To measure heart rate during exercise, pause at the level of maximum intensity, find the heart rate, and count the number of beats for 15 seconds, according to the American Heart Association. Multiply this number by four for the maximum heart rate. Repeat the procedure several times during a workout.
Target heart rate can be estimated by first subtracting age from the number 220, according to Mayo Clinic. A 45-year-old person has a maximum heart rate of 175. Multiply that number (175) by 0.7 to 0.85 to calculate a target heart rate, which in this case, is 123 to 149 beats per minute.