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By Tim Kettenring, Founder of Real Human Performance
Lots of chatter from the Dunning-Kruger club about “Zone 2” training. Zone 2 is just above Zone 1, right? Obviously! End of post.
Or maybe it’s more complex than that…
Let’s dive into all things heart rate zones.
There are several HR zone systems; the most popular being the 5 zone system and the 7 zone system.
The two main variations of the 5 zone system look like this:
Zone 1 - Very Light: 50-60% of max heart rate
Zone 2 - Light: 60-70% of MHR
Zone 3 - Moderate: 70-80% of MHR
Zone 4 - Hard: 80-90% of MHR
Zone 5 - Maximum: 90-100% of MHR
Zone 1 - Recovery: 55-65% MHR
Zone 2 - Aerobic/Base: 65-75% MHR
Zone 3 - Tempo: 80-85% MHR
Zone 4 - Lactate Threshold: 85-88% MHR
Zone 5 - Anaerobic: > 90% MHR
Zone 1 - Recovery: <65% MHR
Zone 2 - Aerobic: 65-75% MHR
Zone 3 - Extensive Endurance: 75-80% MHR
Zone 4 - Intensive Endurance: 80-85% MHR
Zone 5 - Anaerobic Threshold: 85-90% MHR
Zone 6 - Maximum Aerobic: > 90% MHR
Zone 7 - Speed: ~100% MHR
As heart rate monitors proliferated in the 1980s and endurance coaches started prescribing heart rate-based training to their athletes, they estimated different zones based on how much blood lactate was being accumulated, leading to the discovery of the “lactate threshold” (LT).
The first training zone system was based on the adaptations elicited in each range and had 7 zones (bottom) and was simplified to 5 zones (middle) which are generally in alignment. The 5 zone system was then distilled further to a percentage-based - as opposed to adaptation-based - 5 zone model (top).
Much of this oversimplification is due to the now mass consumption of heart rate monitoring devices, many grossly inaccurate.
For the purposes of this conversation, we will use the original 5 zone model.
I hate this phrase almost as much as I hate people using the term “cardio” and “lactic acid.” The ubiquitous nature of each in casual exercise circles denotes how poorly those of us in exercise science have communicated fundamental physiological principles.
When training at higher heart rates - think Zone 4 from above - the breakdown of glucose to make ATP produces pyruvate within muscle cells; a process which yields lactic acid, but LA by itself does very little as it’s quickly separated within the cell into hydrogen ions and lactate. The hydrogen ions cause the “burning” sensation within the muscle fibers themselves.
Mitochondria within slow-twitch muscle cells use pyruvate to generate more ATP at lower intensities - zone 2-3 - and can also efficiently convert pyruvate to lactate where it can also regenerate ATP.
The point at which lactate production exceeds lactate utilization is called the lactate threshold, colloquially recognized in the endurance sports community as the point at which the burning becomes nearly intolerable - after all, they wouldn’t be athletes if they couldn’t tolerate this discomfort…
Most endurance competitions take place at or slightly above lactate threshold and is probably where most of the misconceptions come from for everyday trainees.
In the last two decades, exercise physiologists have been researching how endurance athletes can most efficiently train for high outputs with the least metabolic stress, Dr. Stephen Seiler, an American expat living in Norway, being the most prominent. Dr. Seiler coined the term “polarized training” after watching Norwegian cross country skiers walk up hills on long training runs. The theory being that they wanted consistent exposure to the same low heart rate zone - Zone 2 - for long durations to improve muscle cells’ ability to produce and use oxygen instead of lactate and also to improve the athletes’ ability to use fat as a primary energy source instead of carbohydrate.
More efficient fat oxidation allows athletes to delay burning precious carbohydrates at higher intensities in training and in competition. This concept is critically important, even for coaches of speed/power athletes like soccer, football, and basketball.
The other end of the “polarized” concept is small doses of very high intensity training - usually zone 5.
After Dr. Seiler published a few of his studies, other exercise physiologists and coaches around the world starting to implement his polarized model to fantastic results.
An under-appreciated aspect of polarized training is reduced exposure to high levels of lactate. While lactate can be utilized as a fuel by skeletal muscle, cardiac muscle, and the brain, chronically high levels due to training or cognitive stress cause a cascade of negative effects from mitochondrial degeneration to dementia.
Another benefit of Zone 2 training is its effect on the autonomic nervous system (ANS). Modern society provides us with more “threats” than our evolutionarily-primed autonomic nervous system can deal with, leading to chronic sympathetic nervous system - “fight, flight, freeze” - arousal.
Zone 2 training is not “threatening” enough from an evolutionary perspective to lead to sympathetic arousal, and after training, elicits a parasympathetic nervous system response, acting as a reset for the autonomic nervous system - possibly the most important advantage given how sympathetic-dominant modernity is.
Next week we’ll discuss how to find your maximum heart rate, set subsequent heart rate zones, and the adaptations that take place in each zone so you can maximize your training.
