If you want to get the most out of your work out, monitor your target heart rate. If your goal is improved cardiovascular fitness or strength, it is important to identify how hard you are working from a performance perspective. If your goal or phase is maximal strength, having too high a heart rate, will inhibit your ability to produce force. Conversely, if your goal is extending your time exercising in a particular heart rate zone, using a monitor will help you stay in the parameters of your cardio zone.
How to you find your target heart rate, here is the Karvonen Formula:
(40 year old man) 220-age(40) = Max heart rate (MHR) 180
(40 year old man) Resting Heart Rate (RHR) = 70
Target Heart Rate = (180 (MHR) – 70(RHR) = 110 x .65 (percentage of max heart rate desired) = 71.5+ 70 (RHR) = 141.5