• Introduction
  • The basics of heart rate training zones
  • Zone 1
  • Zone 2
  • Zone 3
  • Zone 4
  • Zone 5
  • Final thoughts
  • Sources
  • Aerobic respiration: producing energy (ATP) with the presence of oxygen.
  • Anaerobic respiration: producing energy (ATP) without the presence of oxygen.
  • Lactate: a byproduct of anaerobic respiration. Known to cause fatigue and nausea.
  • Lactic acidosis: lactic acid production exceeds lactic acid clearance.
  • Maximum oxygen uptake: The maximum amount of oxygen a person can use during intense exercise.

Introduction

Prolonged low-intensity exercises have long been used to improve cardiovascular fitness. However, more recently trainers and athletes have taken a more scientific approach to accurately determine the most effective way to  improve endurance. After several studies, sports scientists were able to pinpoint five different heart rate zones that produced optimal training results. This discovery has later been widely adopted by athletes and recreational sportspeople alike. 

In simple terms, heart rate training consists of using your heart rate to determine your exercise intensity. Because different training intensities have different physiological responses, each zone can be used according to your fitness goals.

This post explains the science behind different heart rate zones, and how they can be used in training.

The basics of heart rate training zones

Your exercise capacity is directly related to your ability to turn nutrients into adenosine triphosphate (ATP) – the energy currency of your body. Most ATP is produced aerobically (with oxygen), although your body switches to anaerobic (without oxygen) respiration as the training intensifies. At low to moderate intensities, ATP is mostly produced from carbohydrates (stored as glycogen in skeletal muscles and liver) and fats (stored in the adipose tissue). At higher intensities, fats cannot be synthesized fast enough and carbohydrates become the predominate energy substrate. Additionally, when oxygen need surpasses oxygen delivery, the body must turn to anaerobic respiration, producing lactate.

Exercise intensity also has an effect on which muscle fiber type is recruited. Slow-twitch fibers (type I) are recruited at lower intensities whereas fast-twitch fibers (type II) activate as the exercise intensity increases. Because type I fibers have a higher mitochondrial density, they are able to better utilize fat for fuel. Type II fibers have a higher capacity to use glucose as an energy source. Thus, each exercise intensity has a different metabolic response that dictates which energy system and energy sources are used, as well as which muscle fibers are utilized.

Between your resting heart rate and maximum heart rate, are five target heart rate zones that correspond to a certain training intensity. In order to determine the optimal heart rate zones for yourself, you must first calculate your own maximum heart rate. There are several ways to do this, but a simple yet accurate method is using an age-based formula by Tanaka et al. (2001): 208 – 0.7 x your age. Once you know your maximum heart rate, you can easily calculate which target heart rate train in. A well-designed training plan includes workouts in all of these five zones in a balanced manner.

Heart rate zone

% of max HR

Description


Zone 1
(recovery/easy)

50-60%

Exercise with minimal stress and exertion. Used for very easy training such as recovery runs, etc. Mainly utilizes fat for energy (aerobic system).


Zone 2
(aerobic/base)

60-70%

An intensity that can be easily sustained for longer training sessions. Builds an aerobic base by promoting mitochondria growth and function, and increasing capillary content. Mainly utilizes fat for energy (aerobic system).


Zone 3
(tempo)

70-80%

Exercise above the optimal aerobic zone but below threshold pace. Consistently training in this zone may cause a plateau in performance (grey zone). Used for tempo runs. Mainly utilizes fat and carbohydrates for energy (aerobic & anaerobic systems).


Zone 4
(lactate threshold)

80-90%

An intensity where your body is processing the maximum amount of lactate without a significant rise in lactate in the blood (lactate threshold). Training in this zone develops the maximum pace you are able to sustain. Mainly utilizes carbohydrates for energy (anaerobic system).


Zone 5
(anaerobic)

90-100%

An anaerobic zone where your cardiorespiratory system works at maximal capacity. Lactate builds up in your blood faster than it can be removed, making the intensity unsustainable for more than a minute or so. Mainly utilizes carbohydrates for energy (ATP-PC & anaerobic systems).


It is also important to remember that your maximal heart rate declines as you age at approximately 0.7-1 beats per year. To better utilize each heart rate zone, it is smart to recheck your maximum heart rate periodically. We have written a full article about it here.

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Heart Rate Zones


Five heart rate zones1 is the least intense & 5 is the most intenseCalculated by using your maximum heart rateEach zone offers different physiological adaptationsUseful when designing a balanced training program

Zone 1

Zone 1 refers to a target heart rate of 50-60%. As such, it is considered a light exercise that can be comfortably maintained for nearly as long as you like. Due to its low intensity, it will not provide you with the same cardiorespiratory benefits as slightly higher intensity exercises. In most situations, exercises in heart rate zone 1 are used for recovery purposes. 

