You must have probably heard of heart rate training zones and how elite endurance athletes, and more recently, a growing number of recreational athletes, use them to guide their training. Training zones can significantly enhance the results of cardio and interval training regardless of whether they are looking to break a triathlon record, get healthier, or lose weight. As a result, they should be top of mind for everyone with a fitness routine.
This blog explains what training zones are, how they are determined, the effects each one has on human physiology, and how they can be used to inform training and nutrition better.
Exercise intensity zones and metabolic response
The human body has a different metabolic response depending on the exercise intensity it’s exposed to. Its metabolic response is defined based on the amount of energy (i.e. calories) and type of fuel (i.e. proportion of fats and carbohydrates) it uses at each particular intensity. For example in zone 2 one may be burning 4.5 kcal/min with a fuel mixture consisting of 75% fats and 25% carbohydrates. These attributes will gradually change as one moves to a higher level of exercise intensity zones because the energy demand increases, and the fuel mixture required to sustain the growing levels of energy release needs to change. To better understand this phenomenon we should first examine the process of energy generation in the human body.
During exercise, your body typically burns a mixture of fats and carbohydrates to release the energy required (i.e., calories) to move. This process typically uses oxygen and is thus also referred to as the oxidation of fats and carbohydrates. Fat releases more energy than carbs when burnt (i.e., 9 kcal per gram of fat vs. 4 kcal per gram of carbs) but has a slower burning process making it suitable for low exercise intensities where the rate of energy demand is low. Carbohydrates, on the contrary, require less time to burn and can therefore support higher exercise intensities where the rate of energy demand is larger.
As a result, the fuel mixture changes from predominantly fats when exercise intensities and energy requirements are low to carbohydrates dominant as exercise intensity increases and the rate with which the body needs to release energy increases. In short, as the demand for energy increases, cells need to rely more on carbohydrates since they release energy faster. On the other hand, cells rely on fat as a fuel source despite its slow energy release process for as long as energy demand remains low.
Figure 1 shows the difference between fat and carbohydrate burn as exercise intensity increases during a treadmill test.
Figure 1 Dark green: fats, Turquoise: carbohydrates, Light green: heart rate
Going from intensity zones to heart rate zones
As described above, exercise intensity zones are defined based on the body’s metabolic response when exercising at a specific intensity. The metabolic response is broadly defined by the calories expended and the contribution of fats and carbohydrates in the calorie-burning process at that particular intensity. To measure calorie expenditure along with fat and carbohydrate consumption, one needs to analyze an individual’s expired oxygen and carbon dioxide using a metabolic analyzer. Since metabolic testing can only be done in a test or evaluation setup and not on every training session or athletic event, one needs to use a proxy metric that can be easily tracked daily to map a person’s metabolic response against it. The most commonly used proxy metric to achieve this is heart rate.
By measuring heart rate during a metabolic test, we can establish a correlation between the different metabolic states the test subject undergoes and heart rate. Since this correlation remains fairly constant, once established, it can be used to infer the subject’s metabolic response just by tracking heart rate during training or athletic events. We refer to this as “getting your personalized training zones.”
How different are my heart rate zones from the person sitting next to me?
The answer is Very! There are many ways to estimate heart rate zones, including predictive equations and wearable devices. However, the metabolic particularities of an individual render these methods highly inaccurate, with some cases reaching up to 50% deviation from one’s training zones. Simply put, unless one uses metabolic analysis to evaluate metabolic response in conjunction with heart rate, it’s impossible to identify their true heart rate zones accurately.
Do training zones change over time?
Yes! Training and nutrition will impact the way your cells work and consequently will affect your metabolism. In broad terms, this means that you will burn a different number of calories and use a different mixture of fat and carbohydrates at specific exercise intensities. For example, after three months of endurance training, you have likely become more economical in your running. You now burn fewer calories at a given pace and perhaps more fat adapted, meaning that the proportion of fat metabolism in your energy generation process has grown. Since heart rate zones correlate between heart rate and metabolic response, if your metabolism changes, so will its correlation with your heart rate profile. As a result, depending on how rapidly you progress through your fitness journey or training, you should re-assess your training zones. The recommended testing frequencies are the following:
- Every six months for individuals in a static or maintenance routine
- Every three months, individuals progress through their fitness program at an average pace
- Every 4-8 weeks, athletes undergo rigorous training
Moreover, it’s also very important to highlight that training zones also change based on the type of exercise. This is due to the fact that the amount of calories, fats, and carbohydrates you burn are heavily affected by the movement your body conducts. This inevitably means that the correlation between your metabolic response and heart rate profile will change depending on the exercise. For example, you may be burning 30% fats and 70% carbs at 140 beats per minute when running but only 15% fats and 85% carbs at 140 beats per minute when cycling. This may also explain why you feel a burning sensation in your legs when biking at a relatively low heart rate range while feeling much more relaxed when running at the same heart rate.
Training zones and physiological adaptations
Each training zone elicits different physiological adaptations on the human body. These physiological adaptations include improved cellular fitness, heart fitness, lung capacity, VO2max, and more. Each zone has specific effects meaning that spending time in it will improve only a specific set of these systems and not everyone. As a result, when one is looking to get the most out of their cardio training by targeting specific weaknesses their body faces, following a training program with accurate training zones is essential. For example, spending a specific period in zone 2 is the essential part of a training program if one is looking to improve their fat burning efficiency. If this person hasn’t measured their personalized training zones and instead of training in Zone 2 they’re actually training in Zone 3; they may be getting as much as 40% less positive adaptation in enhancing their fat burning capacity.
The five zones system is the most frequently used one, accurately capturing the difference in metabolic states while remaining practical enough for everyday usage. Each zone is used for a different purpose as it inflicts different metabolic adaptations on your body. Here are the adaptations each zone will affect:
Training intensity is typically used for warmup or active recovery (i.e. recovering from intense exercise while moving).
Zone2 raining will develop your mitochondrial function and improve your fat-burning efficiency. It’s highly recommended for long-range endurance sports and individuals suffering from metabolic syndrome (e.g. Type II Diabetes). The improved mitochondrial function will also significantly support recovery capacity helping you to recover faster after intense bouts of exercise.
Zone 3 training can help strengthen your pulmonary muscles and improve cardiovascular function. It’s an ideal intensity when suffering from a lung or heart problem since its moderate- intensity offers a solid stimulus to the heart and lungs without being exhausting or overly strenuous.
Zone 4 training will help improve your VO2max and ability to sustain high-intensity exercise for prolonged durations by improving lactate shuttling. Lactate is a byproduct of anaerobic metabolism, which can also be used as fuel by your muscles. For as long as your body can clear fatigue byproducts faster than their being produced, the exercise intensity remains sustainable. As a result, the greater your lactate shuttling capability the greater your ability to sustain high exercise intensities for long periods.
Zone 5 training will improve your VO2max and peak power output capability (e.g. maximum speed or cycling wattage). This exercise intensity is sustainable for 60 to 120 seconds and requires one to train at full potential.
Regardless of age, gender, and fitness level, every person has one or more systems that limit fitness or health. Targeting these limitations effectively requires the precision that stems from focusing your cardio and interval training in the zone(s) that will bring about the adaptations needed to overcome them.
The metabolic analysis provides gold-standard accuracy in determining your training zones and the plan that puts them to effective use. Understanding how your body responds metabolically and building your program around your metabolism is a foundational step toward maximizing your workout’s efficiency and achieving your health or performance goals faster and with less effort.