Target heart rate, or also known as THR, is the heart rate valu, which determines the intensity. The THR can be used as an intensity guide for large-muscle group and continuous whole-body-activities. The target heart rate should be only used by people who wants to increase there fitness level and lose weight. Bodybuilders and people who need to gain weight shouldn’t use the THR as a guidelines for there workouts. 
The fitness professionals can determine the THR range by taking percentage of the maximal oxygen consumption. Oxygen consumption during exercise is measured by subtracting the volume of oxygen breathed out from the volume of oxygen breathed in. Maximal oxygen consumption is the oxygen consumption at which the body can’t keep any more with the body oxygen needs.
Measuring the oxygen consumption is hard to do and can only be done with special equipment. The oxygen consumption holds important information on what kind of fuel is used during the exercise. For example fat and carbs are used for energy during aerobic exercises (when oxygen is presented). The optimal range of exercise intensities associated with improved fitness level and losing weight are 60% to 80% of maximal oxygen consumption.
Heart rate increases linearly with the oxygen consumption. Heart rate is than plotted on a graph against the oxygen consumption. The target heart rate is then determine by taking the percentages of maximal oxygen consumption at which the person should train( will burn fat and carbs) and finding what the heart rate responses were at those points. That method of determining the target heart rate is called direct method. 
Target heart rate: Indirect Method
The indirect method have been developed to estimate an appropriate THR. There are two different indirect methods for estimating exercise intensity. The two methods are simply guidelines to use in an exercise program, and both approaches must be used as guidelines for more accuracy.
First method is called percentage of maximal heart rate. The advantage of this method is its simplicity. Take 75% to 90% of the maximal heart rate (220-age) and you will have the target heart rate.
Example:How much is the target heart rate of 40 years old man?
220-40=180 maximal heart rate
75% of 180 =135 beats THR
90% of 180=162 beats THR
For best results in these case try to keep your heart rate in between 135 and 162 beats.
Second method is calculated by taking the difference between resting and maximal HR.Calculate 60% and 80% of that number, add each value to the resting HR , and you will have the target heart rate
Example:How much is the target heart rate of 40 years old man with resting heart beat of 75 beats?
180Max HR-75 Rest HR=105
60% of 105=63+75=138 THR
80% of 105=84+75=159 THR
The resting heart rate is measured before any activity. Sit calm and measure your pulse for one minute.
To improve your fitness level you should train 3-5 times a week for hour, hour and a half. Keep your pulse in your THR zone, and you are guaranty to be at the right path to success.