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.