How do I figure out my target heart rate? How long do I need to work out after reaching it?
There is a basic formula that applies to the average population (and is actually pretty accurate). Let's use a 20-year-old as an easy-to-calculate example: The formula: 220-age= MAX heart rate (220-20=200 MHR) 60%-80% of this max: 200 x .6 = 120 beats per min, 200 x .8 = 160 beats per min. Therefore the TARGET HEART RATE range for a 20-year old, working at 60-80% (up to 85% for very fit people) of his/her max heart rate is 120-160 beats/min.
To calculate your heart rate: Find your pulse (on the neck or wrist) and count the number of beats for 6 seconds.
Multiply that number by 10 and compare to your recommended range. Using this method will prevent you from stopping exercise for more than a few seconds. You can take your pulse after you've been exercising for at least 5 minutes. For example, suppose you take your pulse and count 13 in 6 seconds.
Multiply by 10 to get 130 beats per minute. Now you know you're in the right range. If you notice you are lower than the minimum, increase your speed/incline/intensity and try to count again. If you notice you are very high, decrease your intensity in some way.
* If you have high blood pressure, you should not be working out in this THR range. As far as time goes, try to sustain this intensity (not including your 5 minute warm-up or cool down) for AT LEAST 20 minutes (minimum recommendations for health and weight loss), and up to 45 or even 60 minutes.
Of course, you will not start an exercise program with that much endurance, but you'll slowly build up. Aim to reach this time/intensity 3-5 days a week, and you're all set!