A solid aerobic base is fundamental for building cardiovascular fitness and endurance, essential components for disciplines like CrossFit and HYROX. Despite its importance, many athletes sports lack a well-developed aerobic foundation.
Dr. Gommaar D’Hulst, founder of Wodscience and senior scientist at ETH Zurich, Switzerland, offered insights into the role of aerobic base training and how to develop it.
Zone 2 Training & How It Relates to Training
Zone 2 training was coined by Joe Friel and Phil Maffetone, two applied researchers and cycling coaches. They proposed a model comprising five zones based on the maximal heart rate.
Physiologically, Zone 2 represents the point at which there is a significant increase in anaerobic metabolism above the intensity threshold. Zone 2 training can be viewed physiologically in two ways:
- Lactate
- Muscle Oxygen
[Related: How to Identify Aerobic Vs. Anaerobic Exercise (And Why it Matters)]
Lactate
According to D’Hulst, when an athlete runs on a treadmill with increasing intensity every three to four minutes, their lactate levels start low, rise steadily, and spike exponentially. This occurs because lactate is a byproduct of anaerobic metabolism, and as intensity increases, so does lactate production in the bloodstream.
D’Hulst has extensively tested moderately trained individuals and elite CrossFit athletes on elliptical bikes. His findings reveal a common issue: many CrossFit athletes lack a solid aerobic foundation.
Athletes struggle to sustain Zone 2 training without a proper aerobic base, emphasizing endurance at low-intensity effort. This is mainly because their training often skips genuine Zone 2 work, making aerobic or pure Zone 2 training especially challenging when paired with functional movements.
It really depends on the athlete, but most moderately trained two RX athletes don’t have this nice little increase in lactate.
—Dr. Gommaar D’Hulst
Muscle Oxygen
Zone 2 training can be monitored using a Near-Infrared Spectroscopy (NIRS) device, which measures muscle oxygen levels. This technology provides insight into the relationship between how the muscles utilize oxygen and how the heart and cardiovascular system deliver it.
Studies conducted by D’Hulst revealed that the muscle oxygen levels of a highly trained elite CrossFit athlete remained stable during the initial stages of exercise on an elliptical bike.
This means there is a balance between oxygen delivery and oxygen consumption.
However, the intensity increases and reaches a breakpoint.
This first breakpoint is what we call Zone 2.
Systemic lactate levels and NIRS thresholds do not exhibit a perfect correlation. (1) Ultimately, this holds little significance as it’s akin to comparing apples and oranges.
You’re looking at local breaking points at the muscle and systemic breaking points.
For instance, when D’Hulst assessed an athlete preparing for a long-distance rowing event, he discovered that the biceps fatigued faster than the thighs. This discrepancy was likely influenced by the athlete’s rowing technique, resulting in varying points of muscle fatigue compared to their lactate threshold.
How to Stay in Zone 2 During Functional Movements
Motion sensors can measure instantaneous and average power output during functional movements, such as burpees, air squats, wall balls, and pull-ups. Even when performed at a slower pace, these exercises tend to push individuals out of Zone 2 due to their high power demands, making it difficult to sustain the necessary energy levels for aerobic efficiency.
For beginners, an innovative approach is to focus solely on erg-based workouts for a few weeks or months, maintaining a rate of perceived exertion (RPE) of about three.
Intermediate athletes can combine ergs with functional movements to diversify their training while prioritizing Zone 2. Functional movements can be included for elite athletes aiming to stay in Zone 2, but they should be performed with light weights and at a relaxed pace. Staying within the aerobic zone is key to building efficiency in these movements while improving endurance.
Beginner Zone 2 Workout Sample
Perform three rounds of the following for time:
- 40/35 kcal Bike Erg
- 40/35 kcal Row
- 40/35 kcal Ski
Intermediate Zone 2 Workout Sample
Sample One
- Accumulate 300/250 kcal on an Airbike (Every Two Minutes)
- Five Dumbbell Hang Clean & Jerk (Light)
- Four Burpees Over a Dumbbell
Sample Two
- 1000 m Runs (Four Rounds)
- Two Rounds of Cindy
Elite Zone 2 Workout Sample (EMOM for 44 Minutes)
- 18 Wall Balls (6/4 kg)
- 14 Hang Clean & Jerk (42/35 kg)
- 20 Jumping Pull-Ups
- 9 Burpees
“It can be done with functional movements, but think about your level and how efficient you are in all the movements because you can be kicked out of this aerobic zone easily, and then completely miss the usefulness of the training,” D’Hulst stated.
Aerobic Base for High-Intensity Sports
Building a strong aerobic base is essential for excelling in high-intensity sports. Training volume and intensity significantly impact the function and density of mitochondria. (2) Put simply, the more you train, the greater your body’s mitochondrial density.
Athletes in disciplines like CrossFit and HYROX are known for pushing their training intensity, which ensures their mitochondria are highly efficient and functional. However, their overall mitochondrial density often falls short due to insufficient weekly training volume.
D’Hulst emphasized that long-term progress in high-intensity sports requires a gradual increase in training volume over time, accompanied by an increase in intensity. Athletes should integrate lower-intensity sessions into their weekly routines to optimize mitochondrial performance. This approach adds much-needed volume without causing excessive fatigue, ultimately creating a more balanced and effective training program.
References
- Arnet, J., Knaier, R., Schoch, R., D’Hulst, G., Bruggisser, F., Feldmann, A., Leuenberger, R., Westerhuis, E., Infanger, D., Schmidt-Trucksäss, A., & Wagner, J. (2025). Determination of Ventilatory Thresholds Using Near-Infrared Spectroscopy in Recreational Endurance and CrossFit Athletes. International journal of sports physiology and performance, 20(3), 345–354. https://doi.org/10.1123/ijspp.2024-0265
- Granata, C., Jamnick, N. A., & Bishop, D. J. (2018). Training-Induced Changes in Mitochondrial Content and Respiratory Function in Human Skeletal Muscle. Sports medicine (Auckland, N.Z.), 48(8), 1809–1828. https://doi.org/10.1007/s40279-018-0936-y
Featured image via Shutterstock/Ground Picture
