Why training heart rate zones don’t work for ‘you’.

In previous articles we have discussed everything you need to know about physiology testing including maximal oxygen uptake testing and lactate threshold testing.

We have even discussed training zones and some of the reasons as to why people can’t stay in specific zones (namely zone 2) and ultimately why this is not always necessary once you understand ‘who’ and ‘where’ you are as an athlete.

Without doubt two of the most asked questions we get is ‘why I can’t stay in Zone 2?’ (in relation to heart rate zones) and/or ‘why is it that my heart rate is high when my pace/effort level is low?’

Before we get into this, there’s a few questions we need to answer.

1.      How does your heart rate monitor or smart watch estimate your training zones.

2.      What actually is heart rate measuring and most importantly what it is not.

Let’s start with the most important question no-one ever asks…

Why do we actually measure heart rate and what does this really represent?

The reason that this question is hardly ever asked is why we monitor heart rate is because the answer on face value is simple.

Of course it is a measure of effort. Case closed.

Not quite.

We need to understand what it truly represents and its limitations as a measure of effort. Hopefully then we can be less confused by zones and our ‘weird’ responses and inability to stay in them. Most importantly, if and when this is a problem!

The reason we use heart rate zones is that they do indeed measure effort and ‘typically’ heart rate tracks our oxygen demand and uptake. This is why if we measure your oxygen consumption on one of our testing sessions heart rates increases as our demand for oxygen increases…

and our demand for oxygen increases because of increased demand for energy…

and our increased demand for energy comes from needing (or wanting) to move faster.

From lots of data on lots of people we can then use heart rate to look at oxygen uptake zones and then we look at the research to say at what percentage of maximum heart rate do we commonly find the specific training zones, with each training zone being representative of both our effort level and the adaptations we get from training at the level but also what energy systems in the body we would expect to be using.

Simple, right?

But the problem is twofold.

Firstly, for the keen eyed amongst you there is a lot of assumptions being made here from our direct measure oxygen consumption, how this translates to heart rate zone boundaries and then also how these boundaries translate to energy system utilisation.

The issue here is that each zone is given the same amount of ‘weighting’ (50-60%, 60-70% etc.) and what we know is that training influences not just at what percentage of heart rate these zones may or may not start, but also the ‘bandwidth’ of each zone.

These can only be accurately determined with oxygen uptake testing and lactate testing, where lactate adds a second, direct measure of our ‘deeper’ physiological response that helps us to create a clearer picture of what is happening to determine accurate threshold zones.

If you imagine your max heart rate is 190.  Then if we determined 5 zones, with zone 1 starting at 50% of maximum heart rate, then each zone would be around 20 beats per minute ‘wide’.

The reality is that if you did a lot of base endurance work with very little of anything else, you might find your zone 1 and 2 (easy effort training) zones are wider, so whereas your HR monitor might tell you zone 2 ends at 130 beats per minutes (90+40) it may, in fact, end at 140 or 150 beats per minute.

With other higher zones being ‘squashed’.

Secondly, and surprisingly commonly, is that peoples heart rate response (and even O2 and lactate responses) may not be ‘typical’ depending on their training history and how they respond to switching on as a response to exercise and training.

When we think about heart rate being a proxy for the increased demand for energy and oxygen delivery. What we are actually missing here is an idea called Cardiac Output.

This is how much blood (and in turn oxygen) we are pumping from the heart each minute, and this is the result of heart rate (HR) and stroke volume (SV). Stroke volume is how much oxygenated blood the body pumps with each contraction of the heart.

Cardiac Output = HR x SV

Typically, what we expect to happen is that both HR and SV increase in unison to increase cardiac output and deliver more glorious blood and oxygen around the body.

However, in some people there may be a (completely normal and un-concerning) a slightly out of synch response.

SV may ‘lag’ heart rate so to get the same cardiac output the heart beats a little faster then expected even though at a metabolic level energy/oxygen demand is still low.

We see this a lot in athletes who need to be ‘explosive’ or work a lot at high effort levels, the heart rate jumps the gun as a response to even lower levels of demand as it is ‘used’ to working at high heart rates, so this becomes the dominant response when triggered.

