Heart Rate Zones

The effort level of a sports session can be quantified using heart rate. To do this, we use heart rate zones as an analysis tool, defining effort ranges associated with heart rate ranges. These zones are built from our personal aerobic and anaerobic characteristics, describing the state in which our metabolism is operating. Before calculating these zones, let's start with a quick look at the different modes of our metabolism.

Aerobic and anaerobic metabolism

Our metabolism can be described as the tool that converts the food we eat into the molecules our cells use to function. These fundamental molecules go by the lovely name of Adenosine Triphosphate, or more commonly ATP. Remember these three letters well, because a large part of training consists of expanding our ATP production lines in order to go faster and longer. The adenosine triphosphate production lines are organelles called mitochondria. These cells produce ATP from oxygen and from the nutrients coming from the digestive system (fats, carbohydrates, proteins). They can also produce ATP from the leftovers discarded by cells. Our body does not have a large reservoir of ATP and production runs almost just-in-time. This is not bad news in itself, since the recycling rate is very high. On the other hand, the faster our organism is able to generate ATP, the faster the muscles are fueled, with an increase in performance as a consequence. ATP production is possible through either an anaerobic or an aerobic process. When our metabolism runs anaerobically, ATP is produced from glucose, derived from the carbohydrates coming from the digestive system. When the process is aerobic, ATP is produced from oxygen and from post-digestion fats, carbohydrates (sugars) and proteins. As long as the muscles' ATP consumption is low, our metabolism favors the aerobic production scheme. The big advantage of the latter is that it can run on the fat stored by our body, with a nearly infinite source of fuel (\> 100,000 kcal). The aerobic process is nevertheless limited in production speed. When the muscles' consumption exceeds maximum aerobic production, our metabolism has to meet the demand with the help of the anaerobic process, whose ATP production speed is far higher. But this comes at the cost of several drawbacks. First of all, anaerobic ATP production also produces lactate, in proportion to the intensity of the effort. With this lactate come hydrogen ions that acidify the blood. When the effort is too great, we cross a threshold of lactate accumulation in the blood and our organism forces us to slow down through fatigue in order to restore balance. This fatigue, which sets in after a few minutes, is the consequence of a drop in ATP production. Then, the amount of glucose available is limited to what the digestive system is able to supply, and that is well below the stock that can be tapped aerobically. Now, the very good news is that lactate is the aerobic process's favorite source for forming ATP, and thus for keeping the effort going. This recycling capacity is trainable, since the processing centers are a certain type of muscle fiber, known as slow-twitch. To put it crudely, the more slow-twitch muscle fibers we have, the more endurance we have. All that remains is to do gentle efforts for a long time, a very long time, in order to increase the quantity of this type of fiber.

Heart rate as an indicator of the metabolism in use

Several methods exist to determine whether our body is working aerobically or anaerobically. The most practical one for mountain sports is measuring heart rate (HR), whose unit is the number of beats per minute (bpm). Ideally, this rate is measured with an electrode chest strap. Optical measurement devices exist on the market, but their precision is not yet sufficient to make use of the data. coming soon Our heart rate has a minimum value, the resting heart rate (RHR), as well as a maximum value, the maximum heart rate (MaxHR). Between RHR and MaxHR, two other thresholds are of great interest to the athletes we are. First there is the aerobic threshold heart rate, noted AeT, below which the organism runs on the aerobic pathway. It also corresponds to the threshold beyond which heart rate is no longer a linear reflection of effort and, in most cases, the critical point where lactate concentration exceeds 2 mmol/l. Then there is the anaerobic threshold heart rate, noted AnT, above which the anaerobic pathway takes over. AnT is the threshold at which the gap between production and absorption of lactic acid is sustainable for one hour. Beyond AnT, the effort is such that lactate production exceeds absorption capacity, and the athlete is forced to slow down. Note that AeT and AnT can be shifted with training, the goal being to raise them as much as possible. It is very interesting to track their evolution over the years of practice and to modify your heart rate zones accordingly. Heart rate zones are built from RHR, MaxHR, AeT and AnT, in order to determine the intensity level of the training session. They are very useful for steering our training. The number and the size of the zones are completely arbitrary. What you should keep in mind when choosing is that the greater the number of zones, the smaller their size, and therefore the more sensitive they are to measurement precision. A good compromise is to go with a choice of five zones, defined as follows:

Zone 0: [RHR ; AeT-20 %]

This heart rate zone is the so-called rest zone. It can be put to use to speed up recovery after an intensive session. But the sport practiced must be different from that of the big session, gentle, and must not engage the same muscle groups. The duration of the session depends heavily on the athlete's preferences.

Zone 1: [AeT-20 % ; AeT-10 %]

This heart rate zone is the zone in which the body can develop its aerobic capacity, via an increase in blood volume, mitochondrial mass, capillary density and aerobic enzymes. Long sessions in Z1 also develop the fat-burning metabolism. This zone is the very quintessence of endurance training. Z1 sessions must last at least 30 minutes, and can go on for several hours.

Zone 2: [AeT-10 % ; AeT]

The effort level corresponding to this heart rate zone pushes the metabolism to the maximum of its aerobic capacities. The greatest benefit is being able to raise AeT, and therefore your endurance level. You reap the same advantages as with efforts done in Z1, but the number of muscle fibers recruited is much greater. A Z2 session should last between 30 and 120 minutes, ideally during the phase of the year when you are building your muscular base. Beginners can already do their entire outing in Z2, thereby skipping Z1. Nevertheless, if AeT is high or the pace already brisk, 10 to 15% of weekly volume is an upper limit not to be exceeded in order to avoid exhaustion.

