creates the after-burn to some degree, research presented at the American College of Sports Medicine's 15th-annual Health & Fitness Summit & Exposition by Michael Bracko, Ed.D., FACSM, revealed that interval training does so longer sometimes up to 24 hours.

Though most exercisers use interval training in aerobic pursuits like walking, running, cycling, or rowing, a variation of interval training can be used in weightlifting. Those doing a form of weightlifting called circuit training can order the exercises in such a way that "easy" exercises for the abs and calves "easy" because they possess a higher degree of slow-twitch muscle fibers and are therefore harder to tire follow "hard" exercises for the larger muscle groups that contain more fast-twitch muscle fibers, like the back, chest, and thighs.