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If you're a cycling nerd like me, or perhaps into running or other endurance sports, you've probably come across the concept of Zone 2 training.
The idea has exploded in the past couple of years, with countless YouTube videos and articles written about the benefits of low-intensity training.
Put simply, Zone 2 is the metabolic state where you burn the most fat to power your exercise. It's also a zone where it's easy enough to exercise for a long time without blowing up.
The Zone 2 idea is that low-intensity training should make up the bulk of your training volume and by doing this you'll increase your overall cardiovascular fitness without having to smash yourself.
It's in direct contrast to the concept of high-intensity training and interval training, which were all the rage for a time, particularly for time-poor people. The idea being that short, sharp, high-intensity sessions could make up your entire fitness regime.
One of the biggest proponents of Zone 2 is Inigo San Millan, a professor in Colorado University's Department of Family Medicine, who specialises in physiology, metabolism, mitochondria and cancer research.
Dr San Millan is the director of performance with UAE Team Emirates, the cycling team of two-time Tour de France winner Tadej Pogačar.
Dr San Millan says Pogačar will spend about 80 per cent of training at Zone 2 and he says that should be the same for all of us.
In order to answer that question I subjected myself to an experiment.
My guide is Antony Stadnyk, a lecturer at the School of Sport, Exercise, and Rehabilitation at the University of Technology Sydney, and a former physiologist with the New South Wales Institute of Sport's cycling program.
"So this is what we for a long time have called the aerobic endurance training zone," Dr Stadnyk says.
"It's got this fancy new name, Zone 2, but it's basically the top end of the low-intensity training zone.
"So it should feel easy, you should be able to have a conversation, but towards the top end, that's when you'll start to lose that ability and need to talk in shorter sentences or a couple of sentences before taking a breath of air.
"Zone 2 and our base conditioning, the aerobic base conditioning, is really the foundation of all our training."
Dr Stadnyk and I are talking after he's performed a series of tests on me to determine my metabolic thresholds and VO2 Max.
We did the tests in October last year.
My experiment is to commit to an extended block of training – the bulk of it in Zone 2 – to see if I get fitter and build my endurance.
I need a goal, so I've committed to riding in the Snowy Classic – a Gran Fondo in the Snowy Mountains in March.
There's a 170-kilometre version, which I think is going to be a stretch too far for me – this year at least – so I'm doing the 107km loop from Jindabyne, which takes in around 1,500 metres of climbing.
Back to the tests — I'm at UTS with my bike on a stationary trainer wearing a heart rate monitor.
Dr Stadnyk's strapped a mask over my head to capture the amount of oxygen I'm breathing in and carbon dioxide I'm exhaling.
After a warm-up, he asks me to ride for three minutes at a particular power output – which is measured in watts.
Dr Stadnyk is capturing all my data in real-time and has made a small pin-prick in my ear to collect a blood sample to measure my lactate levels as I go.
Then I'm off and pedalling. Dr Stadnyk starts me off at a leisurely 80 watts and then every three minutes he asks me to increase my wattage by 20. Each time I go up in power, he takes a blood sample to measure my lactate.
It's pretty easy at first of course, but after 24 minutes I'm at 220 watts and my heart's beating at 153 beats per minute (bpm) – about 91 per cent of my maximum heart rate.
I could go for a bit longer, but Dr Stadnyk now has enough information to determine my various metabolic zones.
And for the data nerds out there, this is what a moderately fit and slightly overweight 56-year-old is pumping out.
Time (minutes) | Power (watts) | Heart rate (bpm) | Lactate (millimole/litre) |
|---|---|---|---|
3 | 80 | 102 | 0.5 |
6 | 100 | 107 | 0.6 |
9 | 121 | 112 | 1.0 |
12 | 140 | 120 | 1.0 |
15 | 162 | 130 | 1.5 |
18 | 180 | 139 | 2.4 |
21 | 199 | 147 | 4.0 |
24 | 221 | 153 | 5.8 |
Two-minute max effort | 322 (265) | 169 | 9.9 |
After I take a breather, I warm up again for an all-out three-minute effort, which Dr Stadnyk is going to use to determine my VO2 peak and maximum heart rate.
At least it was meant to be a three-minute effort, but I made the mistake of going out far too hard and then had to hold on for dear life, while Dr Stadnyk quickly recalibrated it to a two-minute effort.
I've recorded an average wattage of 322 but he warned I should be cautious about the wattage because it was only a two-minute effort – which meant it was mainly anaerobic. He estimated that I would have averaged around 265 watts if I continued for another minute. Next time around I'll go for a more paced three-minute effort and expect a lower mark.
But at least now he has the numbers. My VO2 peak was 45.6 and my maximum heart rate was 169bpm. I'm hoping that at the end of my training block, the VO2 peak, at least, will be higher.
Dr Stadnyk determined that my Zone 2 is somewhere between 115-130bpm or between 130-160 watts.
