Heart rate formulas
If you’ve been to a gym than you probably saw a graph resembling this one:

training heart rate
What does this graph mean? We’ll start with the higher line, the one that labeled Maximum Heart Rate (MHR). Maximum is considered the highest heart rate that can be reached for a short time without causing damage to your heart. For example, for the age 20 the MHR is 200 (beats per minutes). This means higher than 200 bpm can cause permanent damage. Infant’s MHR is 220. Average human’s MHR is decreased by one each year. Therefore the formula: MHR = 220 – age (sometimes 220 is taken for men while 226 for women, there are also other formulas).

While training heart rate is measured as percentages from MHR. In this picture training zone is considered as 70% - 85% (lower and upper bounds) of MHR. Before we answer: “what is the meaning of these percentages?” let’s take another look at this formula. It takes only the age into account. This is its main disadvantage, since the assumption that all people who are of certain age are at the same fitness level isn’t realistic. Karvonen method comes to the rescue. Karvonen additionally to the age takes into account the resting heart rate. Karvonen: THR = ((MHR – RestingHR) × %Intensity) + RestingHR

Example: consider the following parameters: age = 30 years, RestingHR = 80 bps and intensities 70% - 80%.
THR (lower limit) = (((220 – 30) – 80) * 0.7) + 80 = 157
THR (upper limit) = (((220 – 30) – 80) * 0.8) + 80 = 168
The training heart rate should be 157 – 168 (these number can be rounded plus minus 5).

Consider that this person after a year of training checks his resting heart rate and finds out that it’s 60. The updated parameters are: age = 31 years, RestingHR = 60 bps and intensities 70% - 80%.
THR (lower limit) = (((220 – 31) – 60) * 0.7) + 60 = 150.3
THR (upper limit) = (((220 – 31) – 60) * 0.8) + 60 = 163.2
Received new limits are 150 – 163 bpm.

What is the meaning of these percentages?

Following division of MHR percentages and their description:
  • 50-60% - can be called the starting zone. Either you start a workout (warm up) or started training this one is for you.
  • 60-70% - this is a more intense zone than the previous one. Sometimes also called “Fat Burning zone” (especially by heart monitor manufacturers). I don’t like this name since it’s misleading. This zone isn’t better than other ones for burning fat (see: Myths).
  • 70-80% - usually called Endurance Training zone, since its preferred zone if you are training for an endurance event (contributes to cardiovascular and respiratory system and increase the size and strength of your heart). This is the last aerobic zone (see aerobic threshold picture below).
  • 80-90% - this is already anaerobic zone. Main benefit of this zone is improved oxygen maximum and thus an improved cardio respiratory system.
  • 90-100% - usually used only by experienced/professional athletes. You should train in this zone only if you’ve been cleared by a physician to do so.

Note: this is probably the most common division, but there are other divisions also. You should treat the presented numbers (percentages) as good approximations. You can think as if their accuracy is plus minus 5%.
At what percentages (from MHR) should I train?

I would recommend start training with the lowest intensity and gradually increase intensity step by step.

Possible plan: beginner - start with 50-60% of MHR for the first three weeks. Than pass to 60-70% of MHR for four weeks. After the four weeks include 70-80% of MHR workout once a week (rest aerobic workout will still be with 60-70% of MHR). After six weeks … Of course each step of the plan might be updated if needed as time passes.

Note: you can measure intensity not only by heart rate. See also: workout intensity measurements for a better picture.

Related link: http://en.wikipedia.org/wiki/Heart_rate