Improving Your Cardio Fitness by Running: Zone 2 vs Progressive Overload
Interested in improving your VO₂ max? Learn how progressive overload running gets you there quicker than Zone 2 running
Cardio Fitness, or VO₂ max, is a measure of how well your heart, lungs, and blood vessels can transport oxygen to your muscles during exercise. Pretty much every smartwatch can collect this data about you. So, why do you want to know this? Research suggests that Cardio Fitness is strongly correlated with 10-year all-cause mortality: the higher your VO₂ max, the higher your chance of surviving the next 10 years. Improving this figure is one of the key reasons why I started running, and why I want to become a better runner. Read on to learn why, as a beginner runner, I changed my training method from Zone 2 to Progressive Overload and how this significantly improved my VO₂ max.
A bumpy start to my running journey
My running journey started about 6 months ago. I bought a pair of Nike Free RN running shoes. These are barefoot-style shoes with a slightly cushioned sole and a perfectly fitting sock-like upper. I assumed that their lightness, comfort, and ease of putting them on would keep me motivated to run on a daily basis.
My first runs were in the summer evenings or the quiet mornings. They were just a few kilometers at most, and I ran them at a very quiet, easy pace — just a notch faster than walking. This was also a pace that was suggested by well-known experts in the field of sports physiology, like Iñigo San Millán and Phil Maffetone, or longevity experts like Peter Attia.
These guys always keep hammering on about Zone 2 training, which refers to the 2nd out of 5 heart rate zones you can enter when training. Zone 1, here, refers to low-intensity activities like walking, and Zone 5 are efforts close to your maximum heart rate, such as uphill sprinting. Their bottom line is that if you run in Zone 2 intensity for 45 to 90 minutes daily, your VO₂ max will gradually increase.
Personal setbacks and its impact on Cardio Fitness
I had some very unfortunate months ahead, though. I was just a few days into my running journey and I contracted a severe throat infection. I never felt this sick in decades. After taking antibiotics for 14 days, the disease went away, but my VO₂ max kept plummeting down like crazy. To make matters worse, I had to deal with a very serious family crisis in the last week of August, which further degraded my health and mental clarity. My original VO₂ max of 33,4 was already not great and, as you can see in the chart below, bottomed out at 27,1 in the second week of September.
Diligent running, combined with strength training activities, helped my body conquer these setbacks. I did my Zone 2 running in the early mornings and circuit training — which combines strength and aerobic exercises in a single workout — in the evenings. This combination put my VO₂ max in the uptrend again.
Back to a stable VO₂ max and its gradual increase
Mid-October, as the summer faded into Autumn, my VO₂ max started leveling off. My guess is that the setbacks I experienced earlier are completely out of my system at this point. I maintained my training regimen of Zone 2 morning runs and evening strength exercises. As indicated by the sport physiologists and longevity experts I mentioned earlier, doing so would result in a slow, but gradual increase of my VO₂ max. As you can see in the chart below, this was indeed what my smartwatch measured. My VO₂ max improved from 32 to 33,6. However, it took about 9 weeks to get there.
A sudden winter demanded a shift in my running style
I had to adapt my training regimen when it turned December. The winter came to show its teeth in a big way. Temperatures dropped to -10° C/ 14° F and heavy snowfall came thundering down from the sky. I wasn’t unprepared to run in this type of weather. I got myself a pair of Salomon Ultra Glide 2’s and a thermal leggings and shirt set.
However, to conquer this weather, I had to change my mode of running completely. After my first strides, I figured that the Zone 2 style of running I did in summer wouldn’t cut it through the thick layer of freshly fallen snow covering the streets. I felt more like a rally car racing through dirt and muddy roads with exceedingly high energy. Uphill, my heart was easily going 170 — 180 BPM, while I wasn’t going particularly fast. But boy, I had the fun of a lifetime!