Ideally, we'd be using power as a metric during my rides, but I don't have a power meter on my bike, so I'm going on heart rate alone.
He came to those numbers by looking at both my lactate and ventilatory responses. You can see that above 120bpm and 140 watts, my lactate levels jumped from one millimole per litre (mM) to 1.5mM. There's another big jump after 130bpm and 162 watts to 2.4mM.
This graph shows that while my heart rate steadily increased in a straight line with my wattage output, the amount of lactate I was producing increased slowly and then started shooting up at that 162 watts mark – an indicator that I'm starting to burn more carbohydrate and less fat.
So, what exactly are we talking about with all this lactate, heart rate and zone stuff?
Remember that during Zone 2 we're in the metabolic state where we are burning the most fat to fuel our oxygen.
"So, what we have is free fatty acids floating around the body, in the blood," Dr Stadnyk says.
Those fatty acids are converted by the mitochondria in our cells into Adenosine triphosphate or ATP.
"ATP is what our body uses to produce the actual muscle contractions and physiological processes that we need to sustain life," Dr Stadnyk says.
Now, at Zone 2 and below, our exercise intensity is such that the mitochondria can keep pace with our energy demands – burning fat consistently to keep powering our exercise.
Dr Stadnyk explains that when we consistently exercise in Zone 2, over time we get an increase in the number of mitochondria, which means our bodies become more efficient and have a greater capacity to oxidise fat for fuel.
We've been talking a bit about mitochondria, so Dr Stadnyk is taking us back to school:
"Probably back in high school, you've heard of mitochondria referred to as the powerhouse of the cell — that is absolutely true," he says.
"So the greater density and number of mitochondria that we have, the sort of fitter and healthier the system is."
So to summarise: As we train more in Zone 2, we produce more mitochondria, which means we have a greater ability to metabolise fats.
As we become more oxidatively powerful, our bodies can use fat for longer and put off using carbohydrates as a fuel source, saving those stores for when we really need it during high-intensity efforts.
So in theory, the lactate curve in the graph above should shift to the right as I become fitter, but that can only last so long once we start to increase the exercise intensity Dr Stadnyk explains:
"While fats are really energy dense, they can't be oxidised at a rate that is fast enough to maintain that exercise," he says.
"The top end of Zone 2 is what we call the first metabolic threshold — you might hear that referred to as lactate threshold one, or the ventilatory threshold one.
"And that represents the point where we start to have an increase in energy being provided by carbohydrates, which provide less energy [per unit], but can be oxidised at a faster rate.
"And so with that, we start to get an increased production of lactate."
Zone 3 is what Dr Stadnyk calls a tempo zone – we're starting to work a bit harder, and often you'll find yourself drifting into this zone during bunch rides.
"And then above that, we go into our threshold zone – Zone 4 and that crosses the second metabolic threshold," he says.
"And what that second metabolic threshold represents is where we are going predominantly into anaerobic metabolism, so we're not supplying enough oxygen to continue to fuel exercise."
You can see from my data that this happened to me at 200 watts where my lactate jumped from 2.4mM to 4mM.
Zone 4 is what's sometimes called threshold – it's hard work.
If you've ever tried to run a personal best at Parkrun, you would have been running at your threshold – although you would find yourself in the zones 5 and 6 as you pushed towards the end.
Zone 4 is when you're on the red-line, staying just short of the level of exertion where you would burn out in just a few minutes if you upped the intensity.
How long can you stay at the threshold? Again, it depends on training – for me it might be 30 minutes, but an elite cyclist or runner could go for much longer.
Above this we're in the sprint zones, Dr Stadnyk says, where "ATP and phosphocreatine stores will exhaust in 10 to 30 seconds".
Dr Stadnyk uses a six-zone model. Others might use seven, whereas your smart watch, for example, might have five.
OK, that's the science behind Zone 2, now to put it into practice.
Dr Stadnyk hasn't given me a training plan, rather he says my Zone 2 training should make up about 70 to 80 per cent of my total training time.
As for the remaining 20 to 30 per cent of my training time – he says I should do some work in the higher zones.
"A little bit in that Zone 3 and 4 area and then a smaller amount in that Zone 5 or 6 — if you want to put in a couple of extra sprint efforts that can be beneficial for the anaerobic power — you're going to get all of your zones ticked off," he says.
"A little bit of training variation is always useful, but also you're going to be stressing your body a little bit different and getting that anaerobic stress as well.
"So, the ability to actually use glycogen, and carbohydrates well, can be affected by that."
OK, that's the plan – let's say three sessions of riding per week at Zone 2 and one session at a higher intensity or maybe some intervals at Zone 4.
One thing to note about Zone 2: It's easy, but it's not just a gentle ride down to the shops.
When you're in Zone 2 you are definitely working up a sweat and pushing yourself.
It's often said that a good way to determine your Zone 2 is if you can have a conversation, but it's kind of laboured. You might need to stop in the middle of a sentence to take a breath.
So, off I go: Riding, riding, riding.
