THE CORE IDEA OF TRAINING IN DIFFERENT INTENSITY ZONES
Not all cardio delivers the same results. The intensity at which you train determines the type of adaptation your body will make. To better understand this, we use the concept of ‘training intensity zones’ and typically distinguish 5 different training zones. Each training zone is defined by its individual level of effort and its specific training benefit. This concept can be applied to all sorts of activities. Whether you are walking, cycling, rowing, or running doesn’t matter much for your heart, it mainly cares how much it needs to work during the activity.
The idea of training in different intensity zones is very important because your body has different fuel sources to create energy: At lower intensities, the body primarily uses fat as a fuel source. As intensity increases, it relies more on carbohydrates, which can provide energy more quickly but are more limited. Over time you will be able to create a higher output (e.g. running or cycling speed and power) while still operating efficiently.
HOW TO DETERMINE THE TRAINING ZONE
There are several ways to define your training zones, ranging from simple estimates to more precise measurements.
Simple method
A very simple way is based on your estimated maximum heart rate (HRmax). For midlife men, the best formula is 208-(0.7 x age) and for midlife women it’s 206-(0.88 x age). While easy to calculate and correct on average, this approach can be inaccurate on an individual level.
Better method
A more refined method is the Karvonen formula, which is also based on the estimated HRmax but takes your individual resting heart rate (RHR) into account to calculate the usable range called the ‘heart rate reserve’ (HRR = HRmax – RHR).
Best method
The most accurate way is to measure your lactate threshold in a laboratory. This identifies the exact points at which your body transitions between different metabolic states. While providing highest accuracy, this level of testing is not available for everyone and might only become relevant for you if you are really performance-focused.
In practice, the simplest method is sufficient most of the time: The talk test. If you are able to talk comfortably in full sentences your training level is in a lower zone (1-2). You are purposefully moving, but not struggling. Once you increase the intensity level, your breathing gets deep and rapid and you are no longer able to pass the ‘talk test’: you can’t say more than a few words at a time without stopping to catch your breath. This is considered ‘vigorous intensity’ which takes place in higher zones (4-5).
THE 5 ZONES OF CARDIO TRAINING INTENSITY
Recovery and Regeneration
Zone 1 is the recovery and regeneration zone (50-60% x HRmax or RHR + 50-60% x HRR). The effort feels very easy, and you can hold a full conversation without difficulty. At this intensity, the body increases circulation and supports recovery processes without placing stress on the nervous system. This zone is ideal for warm-ups, cool-downs, and active recovery days. In midlife, it is often overlooked, but it plays an important role in maintaining consistency without accumulating fatigue.
The Aerobic Base
Zone 2 is the aerobic base, often referred to as the longevity zone (60-70% x HRmax or RHR + 60-70% x HRR). The effort in Zone 2 feels comfortable and sustainable. You can still speak in full sentences. This is where the body improves mitochondrial density, enhances fat metabolism, and becomes more efficient at producing energy aerobically. A large portion of the long-term health benefits of cardio training are achieved in this zone. It is best used for longer, steady sessions and forms the foundation.
The Grey Zone
Zone 3 is commonly called the “grey zone” (70-80% x HRmax or RHR + 70-80% x HRR). This feels moderately hard. Breathing becomes more noticeable, and conversation is less fluid. While it does contribute to aerobic capacity, it is a bit of a tricky zone: it is too intense to allow for optimal recovery, yet not intense enough to maximize the benefits. Hence, we want to minimize training in Zone 3! However, Zone 3 is where a lot of people spend their training unintentionally. They aim for Zone 2 but go too fast, since the zone still feels quite comfortable and productive. But it creates unnecessary fatigue with limited benefit. That’s why we want to avoid Zone 3 and the talk test is quite useful here: if you no longer can talk comfortably in full sentences, you are no longer doing Zone 2 training. Slow down!
Threshold Effort
Zone 4 is the threshold zone (80-90% x HRmax or RHR + 80-90% x HRR). Effort in Zone 4 feels hard. Speaking is limited to short phrases but you actually don’t want to speak anymore. This training intensity improves your ability to clear and tolerate lactate. You spend time in Zone 4 during tempo workouts and structured intervals. It is highly beneficial and effectively raises your performance level. However, since it creates significantly higher recovery demands, you should apply it with control.
Maximal Effort
Zone 5 represents maximal effort (90-100% x HRmax or RHR + 90-100% x HRR). This is going ‘all out’ and can only be sustained briefly. This zone targets your VO₂max, improving your overall cardiovascular ceiling. It is typically trained through short, high-intensity intervals and is very effective, but it comes with very high fatigue cost and requires adequate recovery. This is a tool to use sparingly.
HOW TO APPLY THE INTENSITY ZONES IN YOUR TRAINING
Research consistently supports the “80:20” model: around 80% of training should be performed at low intensity (Zones 1 and 2), while about 20% should be performed at high intensity (Zones 4 and 5). This combination has been shown to produce strong results for endurance and cardiovascular health. Zone 3 is to be minimized, because it is the ‘in-between’ zone: neither maximizing low-intensity benefits, nor high-intensity adaptations – but generating excessive fatigue.
Gender-Specific Aspects in Training Intensity
Women may benefit especially from high-intensity because this counters the drop in vascular elasticity after perimenopause and menopause. This is why women might want to emphasize slightly more high-intensity work than in the classic 80:20 model.
On the other hand, men should focus on the endurance foundation. Many men tend to overemphasize high intensity and often lack the Zone 2 base. Building the “endurance base” allows them to use their higher cardiac ceiling more effectively. Therefore, men might want to shift the ratio slightly more towards Zone 2.
How do you put this all together in your training week?
Most people benefit from 2-4 cardio sessions per week in Zone 2, lasting between 30 and 60 minutes. Zone 2 should be your priority, it builds your aerobic base. In addition, 1-2 sessions per week can include higher intensity work in Zones 4 or 5, typically in the form of intervals or tempo efforts.
Even a minimal approach of 2 sessions per week can deliver meaningful benefits. If you go with the minimal approach, spend 80% of your time in Zone 2, minimize Zone 3 and try to incorporate some short high efforts from time to time.
More training can improve results, but in midlife it’s crucial that you properly manage your recovery.