High-intensity interval training is one of the biggest trends in fitness, but it has always seemed a bit scary to me. To a mere mortal with achy knees and an aging body, even the acronym ó HIIT ó sounded intimidating.But recently, I overcame my fears of intense interval training, which involves bursts of all-out exercise followed by brief periods of rest. When it was over, not only did I feel great, but to my surprise, I enjoyed it.So how did I overcome my fears about high-intensity training?It began with a heart rate monitor. Alex Chriest, a fitness coach at the Johnson & Johnson Human Performance Institute in Orlando, showed me how using a heart rate monitor could make high-intensity exercise not only easier, but more fun.Chriest, who has a doctorate in education, recommends a Polar heart rate monitor with a chest strap sensor and watch, though there are many other good brands that include a strap. Just set the watch with your weight, height, age and gender and put it on your wrist. I wrapped the chest strap just at the base of my sports bra, and almost immediately, the watch began reporting my resting heart rate, which is around 80 beats a minute. (A normal resting heart rate ranges from 60 to 100. A lower rate suggests better fitness and more efficient heart function.)"A lot of people are very competitive with themselves," Chriest said. "When they see on the heart rate monitor that they are in the range, it is almost like a game. Itís not a subjective feeling of ĎI think Iím pushing myself hard enough.í You walk away knowing you got a lot of benefit from it."Chris Jordan, director of exercise physiology at the Human Performance Institute, agrees that high-intensity interval training suffers from a public image problem."As soon as you hear Ďhigh intensityí it sounds aggressive, demanding and for athletes only," he said. "I like to get people away from terms and remind them this means oscillating between relatively hard and relatively easy for you exercise.""My 82-year-old mother is doing interval training," Jordan said, by walking around the ground floor of her house and then exerting herself a little more by walking up and down the stairs. She repeats the sequence a few times."Itís no different than an elite athlete performing interval running," he said. "The principle is the same. It has to be the appropriate intensity level for you to make sure itís safe and effective."And thatís where the heart rate monitor proves so useful. Instead of guessing whether youíre pushing yourself hard enough, you can see exactly how hard your heart is working and how easily you recover. Based on my workout experience, Chriest set my target heart rate range goals between 145 and 166 ó the low and the high numbers I posted during my stationary bike workout, which represented between 70 and 90 percent of my maximum heart rate. While you can do the same workout to determine your target range, make sure you check with a doctor first.How do you know if youíre in the 70 to 90 percent range? Jordan offers additional guidance on how to figure out your own target heart rate and what it means to push yourself to the "discomfort zone."Perceived exertion: Rate your effort on a scale of 0 to 10, with 0 being rest and 10 being maximum effort. A 5 or 6 suggests moderate exercise, and a 7 or 8 is the vigorous exercise zone youíre aiming for.Talk and sing test: If you still can talk while exercising, but canít sing, youíre in a moderate zone ó the pace you should use during recovery intervals. If you can speak only a few words (gasp) before pausing (gasp) for breath, then thatís the high-intensity zone.Fatigue LEVEL: After my workout, I was pretty spent. That said, I could have done another round even though I really didnít want to. I didnít feel like I could push my heart rate much higher. At the same time, I wasnít falling off my bike with exhaustion or nausea. That suggests that I was exercising in the discomfort zone and that the low and high numbers on my heart rate monitor likely represented 70 to 90 percent of my maximum heart rate.Using those three elements ó perceived exertion, the talk and sing test and fatigue level after the workout ó combined with the numbers shown on your heart rate monitor will help you determine your own 70 to 90 percent target heart rate zone.Some things to remember when using a heart rate monitor and calculating your target heart rate.Listen to your body: "If you feel sick and dizzy or nauseous, itís irrelevant what your heart monitor says," Jordan said. "You should slow down and get off the machine. Itís important everyone listen to their body as well as look at the heart rate monitor."Target heart rate is different for everyone: Donít pay attention to my numbers or anyone elseís. Your target range could be much higher or lower.Maximum heart rate doesnít change over time: Your maximum heart rate is genetically predetermined and, like height and eye color, doesnít change over time. Instead, as your fitness levels increase, your heart wonít beat faster, but it will become stronger, pumping out more blood per beat and allowing you to do more (such as pedal or run faster) at the same heart rate.Standard formulas are unreliable: A common formula for determining maximum heart rate is 220 minus age. But this is often inaccurate. In my case, the formula says my maximum heart rate is 169, putting my target range at 118 to 152, far lower than the range of 145 to 166 that I posted on my bike test.Not all heart monitors are equal: While the heart rate monitors built into gym equipment can serve as a guide, Jordan notes that they are less accurate than a watch with a chest strap. "Just 10 beats off is quite a difference," Jordan said. "It can be the difference between feeling sick and working out within the appropriate level of intensity."Since leaving the Human Performance Institute, I have dutifully continued interval training, using a heart rate monitor at every workout. Weeks later, high-intensity interval training remains surprisingly fun and satisfying.While Iím sold on high-intensity exercise, I still wish we could come up with a better name for it. How about fun, fast interval training, or FFIT?"High-intensity interval training makes time go by faster, and it is more fun," Chriest said. "Itís not jogging for an hour. I like it because the rest periods go by fast and before you know it, itís over."