And then somewhere along the way, the real world kicked in.
As much as I tried to do that volume of riding, I also wanted to maintain other forms of exercise, like social football once a week and a couple of trips to the gym.
Christmas might have got in the way as well.
Come February, I've been probably averaging about three sessions a week on the bike.
I try to make most of them Zone 2 sessions, but the reality is it's very hard in the real world to maintain a steady heart rate.
When you go uphill your heart rate might go higher, downhill it goes slower, and when you stop at a traffic light, you have time to recover even more.
So my Zone 2 riding is really an average. I do my best to stay at a consistent heart rate, but there are necessarily natural variations.
I also do my one session a week at a higher level or do some intervals at the end of a Zone 2 ride.
Over the months I gradually increase my kilometres per week. I've joined a cycling group and go out with them once or twice a week and do a long ride each weekend and throw in some hill sessions in preparation for the Snowy Classic.
I feel I'm getting fitter, but exactly how much is hard to tell because it's all based on feel and I'm not measuring myself against a given known – like how long it takes me to ride a 40km route.
I'm just going out and riding at a consistent heart rate.
That said, I find I'm able to ride for longer at my Zone 2 level and find it fairly comfortable.
As the Snowy Classic gets closer, I try to throw in more climbing training, so this is where I'm definitely jumping out of Zone 2 and getting into zones 4 and 5.
And I do a couple of 100km rides – just to know I can.
The Snowy Classic was tough. A 3km climb with gradients of up to 15 per cent at the 86km mark was a leg killer. A lot of people walked, but I managed to stay on the bike and hang on for the finish, in utter awe of the people who went on to ride the full 170km course, which included another 1,500 metres of climbing.
Since my first test about five months earlier, I'd done about 2,400 kilometres and spent about more than 100 hours in the saddle.
Despite the kilometres in my legs, I had some trepidation as I headed back to the UTS for my follow-up date with Dr Stadnyk and my second lot of testing.
It felt like a lot of riding but was it enough riding to see some improvement in my levels of fitness and my VO2 max?
The answer was not as clear-cut as I would have liked.
Disappointingly, and despite feeling strong on the day, the data showed my basic metabolic response hadn't changed despite all the training.
There were differences however: my VO2 peak was higher – it went from 45.6 to 45.8 and my maximum heart rate increased from 169bpm to 170. At my age of 56, I'll take any changes as a positive.
And there's no question I felt much stronger in the second test. I was breathing through my nose throughout the test, which wasn't the case back in October.
And unlike the first test when I didn't even manage a three-minute effort because I went out too hard, this time I managed it perfectly and increased my power over the three minutes.
As Dr Stadnyk says afterwards: "We did see a change in how you completed the tests – you definitely looked a lot smoother … you looked more comfortable; your breathing was a lot more controlled.
"And what we saw from the VO2 data was that there was probably an increase in your efficiency on the bike. So you aren't using as much energy in each pedal stroke or to produce each pedal stroke.
"One of the main changes at the higher end was that you weren't producing lactate as much, or you were able to clear it more effectively and that's obviously beneficial as well."
So why didn't we see the metabolic changes in my use of fat as an energy source?
One of the answers could be as simple as what I ate. In October I had some cereal three hours before the test, whereas in the second test I ate some muesli only 90 minutes before.
I also ate a lot of carbs the day before the second test.
So I might simply have had more carbohydrates floating around in my bloodstream.
This is borne out by the figures, which show my lactate levels were twice as high when I started my second test as they were in the first, and the amount of CO2 I was breathing out was also higher at the start of the test.
That might have reflected my diet or simply some natural variation.
But what we also saw was the lactate level was quite steady in my second test (despite the high levels at the beginning) until a big jump at 160 watts, which was 20 watts higher than in my first test.
In other words, it's possible that there was a metabolic change, or maybe it has something to do with my physiology.
Perhaps what I've really identified is the problem of doing an experiment with a sample size of one, as opposed to a much larger number doing the same tests.
Clearly I should have been more careful about ensuring the conditions were the same for the two tests.
And the other possible variation was the intensity of my riding over the three months. Going on heart rate alone isn't ideal, because it can vary according to heat, hydration and other factors, whereas power tends to be a more reliable measure.
We simply don't have enough data to come to a definitive conclusion about the benefits of Zone 2, but what I have determined is the five months of Zone 2 riding have left me feeling a lot stronger and a lot fitter – even if that's not obvious in the numbers.
As Dr Stadnyk says: "We know the benefits of aerobic endurance training, we have decades of evidence to show that it works.
"Get out and do exercise really, every little bit counts. It's definitely worthwhile spending a large volume of time at the lower intensity."
But he also says there's a benefit to doing a bit of work in all your power zones so you get the adaptations at both ends.
And that's what I'm going to do. I'll continue to do most of my riding at Zone 2 but be sure to put in some harder efforts on at least one day a week.
And above all, I'm going to keep riding.
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