What is the Aerobic Threshold and How to Train It
CrossFit is more than just a high-intensity workout; it’s a multi-faceted training method that challenges your cardiovascular, musculoskeletal, and respiratory systems to enhance overall athletic performance.
However, many athletes struggle with sustaining higher intensities throughout their workouts or managing fatigue, limiting their ability to push their limits and improve fitness levels.
You might find yourself hitting a wall during a MetCon, unable to maintain the pace or feeling exhausted sooner than expected.
Instead of sustaining your pace and finishing strong, you’re left with lingering fatigue, preventing progress and poor metcon performance.
In this article, we’ll explore how CrossFit demands more from your body than just muscle strength, and how strategies like Zone 2 training, threshold intervals, breath control, and limiter-specific training can boost your endurance and help you train harder, longer, and perform better.
The Energy Systems and Aerobic Threshold
Your body relies on three primary energy systems to fuel movement: the ATP-PC system, the glycolytic system, and the oxidative system.
Each plays a unique role in sustaining different levels of effort. (1)
The ATP-PC system provides immediate energy for short, explosive efforts lasting up to 10 seconds, such as a max-effort snatch or a powerful start in a sprint.
This system doesn’t require oxygen and relies on stored phosphocreatine, making it perfect for quick bursts of power but unsustainable for long-term efforts.
The glycolytic system takes over when your efforts extend beyond a few seconds to about two minutes.
It breaks down stored carbohydrates through anaerobic glycolysis, producing energy quickly but at the cost of accumulating lactic acid.
This buildup contributes to fatigue, limiting how long you can sustain high-intensity work.
The oxidative system is your primary energy source for anything beyond two minutes, relying on oxygen to generate ATP from carbohydrates and fats.
While it produces energy more slowly than anaerobic pathways, it is highly efficient and supports endurance activities like long runs, steady-state workouts, and CrossFit WODs that extend beyond a few rounds.
Where the Aerobic Threshold Fits In
Your aerobic threshold is the intensity at which your body shifts from primarily using fat for energy to increasingly relying on carbohydrates.
It marks the point where your oxygen supply is still sufficient to meet energy demands without lactic acid buildup. (2)
When you train below your aerobic threshold, you stay in a sustainable intensity range where your muscles rely on aerobic metabolism.
But once you cross this threshold, your body starts depending more on anaerobic metabolism, increasing lactate production and gradually pushing you toward fatigue.
Your anaerobic threshold—the second threshold—occurs at a higher intensity when lactic acid accumulates faster than your body can clear it.
If you want to improve endurance and sustain higher intensities, training at or just below your aerobic threshold is key.
Why It Matters for CrossFit
In CrossFit, you constantly transition between energy systems.
Whether you’re pushing through high-intensity intervals or grinding through a long AMRAP, your ability to sustain effort depends on how efficiently your body manages oxygen and energy production.
If your aerobic efficiency is underdeveloped, you’ll fatigue faster in long WODs and struggle to recover between high-intensity bursts.
By improving your aerobic threshold, you can delay the point at which fatigue sets in, allowing you to maintain higher intensity levels for a longer time.
Elite endurance athletes prioritize aerobic threshold training to enhance performance, and CrossFit athletes can benefit from the same approach.
Testing and Measuring Your Aerobic Threshold
Heart Rate-Based Methods
One of the easiest ways to estimate your aerobic threshold is by tracking your heart rate. (3)
The heart rate drift test measures how much your heart rate increases during a steady-state workout.
If your heart rate rises significantly over time while maintaining the same intensity, you may be exceeding your aerobic threshold.
Another method involves ventilatory markers. Your first ventilatory threshold (VT1) aligns closely with your aerobic threshold.
This is the point where your breathing noticeably deepens but remains controlled.
The “talk test” can help identify this threshold—if you can speak comfortably but start feeling slightly breathless, you’re likely near VT1.
