The Different Training Intensities

What are the different intensity zones for effort? What do they correspond to in terms of sensations and heartbeats per minute? Or beneficial effects on physical condition? These are questions we will try to answer, and which can help you better understand some of our other articles dedicated to training.

By Guillaume Judas – Photos: Pixabay / Maratona Dles Dolomites / 3bikes / DR

This article is intended for less experienced endurance athletes, for cyclists and triathletes who ride, run, or swim by feel, and who have few tools or knowledge to make the most of their investment in sport. So that they know what we are talking about when they hear “active recovery”, “endurance“, “threshold”, “MAP” (or VMA for runners), “power” or “watts”, or even “ESIE scale” (Subjective Scale of Effort Intensity). Don’t worry, this last term corresponds to something very easy to understand in the increase of exercise intensity.

How to calibrate effort intensity?

Effort intensity is calibrated in different ways. First, by sensations. And/or with a heart rate monitor (which measures the number of heartbeats per minute), and/or with a power meter (which measures the watts produced while pedaling).

Sensations

Because we are all endowed with a minimum of practical intelligence, and despite some very legitimate management errors when you start, you quickly understand how to manage an effort based on its length and your current physical capabilities. It is based on this observation that the now world-renowned coach Frédéric Grappe developed the ESIE scale in 1999. It is a seven-level graduation of what you feel when exerting an effort, named from I1 (Intensity 1) to I7 (Intensity 7). These terms are now used by all cycling coaches.

I1: light intensity, easy effort in relaxation, easy conversation, corresponding to < 50% of MAP (Maximum Aerobic Power) power, or < 75% of Max HR (heart rate). Training objective: long hours of effort, relaxation, regeneration.
I2: medium intensity, corresponding to a pure endurance effort, no muscle pain, easy conversation, fatigue occurring after 3/4 h of prolonged effort, 50 to 65% of MAP and 75 to 85% of Max HR. Training objective: increase endurance level, active waste elimination.
I3: sustained intensity, appearance of muscle tension, difficult conversation, exhaustion after 2h, 65 to 75% of MAP, 85 to 92% of Max HR. Training objective: maintain the average intensity level of an event over time without difficulty.
I4: critical intensity, progressive increase in muscle pain, very difficult conversation, exhaustion from 20 minutes, 75 to 80% of MAP, 92 to 96% of Max HR. This is the pace of a 20-minute time trial, or what is called the anaerobic threshold intensity. Training objective: withstand high competition intensities, or course difficulties (elevation).
I5: super-critical intensity, very rapid and quickly unbearable increase in muscle pain, very complicated conversation, exhaustion between 3 and 7 minutes, around 100% of MAP and between 96 and 100% of Max HR. This is the pace of a climb ridden all out. Or what is called MAP. Training objective: increase pain tolerance threshold (for key phases of an event, to follow or make a difference).
I6: sub-max intensity, extreme suffering during effort, close to nausea, exhaustion between 30 seconds and 1 minute, around 150% of MAP, HR not significant. Training objective: lactate tolerance (puncher capabilities, uphill sprint).
I7: maximum intensity, feeling of effort in apnea, very short, no muscle pain, hyperventilation at the end of exercise, max effort of 7 to 10 seconds, power peak or 250% of MAP, HR not significant. Training objective: develop maximum strength, improve motor skills.

Heart rate measurement

Heart rate (indicated in heartbeats per minute) is an indirect reflection of your body’s activity level. Every movement or effort implies a need for energy from the muscles, supplied by an increased blood flow. It is the heart that is responsible for increasing this flow, by pumping more or less quickly to send oxygen-rich blood to the muscles, which implies variations in its beating frequency.

Every movement or effort implies a need for energy from the muscles, supplied by an increased blood flow

After a laboratory stress test or by cross-referencing data from your experience, you can use heart rate as a kind of regulator to calibrate exercise intensity, and with regard to the HR zones mentioned in the previous paragraph. However, it should not be forgotten that this method is less precise than a power meter, because there is a delay between the beginning and end of an exercise with the rise or fall of heart rate, and external factors can lead to what is called cardiac drift (temperature, altitude, dehydration, infection…). The fact remains that the display of heartbeats provides a good indication of the energy sources used, and helps in managing effort.

The heart is a muscle that ensures blood circulation, and like any muscle, it develops and is maintained. With training, the heart sees its volume and weight increase, with a thickening that causes an increase in flow capacity and an economy (slowing of beats) for a given intensity. To ensure vital functions, its size and power also allow it to provide more blood flow with each beat at rest. Resting heart rate therefore decreases with regular endurance training. A sedentary person has a resting heart rate around 70 bpm (beats per minute), while a high-level endurance athlete can drop below 40 bpm.

