When most of us begin to explore running, cycling, swimming, or any physical activity for that matter, we usually do not understand concepts like heart rate, power, or zones.
We may have a basic feeling or understanding of pacing, from watching others do the sport, or simply knowing how long it takes us to complete a certain distance.
This does not really tell us much other than how fast we are going relative to how fast we think we should go, or how fast our friends or pro athletes go.
We also have a feeling of how much it hurts, how heavy our breathing gets, how the muscles burn, how we cant talk till we catch our breath, or how we can talk if we simply slow down. This feeling is referred to RPE or rate of perceived exertion. It is a very subjective or simplistic (which can still be effective to a certain level) method of training.
The next step is to link specific paces we know we can run based on a 1 to 10 scale, where 10 is an all out effort that can only be held for a few seconds, and 1 is moving forward but at such a slow pace, we are actually regaining breath and our muscles are recovering. Many of us have been to a spin class where the instructor will tell you go at level 8/10 and then 6/10. Now, based on our energy levels for that specific day, and completely on how we are feeling at that moment, how stressful work was, how long ago we ate, how hard the previous days training was, 8/10 on a certain day may be equal to 6/10 on another. It is therefore necessary to have a more objective measurement of the level of exertion we are training at. Almost all coaches today require a heart rate monitor at the very minimum and a power meter is highly recommended as well.
The HR monitor is a physiological quantification of the exertion, it is a measure of the bodies response to a specific level of effort. It tells you how hard your body is working, and the measurement is expressed in beats per minute. It is much more objective than RPE. The power meter on the other hand is an external measurement of the force that is used to turn the pedals at a specific cadence (rotational speed) and drive a person forward on the bike (or force of propulsion off the ground in the case of running power meters which are now available). In other words, it is a measure of the power exerted by the muscles. The units of power are watts. The power meter is an even more objective measurement of intensity since HR may be affected by fatigue or stress or even temperature, while the power measurement has almost no variables.
Now, in order to classify how hard to go in a workout, we need to know what heart rates or power readings correspond to what level of effort for us and we need to do this objectively without using a RPE scale. This is where the concept of training zones comes into play.