In the days after, the snow firmed up quite a bit. This allowed me to pick up speed. The Salomons felt very confident when running fast on snowy and icy trails. But I also felt that I needed to go at speed to maintain a firm grip on this surface. My heart was going yet again north of 170 BPM on long, straight trails, but it didn’t feel like a struggle at all. I wasn’t even consciously aware that I was running, I felt more like an aerial drone buzzing through the forest. I was truly amazed by the beauty of the snow-covered landscape and the opportunity to run in these conditions.
This adventure wasn’t without injuries though. My toenails went black, my muscles felt sore, and I needed some quality recovery days. When reflecting on how it impacted my Cardio Fitness levels, I saw that my VO₂ max shot up like a hockey stick, going from 33,6 to 34,7.
Reflecting on my considerable VO₂ max gains
After the snow melted and I felt fully recovered, I went back on the road again. During these runs, I noticed that I could run a whole lot faster for the same heart rate. Where I used to run 7:30 to 7:45 mins per km in Zone 2, this easily became 6:45 to 7:00 mins per km.
This speed increase reminded me of my strength training exercises— train a week with weights that feel challenging at first, i.e. use weights that allow you to feel a proper muscle burn within 10 to 12 reps max. Once these weights no longer feel like an effortful task, or you need a lot more reps to experience the same effect, you progress to heavier weights. This training method is called Progressive Overload.
Based on that idea, I started to analyze my snow adventure runs to come to a clear understanding of my running mechanics, so that I can organize my running around the concept of Progressive Overload rather than Zone 2. This resulted in the following parameters for planning my runs:
- Intensity — I found that I have three “Running Zones”: Easy (120 — 150 BPM), Tempo (150 — 170 BPM), and Sprint (170 — 190 BPM). Varying between these zones with the aim of pushing intensity thresholds made a positive contribution to my VO₂ max.
- Endurance — I was running much more than I typically did. This was largely because I was truly enjoying the opportunity to run in the snow. Nonetheless, pushing my endurance threshold made another positive contribution to my VO₂ max.
- Injury risk/Excitingness — “Run the mile you’re in,” is a common mantra in the running world. It means to live in the moment and do what makes you feel happy. I’ve quite been pushing my intensity and endurance thresholds during my snow runs. However, in retrospect, I should have been more mindful of the injury risks I’m taking.
Applying the idea of Progressive Overload to my running
In the days that followed, I took a more structured approach and planned how I should push my Intensity and Endurance thresholds across my runs. When planning, I was trying to estimate which levels would cause mild muscle discomfort and stiffness after running — an indicator that my muscles are adapting to the stresses placed on them. For instance, I would plan a 10 km run as starting off easy, increase the tempo whenever it feels comfortable to do so, and do some sprint work at the end. Or, I would increase the distance of my easy long runs from 12 to 15 km, because 12 km no longer felt challenging.
I did about 10 runs in the 10 km+ range based on this strategy. This resulted in a similar increase of my VO₂ max as my snow adventure, confirming my hypothesis that Progressive Overload would be more gainful than Zone 2. This time, it went from 34,7 to 36,1.
Discussion of outcomes
So, are there any good reasons to train in Zone 2? Let me discuss both training methods across the Lactate, Fat Burning, and Injury Risk and Recovery training parameters.
Lactate
When training in Zone 2, your body doesn’t produce a lot of lactate, which is an acid that causes a burning sensation once there’s too much of it in your muscles.
Peter Attia defined Zone 2 not as a heart rate zone, but as the amount of lactate that is produced being “below the threshold where your body can no longer clear away lactate.” I find this definition very useful, as it provides a larger range of intensities for Progressive Overload running. Passing this lactate threshold causes a sense of muscle fatigue or weakness during exercise — you feel that your body is no longer able to keep up with the intensity level you’re in.