Zone 3: [AeT ; AnT]

Z3 sessions are perceived as fun, and the gains in fitness are most often spectacular. But these gains do not hold up over time. In Z3, the main source of ATP production comes from glucose. The body calls on both slow- and fast-twitch fibers. It also uses both the aerobic and anaerobic metabolisms. The impact will be a production of lactic acid, staying within proportions that are sustainable for one hour at the very most. Z3 sessions are a powerful lever for developing aerobic capacity and the glycolytic system (production of ATP from glucose). For beginners, these sessions also help build muscular endurance. Nevertheless, time spent in Z3 must not exceed 10% of annual training volume.

Zone 4: [AnT ; MaxHR]

Z4 is the maximum effort zone. The perceived effort here is hard. The gains are an increase in maximum aerobic power, in strength and speed endurance, in technique and in efficiency. The aerobic and anaerobic pathways are running flat out, and it is difficult to sustain this effort for more than 10 minutes.

How to measure the aerobic threshold AeT

Several methods exist. The most effective is a laboratory test, but it is complicated to set up. More within our reach, there is a treadmill method and a track method. But both require you to have a rough idea of where your AeT sits. If you have never known it, there is a magic formula. Phil MAFFETONE established a rough calculation method based on observations of a large number of athletes, giving a quick estimate of AeT. It works more or less well for more or fewer people. This formula is 180 – your age. It only works if you have been training regularly for at least two years. With a few corrections:

This magic formula must not be used to set the heart rate zones for a plan spanning several months, but it gives a quick idea of where your AeT should sit.

Treadmill test

The treadmill test provides a controlled test environment. This method is useful for minimizing the impact of outside conditions and for observing the evolution of your AeT over several years. To carry out this test, you need an inclinable treadmill and a heart rate strap paired with a means of recording (watch or phone). The test protocol is as follows:

  1. Start the test after several days of rest.
  2. Incline the treadmill to a gradient of between 5 and 10%. Note this gradient so you can reproduce it at the next test.
  3. Start recording your heart rate.
  4. Set the speed such that you have to walk at a slow pace to begin with.
  5. Increase the speed over the first fifteen minutes to warm your body up properly.
  6. Determine the speed needed to reach your AeT estimate. Once you are there, your heart rate should be stable, you should be able to breathe through your nose only, and the effort should feel easy.
  7. Once you are at the right speed, run for one hour.
  8. Once the test is over, determine the average heart rate over the first half hour starting from point 7 (HR1), as well as that of the last half hour (HR2). If HR2 is less than 1.05 x HR1, the starting heart rate from point 7 is below AeT (if between 1.035 and 1.05, HR = AeT). If HR2 is greater than 1.05 x HR1, then this starting heart rate is above AeT.

Track test

The track test is more pleasant than the treadmill test, especially for athletes who love being outdoors. On the other hand, you need to pay attention to the weather conditions, which must not be extreme (rain, cold, heat…). To carry out this test, you need a GPS watch with a paired heart rate strap, a track (!) and a premium account with TrainingPeaks (no sponsorship here, they just have the right analysis feature). The test protocol is as follows:

  1. Start the test after several days of rest.
  2. Warm up during the first quarter hour, gradually increasing your speed until you reach a sustainable effort level close to your AeT estimate. Once you are there, your heart rate should be stable, you should be able to breathe through your nose only, and the effort should feel easy.
  3. Start recording your heart rate and run for one hour, keeping your heart rate at the same value throughout.
  4. Upload the session data to TrainingPeaks (just the test hour)
  5. Look at the Pa:Hr variable in the analysis section of your file. If it is between 3.5 and 5%, your heart rate is AeT. If it is too far below 5%, you were underneath, and the same reasoning applies if Pa:Hr is above 5%.

How to measure the anaerobic threshold AnT

The only way, in our view, to measure AnT correctly is by running up a steep, long slope, or failing that a treadmill whose incline is at least 15%. To carry out this test, you need a GPS watch and a paired heart rate strap. The idea is to dig deep into the engine room for more than half an hour, so be mentally prepared to suffer a little. The test protocol is as follows:

  1. Start the test after several days of rest, or at the very least several days after your last effort above AeT. Make sure you arrive with a full tank of energy, with a suitable meal within the 2 hours before the test, and a cereal bar 45 minutes before the test.
  2. Warm up during the first quarter hour, gradually increasing your speed until you reach an effort level close to AeT.
  3. Start recording your heart rate and run at a level such that you cannot hold on for more than half an hour (45 to 60 minutes for athletes who already have a substantial training history).
  4. If you have not collapsed before the end of the test, the average heart rate of the "all-out" phase is your AnT. If you could not finish the test, you were above AnT.

AeT, AnT and aerobic deficiency

Heart rate zones are defined from the AeT and AnT values, but they are not the only ones to use this data. By looking at the AnT/AeT ratio, we can determine what the priority of our future training should be. If AnT/AeT \> 1.10, we can conclude that we have not maximized our aerobic capacity. If \< 1.10, we will have to stop the Z2 sessions and start adding Z3 and Z4 sessions to pull AeT and AnT upward. Z2 is no longer as useful in this case, because the range is very narrow and we run the risk of hitting Z3 too often when the goal was to maximize aerobic capacity. Skipping Z2 is thus a way to protect ourselves.