Introduction to heart rate zone training and RPE training

Firstly, welcome. If you’re here it’s likely you are wading into the world of cycling training, and trying to wrap your head around the intricacies of duration, intensity, zones, carbohydrates-per-hour and a host of other things that come part and parcel with cycle training.

One of the most accessible pieces of cycling tech nowadays is heart rate monitoring, which is featured with almost all cycling and watch-based fitness computers. In more recent years, power training has been hailed as ‘king’ and has become much more financially accessible for the average cyclist, however heart rate training still has its place in cycling training. Here we will go through a few bits of info to get you started with heart rate based training.

What is the use of tracking heart rate?
How does this differ from power-based training?
What are my heart rate zones?
How is heart rate affected by different things?
How Does RPE fit into the mix?

What is the use of tracking heart rate when exercising?

Heart rate is one of the most accessible modes of tracking training, and can be done via LED sensors within a fitness watch or separate heart rate strap. In more recent years there has been increased interest in long-term heart rate tracking and what’s called ‘heart rate variability’ to measure fitness and recovery, however for the purposes of this article we are discussing heart rate while exercising.

Super basic physiology: your heart is essentially a pump that acts to push blood around the body: the left side punches it out to the head and body while the right side pumps blood returning from it’s magical journey around the body, back to the lungs via the pulmonary artery where it basically grabs some oxygen and drops off the carbon dioxide, returning to the left ventricle to deliver some fresh oxygen to the body.

Low-intensity exercise (we will call it ‘aerobic training’) results in an increase in heart rate as well as stroke volume (SV: the amount of blood pushed out with each ventricular contraction aka; heart beat), which together lead to increased cardiac output (HR x SV+CO). The increase in stroke volume is a result of heart size increase, leading to decreasing resting HR, increasing cardiac output, increased capillarisation, and changes in cellular composition including increased mitochondria.

As we increase the intensity of exercise though the aerobic zones, we hit the point at which there is an onset of blood lactate, often referred to as the first threshold (though there are many studies that will refer to this phenomenon in a few different ways, VT1, LT1, OBLA are all terms that are discussing similar things that probably occur at slightly different times) and then through the second threshold (VT2, LT2, MLSS…) the point past which the bodies production of lactate outstrips its ability to process, leading to increased lactate accumulation. Thus, when you’re exercising ‘very hard’ it’s probably above this threshold, can only be sustained for periods of minutes (as opposed to around an hour for on the second threshold, or multiple hours below it).

The crux of this is: cycling is primarily an aerobic sport and aerobic training stimulates the structural changes run your physiology to make you better at the sport. Keeping in a training ‘zone’ to elicit these changes is where heart rate (and power based) training comes in. Using heart rate is insurance that you are working around the intended intensity that’s prescribed for a session.

2. How does HR training differ from power-based training?

Power-based training differs from heart rate training in a few ways, and both have their merits. Power is measured via a strain gauge in a power metre, which can be on a crank arm, in a chain ring, in a rear hub, through a pedal or on a smart trainer. Power is measured in watts (yes, like a lightbulb) and is an objective, measurable output of workload, compared to heart rate which is measuring your bodies response to the work. In automotive analogies, power is speed and heart rate is the revs.

Two riders can be sitting at similar heart rates and have vastly different power outputs, or at similar power but with different heart rates. Generally, a larger rider will have the ability to sustain larger power numbers due to larger muscle mass and motor unit recruitment, but workload over hilly terrain is more usefully scaled against weight, so a 50kg rider sitting at 200w would be moving more speedily (all other things controlled!) Than a 90kg rider at 330w.

Power is an instant assessment of what’s going on: while heart rate takes a while to respond to workload changes, power instantly assesses if you’re on the pedals or not, and hence one heart rate can tell a very different story if we add in power data to the mix.

Here is an example of a file from a rider that is using heart rate and power: red line is heart rate, purple is power and the yellow line is cadence. This is a cyclocross race so one hour at very high intensity. As you can see the power is up and down the whole time, each lap has a discernible rhythm, while HR varies very little: it’s all “full gas”!

While power has been seen to be a ‘gold standard’ as it measures work performed, we shouldn’t forget the humber heart rate, as it tells us the response to work and is still a useful metric for looking at time spent in zones.

3. What are my heart rate zones?

Heart rate zones can be calculated in a few different ways, but the old ‘220 minus your age’ is the one most people will be familiar with. This is, however, one of the least accurate ways to measure heart rate. Using maximal heart rate of threshold heart rate is much more accurate, threshold in this case referring to the maximal heart rate one can sustain for an hour. If you have ever done a criterium, XCO mountain bike race, or cyclocross event congrats: you probably already have some useful data to start with.

When you have assessed roughly what the number is that you can sustain for an hour you can use this handy threshold chart embedded below to delineate your zones.

4. How is heart rate affected by different things?

One of the flaws of heart rate, like much of physiology, is that it’s absolute. We aren’t robots and as such there are things that can affect heart rate readings. In particular, heat can augment heart rate up, as can caffeine and stimulants and anxiety. If you have ever had a hard ride on a really hot day you may have already noticed this!

In addition to this, excessive fatigue, circadian rhythm and some medications can push the heart rate down. Any big aberrations in heart rate, low or high, may be a result of arrhythmia and should be checked out by a doctor!

Heart rate is also affected by age, so if you’ve been tracking your heart rate throughout your life you will probably notice a very gradual decline in your maximal heart rate.

5. How does RPE fit into the mix?

For those of you without the inclination to use heart rate or power, there is another effective way to train and it’s the way we all used to train before exercise technology was prolific: RPE or rate of perceived exertion. Originally based on the 6-20 model called the ‘Borg Scale’ RPE was used in lab testing in order to understand the relationship between exertion and how things felt, and what was happening physiologically. 6–20 was selected as it was said to pick where an athlete would be between resting (heart rate of 60) and maximal heart rate (200).

Most RPE levels offered now have brought this down to 1–10 for simplicity, but understanding ‘feel’ and RPE can be a bit of a challenge in itself. Using the zones outlined above, Zone 1 would be 1–2, Zone 2 would be 3–4, zone 3 at 5-6, Zone 4 at 7 and zones 5+ ay 8 to 10. While RPE isn’t the most accurate way to train, it is a good way to work on understanding how endurance training and higher intensity training FEELS and is good to keep track of overall session RPE in order to track changes in fatigue and freshness (ie: if you were turning the pedals at RPE 2 but felt awful).