The type of muscle “burn” that I’m aiming for has little to do with this. First of all, it doesn’t occur during, but after exercise. It feels like stiffness and soreness, typically the morning after, and may come with mild swelling and inflammation in the affected muscles. This is a normal response to exercise, and indicates that the muscles are adapting to the stresses placed on them. As a result, the muscles get stronger and more resilient. I presume that the large increases in my VO₂ max come from this muscle development.
From Attia’s perspective, what I’m doing would still be Zone 2 training. However, to me, this definition sounds overly stretched as my lactate levels only take a toll on my body at exceedingly high 175+ BPM heart rates. I’m really sprinting at this point, which is well beyond what’s typically considered a Zone 2 activity.
Fat Burning
Another argument for Zone 2 training is that your body burns fat for fuel rather than glucose and that you’re training your cells’ mitochondria to get better at this. Iñigo San Millán, a well-known sports physiologist, studied fat and carbohydrate oxidation in professional athletes, moderately actives, and people with metabolic syndrome. Here, he demonstrated that the consumption of fat and carbohydrates are indeed inversely correlated the more exercise intensity increases.
The chart below comes from his research paper and shows the results for moderately active individuals. I most likely fall into this category considering my current VO₂ max and athletic ability. Blood lactate levels could be seen here as a marker for carbohydrate oxidation. Furthermore, it could be reasonably argued that the participants’ Zone 2 would be between 125 and 150 watts. If they would weigh 75 kilos on average, with 2 watts per kilo, this would indeed correspond to moderate-intensity exercise.
However, this chart does not suggest at all that the fat burning benefits cease to exist if I would go beyond my Zone 2. Yes, they decrease by 25%, but I’d improve my VO₂ max more effectively. Based on this chart, I’d argue that Progressive Overload running improves my mitochondria as well as my VO₂ max. If you seek to reduce your body fat, though, frequent Zone 2 is the way to go.
Injury Risk and Recovery
Progressive Overload running requires thoughtful planning and conscious awareness during running of the amount of injury risk you’re willing to take on. Overshoot this, and you’ll end up with runner’s knee, ankle tendonitis, or other issues that would require days of rest and immobility.
Zone 2, in this regard, has a low injury risk by comparison. It allows you to complete a moderate-intensity exercise that you could repeat day after day. However, if you run much longer distances than you ordinarily do, you’d still risk overuse injuries. Increasing your running distance by 10% weekly would still be recommended here.
Lessons Learned
Four weeks of Progressive Overload running did far more for my VO₂ max than 9 weeks of Zone 2 running. Based on the “Patient Survival by Performance Group” chart that you can find at the top of this article, my journey to increase my VO₂ max improved my chance of surviving the next 10 years from about 75% to 95%, so I accomplished what I was aiming for in about 6 months time.
I may have had a “beginner’s advantage” here. I started with a “Below Average” VO₂ max, which dropped firmly into the “Low” range after illness and other setbacks, after which I managed to pull it upwards to near “Above Average”. It may well be the case that I won’t see similar gains when going from “Above Average” to “High” for the same increases in my Progressive Overload regimen. That is to say, if, for instance, a 25% increase in running distance (from 12 to 15 km) contributed to my VO₂ max going from 34,7 to 36,1, would the same increase in running distance from 20 km to 25 km, improve my VO₂ max from 37 to 38,4? I think that’s unlikely. The research paper of Mandsager, et. al. (2018) shows that the health benefits of different VO₂ max performance groups scale logarithmically the more you go up. It would be fair to assume that the effort required to further increase my VO₂ max would scale in a similar, exponential way.
Nonetheless, my current running performance and VO₂ max make me feel confident that I can participate in the 2024 Marathon season. My first gunshot moment will be on the 6th of April at the Prague Half Marathon. I registered myself to complete the race in 2 hours and 30 minutes. But with 3 months of Progressive Overload training ahead of me, I have the ambition to complete it in a faster time.
If you want to learn whether my confidence, ambitions, and predictions are misplaced or not, click the red Follow button at the top, and the latest updates about my running journey will come your way!
