Proper development of this system is highly related to how hard you train. Verbal description does not always convey a clear definition of intensity level. How many levels of hard are there? In order to make this definition of intensity consistent, it makes sense to establish a systematic method to monitor intensity by creating progressing levels that are dependant on objective measurements as well as one's individual physiological traits.

The two criterions are: Rate of Perceived Exertion (RPE) and Heart Rate (HR). The heart rate monitor will give you an accurate reading of what your heart is doing (and allows you to correlate HR with a given training level/zone). Your perceived exertion allows you to evaluate degrees of difficulty by assigning numbers to your perception of each incremental level of exertion. Heart rate monitors provide an external, objective measurement, which (though an excellent tool) can°Øt account for factors like muscle fatigue, and psychological distraction. RPE enables you to subjectively measure your level of effort, and when combined with a heart rate monitor, provides a more complete and accurate picture of one's level of effort. Because of these points, it makes sense to combine the two when communicating the varying intensities of workouts.

Rating of Perceived Exertion (RPE)
In the early '70s a Swedish scientist named Dr. Gunner Borg developed a scale that we can use to express our subjectively perceived effort. He determined that athletes could accurately predict how hard they were exercising based on how they felt through a numbered scale of 6 to 21. These numbers correspond quite accurately to their heart rate. Some experienced athletes have sharpened their awareness to the degree that they can identify their lactate threshold exactly from their subjective feelings. Since the 70s the scale was modified to a scale of 1 to 10 which most people can better relate to. As in anything in life it will take some time before you can correlate your subjective evaluation of intensity, or RPE, with the objective method of taking your pulse.

Heart Rate training (HR)
Your heart never lies. Heart rate is your most accurate measure of effort. Two factors are essential when using HR for training; your maximum HR, which represents our maximum intensity/ability, and resting HR which represents the minimum intensity/maximum rest.
Athletes and coaches have recognized long ago that heart rate monitors will accurately reflect objective effort. The ability to know what your heart is doing (what intensity level you are in) without stopping really helps athletes focus on training, and not trying to find their pulse.
Maximum Heart Rate (MHR)

The highest attainable heart rate; this value is genetically determined, finding the value of your MHR is best done by going out and achieving it. For the run, go to a track, warm up for 15 minutes or 2 miles, than begin to run progressively faster increasing your speed every quarter mile, picking up the pace until you are sprinting all out for the final 100 yards. Check your heart rate immediately after you finish. This is your MHR. Remember to cool down at least 10 minutes (jogging or walking). MHR is not a trainable level _ it is what it is.
Some suggest using the formula 220-your age. That formula has a standard error of 10 beats per minute (bpm) and can really skew the measure of your effort. It is probably as accurate as assuming that all people at a certain age have the same height.

Resting Heart Rate (RHR)
The lowest HR you have. Average RHR range is 60 to 80 (bpm). In highly conditioned endurance athletes RHR ranges between 28 and 40 bpm. It is easy to determine, especially first thing in the morning. When you wake up, and before you get out of bed, feel for your pulse by sliding your middle and fourth finger into the groove at the base of your neck. Relax, then take note of time using a clock or a wristwatch and count the number of pulse beats for 15 seconds. Multiply this number by 4 to identify your RHR. Check it regularly to monitor fitness and detect signs of overtraining or impending illness.

Anaerobic Threshold (AT)
Anaerobic threshold is considered by many scientists as the point at which blood lactate begins to accumulate above resting levels during exercise of increasing intensity. Muscles produce lactate well before the lactate threshold is reached, but it is being removed from the muscles by the blood. AT is expressed as % of maximum oxygen consumption (VO2 max) or in our case, % of maximum HR. It is considered as the point where the blood can no longer clear lactic acid from the muscles faster than the muscles produce it. Accumulation of lactate acid will cause the muscles to fatigue and slowdown. Consequently, athletes that have an AT of 80% of MHR will be able to tolerate exercise better than an athlete that has AT of 60% of MHR even if both have the same MHR.

Many exercise physiologists believe that raising your AT is the most important factor in enhancing your performance in endurance events. Raising your AT allows you to go faster with less reliance on anaerobic metabolism so that lactic acid will accumulate at a slower rate. This is the focus of this program.

Finding your AT could be a bit complex .One common and practical way is to go out for a 10k race. You should be running at somewhat uncomfortable pace, actually racing. Your average HR for the race is a very close estimate to your running AT.

Using the AT in training
Now that we are familiar with these terms of intensity (heart rate, perceived exertion and anaerobic threshold) we can combine them for the purpose of communicating how hard or easy a particular workout is to be performed:
As we mentioned many times, triathlon is predominantly aerobic; therefore you should spend most of your training time in the aerobic zones; level 1 and 2. However, levels 3 and 4 also have an important roll in developing the necessary conditioning for triathlon.

Training intensity table
(This is a modified intensity table that was originally develop by the coaches at multisport.com)

HR zone 1
This range is less than 78% of your AT, or 70% of your MHR, also known as "active rest". Training within this zone maintains basic endurance and recovering from hard work. Another advantage to running in this zone is that while you are happily burning fat, you allow your muscles to re-energize with glycogen, which has been expended during those faster paced work-outs. One of the biggest problems for athletes is the inability to go easy and to allow one's body to recover training at this level, maximizing the benefit of your training.

HR zone 2
This range is 78-90% of your AT or 71 to 80% of your MHR. This is a good zone to stay in for your long rides and runs, developing your cardiovascular and respiratory system, allowing muscles to work more efficiently. This is a relatively easy zone to be in, but requires steady, moderate effort.

HR zone 3
Your lactate tolerance heart rate zone is 90-100% of your AT or 81 to 90% of your MHR. This could also be called 'Tempo'. Depending on one's level of fitness, the frequency and duration spent in this zone will either drop you into an over training abyss or gradually maximize your athletic potential. As you raise your AT, this zone should feel easier and easier. It is not an "easy" level of intensity but one that you should be able to hold for long periods.

HR zone 4
Training above your AT or above 91% of your MHR, you use this intensity level during your race phase. Used in the right amount, this is where major gains in anaerobic capacity for shorter races can be made. Too much time spent in this zone will quickly lead to diminishing returns. In this case, abuse doesn't take long to manifest itself into symptoms of over training.
(The above training zone explanation is a modified explanation that was originally develop by the coaches at multisport.com)

Calculate your training zones
The calculation of a zone value, X%, is performed in the following way:

• Subtract your RHR from your MHR giving us your heart rate range (HRR)
• Calculate the required X% on the HRR giving us "Z"
• Add "Z" and your RHR together to give us the final value

Example: The athlete's MHR is 180 and her RHR is 60 - determine the 70% value

• MHR - RHR = 180 - 60 = 120
• 70% of 120 = 84
• 84 + RHR = 84 + 60 = 144 bpm

Talk test
If you do not have a heart rate monitor use the talk test to define intensity.The talk test measures perceived exertion and is considered a fairly accurate estimate of exercise intensity. It can also be used to harmonize calculated exercise heart rate with actual exercise intensity.

Zone 1 and 2 - An individual should be able to breathe comfortably and rhythmically throughout the entire exercise session. You should be able to carry on a conversation during your workout.

Zone 3 - Breathing is significantly more strenuous than at rest. One is still able to talk, but finds it difficult to speak in full sentences. You should just about be able to carry out a 'snatched' conversation.

Zone 4 - you can barely answer your own name.