Recently I was asked how accurate and safe it is to calculate your target heart rate based on age during exercise. This question is a very good one and one not easily answered.

First let's look at the standard rule of thumb "Target Heart Rate" pulse count by age:

Age 20 - 50% max. 100 75% max. 150

Age 30 - 50% max. 95 75% max. 142

Age 40 - 50% max. 90 75% max. 135

Age 50 - 50% max. 85 75% max. 127

Age 60 - 50% max. 80 75% max. 120

Now the idea is to identify a safe target heart rate as it relates to your age and when you use the standard formula you will notice it does not conform to the "Target Heart Rate" chart above. Let's take a look, subtract your age so we will use age 50 from 220 = 170. Take 170 and multiply it by .6 and you get 102 or your lower exercise range.

Multiply 170 by .8 and 136 is your higher exercise range. This range defines that a man age 50 has a 50 percent to 75 percent of Max heart rate for his age group. So for a 50-year-old male the formula assumes 170 is your maximum heart rate beyond that 170 can be unsafe, even damaging.

The problem with generalized formulas and standardized "Target Heart Rate" charts is they have large standard deviations in them. They also do not account for each persons fitness level and other factors like gender, medical conditions or illnesses along with body weight, height, etc. These health and fitness variables can affect all ages equally either positively or negatively.

Many health and fitness professionals do not believe that a maximum heart rate can be defined accurately based on age. They measure it by where the individual's body starts to operate at its Lactate Threshold Heart Rate (LTHR). Your lactate threshold or sometimes referred to as aerobic threshold is where the intensity of your workout starts to accumulate lactic acid in your blood stream faster than it can be metabolized from your body.

The Lactate Threshold Heart Rate (LTHR) is generally figured more at 85 percent of max heart rate. This means if a man age 50 has a traditionally calculated max heart rate of 170 their Lactate Threshold Heart Rate would be around 144. The problem with Lactate Threshold Heart Rate is it can only be measured by testing your lactate levels in your blood. No matter your age if you're in excellent health and at a high fitness level your Lactate Threshold will define your level of exertion. This exertion level for most is where you're feeling muscle burn, you can't speak very easily and you're getting to a point during a working out where you feel like you're shutting down.

The Lactate Threshold Heart Rate (LTHR) can develop quickly for example during interval exercises or develop more slowly as seen in a paced run or jog. Typically your lactic acid levels in your blood tells you your limits before you reach your maximum heart rate. This is why many health and fitness professionals see Lactate Threshold Heart Rate as a more precise way to measure your cardiovascular conditioning.

Getting back to the question how accurate and safe it is to calculate your target heart rate based on age? When it comes to health and fitness levels we are all different so it is very important not to compare yourself to others when dealing with heart rate levels. Age is a very important factor in finding the right number, regardless of one's fitness level. For example, a 67-year-old's range is 80 to 120.

A 24-year-old's range may fall between 118-157. Some people naturally have faster or slower heart rates than others too. If you find that your range is slightly abnormal, you will eventually become familiar with what range is normal for you. But you should recognize these formulas and/or charts are just simply a generalized guide to give you a starting point. What really matters is listening to your body and taking queues on fatigue, lactic acid build up and oxygen recovery.

If you are just getting back into fitness and exercise you should keep your heart rate lower and don't push yourself toward your max levels. As you increase your Cardio conditioning, you will strengthen your heart muscle and respiratory system and your body will naturally perform at higher levels without the need to push yourself to the extreme.

Please remember that when starting any exercise regimen, getting a physical and consulting your doctor is very important regardless of your age. There are many health factors that must be considered. One good example: if you're taking blood pressure medication, you may need to keep your heart rate below a certain level. Whether you're a beginner or off the hook fit, be aware of the warning signs or symptoms that may indicate a problem: chest discomfort and/or pain in arms, neck or jaw, unusual shortness of breath, dizziness or light-headedness that occurs often during exercise, and heart rhythm abnormalities. If one of these symptoms occur, medical attention should be sought immediately.

Finally, cardiorespiratory fitness is one of the most important components of health and fitness. It can be one of the best indicators of overall health for adults. Cardiorespiratory health is necessary to sustain the energy requirements of a normal life. Effective cardiorespiratory endurance levels help maintain recommended body weight and reduce many other risk factors.

Judd Jones is a director for the Hagadone Corporation.