Heart Rate Training Simplified: Unveiling the Two Key Zones for Efficient Running

Most modern runners will utilise heart rate to control the intensity of their training sessions but there are a number of schools of thought around how to calculate the different heart rate training zones. The most common model consists of five different zones:

• Zone One (recovery) – 55-65% of maximum HR
• Zone Two (aerobic) – 65-75% of maximum HR
• Zone Three (tempo) – 80-85% of maximum HR
• Zone Four (threshold) – 85-90% of maximum HR
• Zone Five (anaerobic) – 90%+ of maximum HR

I argue that having five, clearly demarcated zones based on our maximum heart rates is over-complicated and not that accurate.

Firstly – Anyone who has trained with a heart rate monitor will be aware of the phenomenon called ‘Heart Rate Lag’. Simply put, this is the delay in our measured heart rate reaching the level of our exertion. Our measured heart rate takes time to reach the level of our exertion so, when performing shorter intervals or starting a longer tempo run, we need to use experience and perceived exertion to control effort and pace so that we don’t end up starting harder than we want and then having to back off once we reach the heart rate zone that we are looking for. That is not a big problem once we have a bit of experience training with heart rate monitors but what it does mean is that aiming to hold training zones that are too narrow becomes problematic, especially on any training route other than a flat track, field or treadmill.

Secondly – maximum heart rate is not a good determinant of exertion. The more trained runner will be able to hold heart rates closer to their maximum for longer. Their bodies adapt to training and become more efficient at higher heart rates while their maximum heart rate will stay the same. Over the years, our maximum heart rates decline. Starting out unfit, using the five zone model above will produce different exertion levels than using the same model a year later in a considerably improved state of conditioning. A more effective approach is to use the two significant thresholds that occur as we increase our effort while running. These are pretty easy to calculate. Well, when I say easy I mean, the calculation is simple but the test will be fairly strenuous.

The first heart rate threshold we are looking for is the ‘Anaerobic Threshold’. Theoretically, this is the heart rate that coincides with the level of exertion where the body is just able to keep up with the muscles demands for oxygen and clearing and conversion of metabolites that result from the breaking down of carbohydrates for energy in the muscle cells. So, at the anaerobic threshold heart rate (AnT), the body is working furiously to provide everything required to hold that effort. A little bit more effort and our bodies fall behind and we go into deficit which will need to be repaid sooner rather than later and to do that, we would have to ease up and slow down. Running at our AnT is our optimum, sustainable pace. In well-trained runners this corresponds quite closely to their race pace average heart rate for a half marathon.

Determining AnT can be done using a ramp test on a treadmill or track where the runner starts slowly and gradually speeds up at regular increments until failure. Heart rate generally rises in a linear fashion until AnT is reached, at which point a gentle deviation away from the straight line can be seen.

A far simpler method is to perform a rested 8km time trial on a flat route. If the effort is as close to a full gas effort as we are able and we came into the session with fresh legs, the average heart rate for the TT should correspond quite closely to our AnT. Close enough for the purposes of determining training intensities anyway and there are will be no need for interpretation of heart rate graphs or messy calculations. Once we have our AnT heart rate the other threshold is even simpler. This is our ‘Aerobic Threshold’ (AT), which we can calculate as AnT-20bpm.

Why are these two thresholds significant to our training? Good question… They let us know how our body is fuelling our working muscles to produce that effort and speed. We have already discussed that AnT is our maximum sustainable effort. At this level of exertion we are almost exclusively burning carbohydrates in the form of stored glycogen. AT is a much lower exertion level and the body therefore has more time to break down and utilise fats for energy. Our skeletal muscles actually prefer to use fats as fuel in order to save the more rapidly metabolised carbohydrates for tasks that it deems more important than running our marathon PB. Things like brain function and generally staying alive. Even the leanest among us has an almost inexhaustible amount of energy stored as fat. The harder we run though, the more the body is forced to use carbs, although this is always a ratio. Higher fats to carbs closer to AT, changing to higher carbs to fats as we approach AnT. So, that 20bpm zone between AT and AnT is very significant as far as how our muscles are fuelled, which is a big determinant of how long we can hold higher paces.

Knowing these two HR thresholds, how do we structure our training? The fundamental goal of training is to become more and more efficient. A major component of that is how our bodies fuel our efforts. Training at lower HR will allow our bodies to utilise a higher ratio of fats in our fuel mix and, like anything we do, the more we allow our bodies to ‘practice’ that, the better they get at it. Then what will happen is our fats / carbs ratio will change at higher HR. Allowing us to hold higher HR for longer and therefore the higher corresponding speeds. As a result, the bulk of our training should be below AT. Our endurance building runs and our easy runs should all be aerobic and below AT.

Our more quality sessions such as hill efforts and tempo runs are then aimed at the zone between AT and AnT. The longer the effort, the closer it slides to AT and the shorter they are, the more they get up to AnT. This is usually determined by where we are in our training progression and how far we are away from our goal event. The thing to remember with the quality sessions is that they don’t have to end with us on our hands and knees. Once we move above AT and into that 20bpm zone, we are developing efficiency at AnT. So there is an anaerobic training effect happening even just above our aerobic threshold. Obviously, there is much more stimulus at a HR 5pm lower than AnT than there is at one 15bpm below but, recovery from that easier effort is quicker and it can then be repeated sooner than the higher effort. So, a balance needs to be found between effort and recovery and this will change as our form and recovery capabilities improve.

Those are the two HR zones that we need to concern ourselves with. Yes, we could include a third one which is above AnT heart rate but very few of us need to train at that level. That would be adding the final polish of speed to our training progression and the intervals will be of such a short duration that using a heart rate monitor is not really effective because of the aforementioned heart rate lag.