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.

The
terms “LT” and “VO2 max” are used liberally in the endurance sports
realm — but what are they, really, and why do they matter to you? Out of the many metrics that influence athletic performance, it’s
arguable that lactate threshold (LT) and VO2 max are among the most
important. Defining your LT and VO2 max numbers will help you train more
efficiently, and developing these metrics is a surefire way to get
across the finish line faster and stronger. Here’s what you need to
know.

Understanding Your Lactate Threshold

During exercise, lactate naturally spills into your bloodstream as
your body attempts to increase the breakdown of glucose for energy
production. As intensity increases and energy demands can no longer be
met entirely with aerobic energy systems, blood lactate levels begin to
rise, and other biochemical and neurological processes prevent that
intensity from being sustained for very long. If you train your body to
better withstand more intense exercise, you’ll therefore be able to
perform longer and harder. This is why your lactate threshold is so important. Usually defined
by an athlete’s ability to endure 40-60 minutes of maximum steady-state
effort, LT is more precisely measured by your heart rate, power, or pace
when the lactate volume in your blood (generally) reaches 4 mmol/L.
When you accurately define your LT, you can base your training off of
that number which will help you tailor your program to your current
fitness level while you work towards increasing your LT. Doing so will
increase efficiency and reduce the risk of injury and illness,
effectively making you a higher-performing athlete.

The Basics of VO2 max

VO2 max is defined by the maximum volume of oxygen per minute that an
athlete can capture from the air, fix at pulmonary level, transport,
and utilize. Efforts at your VO2 max are well above your LT, and can
approximately be held between 6 and 10 minutes (though this depends on
the amount of training as well as genetics). Though VO2 max has become
widely revered as a top training metric, it is considerably less
trainable than LT, as improvements are usually highly related to body
weight changes. For your reference, the average person’s VO2 max is 35 mL/kg/min
(i.e., 35 milliliters of oxygen is used per minute for each kilogram of
body weight) at rest, and 20-40 mL/kg/min while running. For elite
athletes, VO2 max is reached somewhere from 65-80 mL/kg/min, climbing to
90 mL/kg/min for the highest scorers. It is worth noting that female
athletes’ VO2 max is usually 5-15% lower.

Setting Your LT and VO2 max

Blood lactate tests can be used to set your LT either in the lab,
track, or velodrome. If you don’t have access to these settings, then
field tests such as running or riding 40 to 60 minutes at maximum
steady-state pace/power — equating to 10-15 km for runners or 30-40 km
for cyclists (for the majority of recreational athletes) — can help you
estimate your LT. VO2 max is only truly measurable with a ventilatory test. However, a
maximum-effort test of 8-9 minutes can give a good indication of your
speed, pace, power, and heart rate that corresponds to your VO2 max.

Which is More Important: LT or VO2 Max?

To better understand the dynamics for each of these metrics, consider
the graphs below. The graphs depict two different runners and two
different cyclists in relation to their LT and VO2 max. Note: for the
purposes of this article, LT-2 is the same as LT. In reality, your body
has multiple lactate thresholds (e.g., LT-1 corresponds to the initial
presence of lactate in your blood during low-intensity exercise, while
LT-2 corresponds to the excessive presence of lactate in your blood
during high-intensity exercise. The latter is the more important of the
two LTs for endurance athletes).

Illustration (illustration being migrated)

In the lefthand graph, Runner A’s VO2 max is 19km/h while his LT-2 is
13km/h, which corresponds to 68% of his VO2 max. Runner B’s VO2 max is
17km/h and his LT-2 is 14km/h, which corresponds to 82% of his VO2 max.
Cyclist A and Cyclist B have very similar metrics compared to their
respective runners. If each set of these four athletes were to race each other, it
appears very likely that Runner A and Cyclist A would be stronger in
workouts related to VO2 max that last under 10 minutes. But in longer
efforts, Runner B and Cyclist B would probably be stronger because their
LT surpassed that of Runner A and Cyclist A. The question is if Runner A and Cyclist A were to specifically focus on LT development, would that change the outcome? In the righthand graph, we can observe a new reality. Assuming that
both runners and cyclists continue with the same VO2 max, the improved
LT for Runner A and Cyclist A will not only make them stronger in
shorter efforts, but also in longer workouts or races. As previously
mentioned, LT is highly trainable, but it is limited by VO2 max.

How to Develop Your VO2 Max

As previously mentioned, genetics has a large impact on VO2 max, but
with the right training, it is possible to develop it. Due to the
high-intensity workouts that athletes need to face at VO2 max — which is
close to maximum heart rate for some — good aerobic and muscular
adaptations need to be secured before starting to work at this level. Typical VO2 max workouts are built using repetitions that are
sustained from 1 to 5 minutes, with active recovery between 50% to 100%
of the duration of the repetition (with a total volume of 10 to 20
minutes at VO2 max). The pace is very similar to the 2-3 km race pace
for runners.

Running Examples

Cycling Examples

Depending on the duration of the workout and your current level,
recovery after a VO2 max session will take between 36 and 48 hours.

How to Develop Your LT

LT workouts are less intense than VO2 max workouts, with longer
intervals and shorter recovery periods. The total duration of a typical
LT workout should be between 30 and 60 minutes without recovery
intervals. The typical structure will include repetitions of 4 to 12
minutes and recoveries of 1 to 3 minutes, following an approximate ratio
of 1/3 to 1/5.

Running Examples

Cycling Examples

For LT and VO2 max workouts, recovery intervals should be kept active
by jogging or cycling. Lactate clearance is accelerated by movement, so
this will help your performance. Depending on the duration of the workout and your particular level,
recovery after LT workouts is typically longer than recovery after VO2
max workouts due to higher glycogen depletion, lasting between 48 and 72
hours. Here’s an article I wrote about recovery periods that will help you decide whether you’re recovering adequately. To reach your full potential in running and/or cycling, I advise that
you include frequent workouts to develop LT so that you work this
metric to be as close to your VO2 max as possible. This could follow the
80/20
Rule, which calls for 80% of your workouts to be at lower intensities,
and 20% of your workouts to be at moderate and high intensities. VO2 max should be stimulated as well, but not so frequently. For
example, you can include weekly LT workouts into your training plan with
VO2 max workouts every two weeks. This is a good compromise if your
aerobic base and muscular adaptation is already secured. In trying to close the gap between your LT and VO2 max, the quality
of your high-intensity workouts will be more effective at 15% to 25% of
your weekly mileage (remember the 80/20 rule). The goal of this training
is to make racing at maximum aerobic potential possible for extended
periods of time. Precisely defining and frequently updating your LT is essential for
good planning so that you avoid training at intensities that are either
too high or too low for your current fitness level. For long-distance
endurance athletes, LT should be the priority physiological marker to
develop because it’s less risky in terms of injury and less aggressive
for the body and mind. Even if you don’t have a high VO2 max, keep
training. Those who work on efficiency and consistency are always
rewarded!