Lactate Threshold Testing
For a more precise measurement, lactate threshold heart rate testing provides a clear picture of your training zones. (3)
A blood test taken at progressive exercise intensities measures lactate levels, helping to determine the point at which accumulation begins.
While a laboratory test offers the most accuracy, field tests like max-effort intervals combined with heart rate monitoring can give you valuable insights into your lactate threshold and aerobic efficiency.
Practical Field Tests
If you don’t have access to lab testing, there are several effective ways to assess your aerobic threshold on your own.
A max test, such as a 30-minute tempo run while tracking heart rate and pace, can help estimate your aerobic training zone.
Another approach is the heart rate drift test during a 60-minute steady-state workout.
If your heart rate drifts upward significantly in the second half of the session, you’re likely working above your aerobic threshold.
By regularly tracking these numbers, you can adjust your training to improve endurance and efficiency.
Training Strategies to Improve Your Aerobic Threshold
Zone 2 Training for Aerobic Base Development
One of the best ways to build a solid foundation for endurance is through Zone 2 training. (3)
This type of training involves sustained, low-intensity efforts where you remain below your aerobic threshold, allowing your body to develop greater efficiency in oxygen utilization.
Easy runs, long rides, or steady-state rowing sessions at 60–70% of your max heart rate encourage mitochondrial adaptation, increase capillary density, and improve fat oxidation.
Well-trained endurance athletes dedicate significant time to Zone 2 work because it enhances long-term aerobic performance without excessive fatigue.
Threshold Intervals for Performance Gains
While Zone 2 training builds your aerobic base, threshold intervals push the boundary of your aerobic capacity. (3)
Training just below or at your lactate inflection point strengthens your ability to sustain higher intensity levels without shifting too quickly into anaerobic metabolism.
One effective method is functional threshold power intervals—sustained efforts at a pace that challenges your aerobic system but doesn’t overwhelm it with lactate buildup.
These intervals improve your ability to clear carbon dioxide and maintain power output during long, demanding workouts.
Breath Control and Efficiency Drills
Your respiratory system plays a key role in sustaining performance across different training intensities.
Breath control drills, such as nasal breathing during easy runs or controlled exhales during high-intensity efforts, improve oxygen utilization and carbon dioxide clearance. (3)
You can also use inspiratory muscle training devices to strengthen your breathing muscles.
Research suggests that elite endurance athletes have higher maximal inspiratory pressure, giving them an advantage in sustaining higher intensities with less respiratory fatigue.
By combining steady-state aerobic workouts, targeted threshold intervals, and breath control techniques, you can develop a more efficient aerobic system.
Over time, you’ll delay fatigue, recover faster, and sustain a higher level of effort across multiple training sessions.
Why Your Aerobic Threshold Isn’t the Whole Story
Improving your aerobic threshold is a key factor in building endurance and sustaining higher intensities, but it’s only part of the equation.
Your overall performance is also shaped by three major physiological limiters—your ability to deliver oxygen, utilize it efficiently, and manage fatigue.
While a stronger aerobic system helps delay exhaustion, these other factors determine how well you can push beyond your limits.
To truly optimize your training, you need to understand what’s holding you back and how to break through those barriers.
The Three Trainable Systems and Limiters
1. The Cardiac System
Your heart’s ability to pump oxygen-rich blood directly impacts how long you can sustain high-intensity efforts and how quickly you recover between sets.
CrossFit workouts often push you into higher intensities, where efficient oxygen delivery becomes the difference between pushing through or burning out.
The stronger your heart, the more blood it can pump per beat—also known as stroke volume—which improves your ability to maintain performance in endurance training and short bursts of anaerobic effort.
One of the most significant adaptations to CrossFit is an increase in left ventricular wall thickness and cavity diameter.
This expansion allows more blood to be pumped with each contraction, improving overall cardiovascular system efficiency.
