Your heart rate is a valuable tool for monitoring the intensity of your workouts and estimating the number of calories you've burned. The numbers depend upon so many factors, however, that it is impossible to determine an "average" heart rate for any point in a workout. Your heart rate depends upon you, your workout and the amount of effort you are putting into it.
Your target heart rate depends upon your age -- subtract it from 220 for men or 226 for women to find your maximal heart rate, and aim to keep your heart rate between 50 and 80 percent of this number during your workout. The rate at which your heart rate climbs depends upon how fit you are. An elite runner would have a much lower heart rate than a couch potato after five minutes of an identical workout because the runner's body is trained to endure physical stress. The couch potato's body is "shocked" by the activity and is not accustomed to expending so much energy, so his heart rate climbs as his body struggles to pump blood to the muscles that are suddenly working.
The structure and type of workout matters, too. Five minutes into a leisurely walk will find your heart rate lower than five minutes into sprint intervals, no matter what your fitness level. This is simply because sprinting is a more intense exercise, requiring more energy, more blood flow and more muscular activity. Even within the same sport, differences in workout structure can affect your heart rate reading as well. Five minutes into a tempo run, during which a moderately quick pace is sustained for a long period of time, will find your heart rate higher than it would be if you were to run negative splits. During negative splits, a slower pace is maintained at the beginning of the session to save energy for a burst of speed in the second half.
The amount of effort you put into your workouts determines your results, and it also determines your heart rate. If you simply go through the motions of the same aerobics class you've grown accustomed to, your heart rate won't reach the level it would if you gave the workout 100 percent of your concentration and energy. Even if you're keeping up with the class, you're probably lacking strength behind the moves, quickness of step and explosive power -- all of which contribute to calorie burn. These factors also incorporate many more body mechanics than simply "miming" the moves, which causes your heart rate to rise as your body rushes more blood to your muscles.
The first five minutes of any workout should be a warm-up, where you maintain a comfortable pace of about 50 to 60 percent of your maximal heart rate and gradually increase your intensity from there. A warm-up allows your heart rate to rise slowly, which gives your body time to bring oxygen-rich blood to all of your muscles so they are warm and ready to go when you crank up the intensity. A proper warm-up reduces your risk of injury and may even help your performance.