Conversely, some athletes will have led with a increase in heart contractile ‘squeeze’ increasing stroke volume, keeping heart rate lower than expected, as the big old heart muscles force production is doing most of the work. Then heart rate increases to jump in an lend a helping hand as we work harder.

Side note: This is one of the reasons athletes (especially with an endurance dominant performance profile) have such low resting heart rates, they pump more blood (that contains more oxygen) with each contraction and get better oxygen uptake and this gives them more and wider heart rate gears to go through.

This is also way as heart rate declines with age, we can still ‘compete’ over longer durations as we keep our lower gears in good order, we tend to lose our high end gears and power muscle fibres (this is why we should also keep on top of strength training especially endurance athletes as they age).

Now onto the original ‘question’ about how your tech is producing these numbers.

How does your heart rate monitor or fitness app determine heart rate zones.

The answer, as we have discussed, is easy at present based off of heart rate versus intensity assumptions, but it is getting increasingly more complex to answer.

The reason it is becoming more complex is that with the increasing interest in AI and data manipulation with coaching apps, there are some ‘scary’ calculations that can be done looking at your historic data from whatever apps you use to track your training that can create your zones based off of your paces across those runs.

We have had some very interesting conversations with coaching apps about the way that they can your training history and even the conditions your runs are in and your previous performance to adjust paces, session distances and durations automatically.

However cool and promising these apps are there is still an issue. Firstly, these are still based of an algorithm that is based off of certain assumptions on the complex being that is ‘you’, and also it is highly influenced by the standard to the data going in.

In computer speak ‘if you put garbage in, you get garbage out’.

Using wearable tech has a broad spectrum of accuracy in measurement and no matter how accurate we’d like them to be there are components it will NEVER be able to measure, such as exact conditions under foot and minute to minute weather, clothing, sweat rates, biomechanics and lots of other factors that influence performance.

It is true these limitations still effect real coaches looking through reams of data, and apps may even do a better job, depending on the experience and attentiveness of a coach. Unfortunatley, we are a long way off minute to minute internal and external monitoring of our environment and arguably more importantly our mental state.

Even though physiology testing only provides a snapshot in time that can be influenced by all of these conditions, the main metrics we measure are still relatively fixed.

Paces and powers might not be accurate to determine how these are represented in the real world. They do give us some ‘absolute’ values that are useful and an ability to profile an athlete to understand their unique physiological response to exercise, both in the present moment and also as result of training. When these are combined with an expert eye can determine not just if performance has improved but why and what to do next.

Now I have pleaded the case for physiology testing as it is still (at present) the only true way of profiling you as an athlete and your ‘metabolic biases’, strengths, and weaknesses.

This is one of the reasons why at Athlete Lab UK we look beyond the numbers, we want to know why they are what they are and ultimately be able to accurately map your external measures of performance to the more accessible measures of effort such as perceived effort, and of course, heart rate.

It should be a pretty small step as to see why even more simplistic measures like heart rate still fall foul of issues with accuracy depending on your kit, but what it doesn’t do (for good or bad) is add additional variables and potential sources of error.

The negative aspect of this level of simplicity, is that unless you have a very typical heart rate response that maps are expected internal physiological responses, then trying to base your training zones and paces/powers off this is like throwing a dart with your eyes closed and expecting to hit the bull’s eye.

It’s not that you HAVE to hit the bulls eye to make progress, maybe the dart board will do, but if you are following an approach for a specific reason then we should at least make an attempt to be as accurate as possible.

We can tell you from experience that a high proportion of the athletes we test do not fall into the heart rate bands given by their ‘tech’ when we compare their heart rate data to their physiologically determined thresholds.

This is because most heart rate monitoring devices simply use your maximum heart rate (or predicted maximum heart rate) and then use this to work out the following zones.

However, from testing, we know that when we use lactate and ventilatory thresholds to determine heart rate zones there can be a HUGE discrepancy with what the tech shows.

To summarise…

Your training history, current fitness levels and body size are all going to throw these zones out of whack and are impossible to stay in whilst exercising without seemingly going at such a low effort as to be effectively useless for training.