This heart rate represents an index of your fitness, provided you have some references. There are no absolute rules in this area, but a variation in individual resting heart rate indicates good adaptation to training (if it decreases or remains stable), or on the contrary, the onset of fatigue or overtraining, or even an infection (if it increases).

Heart rate and energy systems

The energy needed by muscles is supplied by the biological transformation of ingested and stored food, mainly glucose, which is stored in a particular form in muscles and the liver as glycogen. But the breakdown of fatty acids (lipids) also allows the synthesis of energy usable by cells. For this energy source to be converted into fuel, it needs oxygen. All of this is immediately converted into ATP (Adenosine Triphosphate). ATP is the main source of energy directly usable by the cell. Depending on the intensity of the effort, the body does not use the same energy substrates and does not produce ATP in the same way. The first energy system (and the main one in endurance sports) is called the aerobic system, and creates ATP by using oxygen, and by breaking down glycogen and fatty acids. This system produces few waste products in the body and its limits depend on VO2Max (Maximum Oxygen Consumption Volume) and glycogen reserves. The other two energy systems are anaerobic lactic and anaerobic alactic. The first produces ATP by breaking down glycogen exclusively, but also by producing lactic acid, which itself is responsible for muscle acidosis, which quickly limits activity (rarely beyond 2 to 3 minutes at this very critical intensity). The second produces ATP by breaking down phosphocreatine, present in very small quantities in the body, for short, explosive efforts of a few seconds. Fortunately, phosphocreatine reserves are quickly replenished, in a few minutes. Glycogen, on the other hand, is not replenished for several hours, which explains why effort and reserve management is so important. For the anaerobic alactic system, heart rate is absolutely not significant (power in watts, yes!). It is a little more significant for the anaerobic lactic system, but the duration and intensity of the effort depend on glycogen reserves (which decrease at high speed) and the ability to tolerate acidosis. Clearly, you have very few cartridges during a training session or a race. It is mainly within the aerobic system that heart rate is important, because according to Fick’s principle (1870), it is directly related to oxygen consumption. In other words, monitoring heart rate and ensuring it remains within the desired values allows for the management of energy potential during long-term events or training sessions. Glycogen is the primary fuel used during exercise, and it is also the most efficient. But reserves are limited, unlike fatty acids, a less efficient fuel but one that allows you to go very far. Improving physical condition linked to regular training and the body’s adaptations allows you to push back the threshold (in intensity, power, heart rate) at which limited glycogen reserves are used in favor of fat reserves.

“Reserve” heart rate

Training has little impact on maximum heart rate. This is individual and can vary greatly from one person to another. It decreases with age. There are several formulas for calculating maximum heart rate, the most famous of which is the well-known 220 – age for a man and 226 – age for a woman (for example, for a 40-year-old man: 220 – 40 = 180 bpm maximum heart rate). This is only an indication; it is best to perform a field test, which consists of a progressive acceleration after warm-up, up to one’s maximum capacity over 3 to 5 minutes, and finally noting the heart rate at the end of the effort. You can then note your reserve heart rate, which is calculated by subtracting the maximum heart rate from the resting heart rate (for example, 180 – 50 = 130). From there, you target a heart rate that corresponds to an energy system to prioritize during the session. The Karvonen formula allows you to calculate it by multiplying the reserve heart rate by the desired intensity percentage, and adding the resting heart rate. To take the example of our 40-year-old athlete with a maximum heart rate of 180 and a resting heart rate of 50, to ride at 85% of maximum capacity, the calculation is as follows: 130 (reserve HR) x 0.85 (target intensity) + 50 (resting HR) = 160 bpm.

Power measurement

Finally, the power meter is the reference tool for professionals or cyclists and triathletes who are fully invested in their discipline. The principle is the same as for sensations or heart rate concerning energy systems, but the equipment is more precise and directly shows the effort production at a given moment. It allows for precise management of an effort in a race or training session according to the system you wish to work on, or according to your qualities.

The reference value is MAP, or Maximum Aerobic Power, which is the power (in watts) at which the cyclist reaches their maximum oxygen consumption. This is the same as VMA (in speed) in running. It is also the maximum power you can sustain for an effort of about 5 minutes. But each intensity level corresponds to a power value depending on the individual level, and provides information on the athlete’s qualities and possibilities. We then speak of a Record Power Curve (the power records achieved depending on the duration of the effort), which can show that a rider is more of a sprinter than an endurance rider (with a very high maximum power, but relatively low power over long efforts), or that another has very strong dispositions for time trialing with a very high 20-minute power compared to their maximum power. But we will come back to this soon!

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