Zone 2

Although all target heart rate zones are needed in a well-balanced training block, zone 2 should form the core of your overall training program. This refers to a training intensity of 60-70% of your maximum heart rate, which is essential for building a solid base level of fitness to improve upon.

Following multiple studies, trainers and athletes often spend 60-75% of all training time in zone 2 depending on their sport. This is especially important for endurance athletes, who should incorporate three to four zone 2 training days per week during preseason. The closer you get to competitive season, the more you should move from high volume training to more high-intensity training. Similar results have been found in numerous different sports.

Training at zone 2 specifically focuses on utilizing slow-twitch muscle fibers, which stimulates mitochondrial growth and function. Increased mitochondrial content helps your body use fat as a fuel source during longer activities. Increased fat oxidation preserves your muscle’s glycogen stores which can provide a significant performance advantage especially in longer endurance events. After all, the leftover glycogen can be used for your last sprint at the end of a race.

Type I fibers are also responsible for clearing lactate, which are mainly produced by type II with poor oxidative capacity. To prevent a dangerous accumulation of acidity in the muscle, lactate must be delivered to well-oxygenated type I muscle fibers. There, lactate can be transported to the mitochondria where it is used to produce more ATP.

Zone 3

Heart rate zone 3 is performed at a moderate intensity of 70-80% of your maximum heart rate, which can also be seen as a slight increase in blood lactate. Because of this, zone 3 is considered too hard for recovery or building an aerobic base, while also not hard enough to improve lactate tolerance or speed endurance. In fact, there is little benefit from training in heart rate zone 3 when compared to the increased aerobic capacity and fatty acid utilization of zone 2 or the VO2max and lactate threshold improvements of zones 4 and 5. This is why heart rate zone 3 is sometimes called the ”grey zone”.

A common mistake for recreational athletes is to consistently run on zone 3. There are numerous reasons for this. For example, trying to consistently run at your ”average pace” or trying to get the most out of your training when time is of the essence. The best advice we can give is to clearly divide your easy and hard days.

Zone 4

Heart rate zone 4 refers to training at 80-90% of your maximal heart rate, also known as your anaerobic threshold or “threshold pace”. At this intensity, most of the energy is produced anaerobically (without oxygen) from glucose, which also produces lactate at a faster rate than it can be removed. This leads to shortness of breath, a burning sensation in the muscles, and fatigue. 

Consistent training in zone 4 improves your anaerobic capacity (the total amount of energy produced from the anaerobic energy systems) lactate threshold, lactate tolerance, and maximal oxygen uptake (VO₂max). Essentially this means that you are able to produce ATP with oxygen at higher intensities and maintain performance for longer in high-lactate situations. This is especially useful in medium-distance events that rely heavily on speed endurance.

Zone 5

Heart rate zone 5 refers to all-out efforts at 90-100% of your maximum heart rate. Because of this, most people are only able to maintain this intensity for less than a minute. This makes it suitable for only a few different sports, such as a 100m or 400m sprints.

Zone 5 relies on two energy systems to produce ATP; the ATP-phosphocreatine system (ATP-PC) and anaerobic glycolysis. The ATP-PC system uses stored ATP and creatine phosphate for immediate energy production. However, these stores are fully used up in as little as 10-30 seconds. After this, anaerobic glycolysis starts taking over as the main energy system. 

These training sessions are often reserved for athletes and very fit individuals who want to improve their speed or speed endurance in quick, very intense bursts of exercise. High-intensity interval training (HIIT) is one example of this type of exercise.

Different heart rate zones help you train at the optimal intensity.

Final thoughts

As you can see, all target heart rate zones have their own specific physiological effects and training implications. Being able to utilize them in the best way possible can provide you with significant improvements in performance.

Heart rate zones can be used in a wide variety of endurance activities such as running, rowing, cycling, and cross-country skiing. For swimming however, it is important to remember that heart rate is approximately 10-15 bpm lower in the water than on land. This is due to hydrostatic pressure that increases the amount of blood that is pumped to the extremities. Thus, heart rate zones should be adjusted accordingly.

In addition to using your maximum heart rate to gauge training intensity, some people also use perceived effort or a threshold-based percentages to determine optimal training effort. Whichever method you end up using, it is important to choose the right zone for your own goals. This will lead to consistent progression in your performance. 

Did you learn anything new about heart rate zones? Let us know in the comments.

Sources

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