The Aerobic Advantage

What is "Zone 2" anyway?

Signal > Noise

With all the conversation around heart rate zones on social media recently, especially Zone 2, we thought it would be a good idea to clarify all things heart rate zones.

HR Zone Systems

There are several HR zone models; the most popular being the 5 zone model and the 7 zone model.

The two main variations of the 5 zone model look like this:

Simplified 5 Zone Model

• 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

Original 5 Zone Model

• 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

7 Zone Model

• 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

Why the disagreement?

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 original “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 5 zone model.

Zone 2 - The Sweet Spot for all the Best Reasons

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. More on lactate below…

The primary physiological adaptation to Zone 2 work occur in the heart muscle itself. The left ventricle, which pumps blood into the rest of the cardiovascular system, stretches to take in more blood, reducing the amount of work done each heart beat, or stroke. This increase in stroke volume coupled with a decrease in heart rate, leads to an improvement in cardiac output, or the overall efficiency of the heart function.

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.

Here’s the kicker: above 75% of max heart rate, the left ventricle can’t fill up completely so controlling intensity during low intensity work is crucial for eliciting the desired central adaptations. This is also why Zone 2 work is so critically important for athletes because relying on high intensity work for conditioning neglects cardiac health.

Lactate - The Physiological Training Linchpin

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 lactate. Accumulation of lactate in the blood is what causes the “burning” sensation athletes feel.

With this context, let’s work backwards to establish why heart rate zones are set how they are.

Lactate is a cellular byproduct of exercise and gradually accumulates in the blood

Mitochondria within slow-twitch muscle cells use pyruvate to generate more ATP at lower intensities - zone 2-3 - and can also efficiently convert lactate into pyruvate for further ATP regeneration.

As research into lactate proliferated, two different lactate thresholds were established: lactate threshold 1 (LT1) where blood lactate accumulation reaches 2 mmol/liter of blood which correlates roughly with 75% of max heart rate or the top end of Zone 2. Lactate threshold 2 (LT2) is where blood lactate accumulation reaches 4 mmol/liter of blood and correlates roughly with 85-88% of max heart rate.

At LT1, muscle cells are able to clear lactate at a slightly faster rate than it is produced. Above LT1, lactate production steadily increases concurrently with intensity to the point where production exceeds clearance capability at which point lactate threshold 2 (LT2) is reached. The range between LT1 and LT2 is called Maximal Lactate Steady State (MLSS) and lactate production/clearance rates stay basically equal if intensity is maintained and not increased. MLSS is roughly heart rate Zone 3 (75-82%).

Above Zone 3 is where we get closer to LT2 and where lactate starts to play a role as an intensity rate-limiter the longer an athlete stays in that range. LT2 is usually reached at the top end of Zone 4/bottom end of Zone 5 and most team sport athletes cannot maintain this intensity for longer than 3-5 minutes.

How We Use Heart Rate Zones in the Conditioning Cohort (and what happens physiologically in each zone)

In the Coaches’ Conditioning Cohort, we use a combination of the two 5 zone models. We use the modified age predictive formula of 211 - .64*age for males and 221 - .64*age for females to establish max heart rate and then set zones as follows:

Zone 1: 50-60% MHR

• recovery zone - clears waste products from muscle cells and tissue

Zone 2: 60-75% MHR

• central adaptations - left ventricular hypertrophy; cardiac output

Zone 3: 75-82% MHR

• peripheral adaptations - lactate utilization, aerobic enzyme production

Zone 4: 82-88% MHR

• peripheral adaptations - capillary density, mitochondrial density

Zone 5: 88-100% MHR

• peripheral adaptations - lactate tolerance + clearance
  

Broadening Zone 2 allows us to prescribe low intensity work for both recovery (60-65%) and stimulus (65-75%) applications. Working in the lower end of Zone 2 is going to be more recovery-oriented and working at the higher end is going to be more for stimulating central adaptations.

Final Thoughts

Heart rate zone models were established to easily and generally identify what adaptations are elicited from various intensities and to make zone-based training more understandable for more athletes. The zones are guidelines and not hard and fast rules and should be treated as such. As long as the training prescription is based on time in zone, the desired adaptations will more than likely be elicited.