Over time, these changes help delay your anaerobic threshold, meaning you can sustain a higher level of effort before lactic acid buildup forces you to slow down. (4)
Signs of a Cardiac Limiter
Recognizing a weak cardiac system is essential if you want to refine your training intensity distribution and maximize performance.
If you notice these issues, your heart might be limiting your output:
- You fatigue early in long, steady-state workouts or struggle to maintain consistent power output.
- Your maximum heart rate climbs quickly, even at moderate intensities, and takes longer than expected to return to normal resting levels.
- You recover slowly between rounds, making it harder to sustain performance across multiple training sessions.
The primary reason for these issues is a low stroke volume (blood pumped per beat), which forces your heart to work harder to supply oxygen.
Without an efficient pump, your muscles don’t get the oxygen they need, leading to early fatigue and reduced ability to sustain anaerobic energy pathways.
How to Improve Cardiac Function
If your heart is your limiting factor, you need to focus on structured endurance training that specifically targets stroke volume and oxygen delivery.
One of the most effective methods is interval work that challenges your functional threshold power and progressively increases ventilatory threshold.
Such as through Max Aerobic Power (MAP) intervals.
This type of training creates the necessary stress to stimulate left ventricular mass growth and improve your ability to sustain a higher intensity over time.
Studies have shown that high-intensity interval training can increase left ventricular mass by 10–20% in athletes, leading to improved aerobic efficiency and greater capacity to clear carbon dioxide during exertion. (4)
To apply this effectively, track your heart rate drift test results and monitor your recovery between intervals.
Over time, you’ll notice your heart rate stabilizing at lower intensities, allowing you to push harder while staying within optimal training intensities.
2. The Musculoskeletal System
Your muscles rely on oxygen to sustain high-rep workouts and long-duration AMRAPs, making capillary density a key component of endurance. (5)
Capillaries are responsible for delivering oxygen to your mitochondria, the powerhouse of energy production in your body.
When you develop a higher capillary-to-fiber ratio, oxygen can diffuse into muscle cells more efficiently, reducing fatigue and increasing power output during training.
If your mitochondria struggle to extract enough oxygen, your body initiates compensatory angiogenesis, a natural process that increases capillary growth.
This adaptation is critical for CrossFit athletes since higher intensity efforts, such as interval workouts, demand a continuous oxygen supply.
Even if your anaerobic system kicks in during short, explosive movements, having a well-developed capillary network ensures faster recovery and better endurance over a long time.
Signs of a Muscular Limiter
If your legs feel like they’re filled with lactic acid halfway through a long chipper WOD, you may have a muscular limiter.
One major sign is early muscle fatigue, especially during steady-state workouts or long aerobic efforts.
If your quads or hamstrings burn out quickly during a long run or high-rep squats, your muscles may not be extracting enough oxygen.
Slow recovery between reps is another red flag.
If you struggle to sustain your training intensity distribution across multiple rounds, your mitochondria may not be producing ATP (energy) efficiently.
This issue is common in athletes who have excellent anaerobic capacity but struggle with aerobic efficiency.
If you feel strong in sprint-style workouts but fade fast in long rides or high-rep circuits, you likely need to improve your aerobic base training.
How to Improve Capillary Density and Mitochondrial Efficiency
One of the best ways to enhance capillary density is through high-rep endurance circuits, such as EMOM air bike sessions.
These workouts create capillary shear stress, triggering VEGF signaling, a process that stimulates new capillary growth.
By increasing your capillary network, your muscles become more efficient at using oxygen, delaying lactate inflection point and improving endurance.
Research shows that capillary density explains much of the differences in athletes’ aerobic capacity.
Well-trained endurance athletes have significantly higher capillary-to-fiber ratios than untrained individuals, allowing them to sustain higher intensities for a long term without excessive fatigue. (5)
To maximize results, prioritize sustained, low-intensity work with high-intensity sessions sprinkled in.
3. The Respiratory System
Your ability to control your breathing directly impacts your power level, endurance, and overall training intensity distribution.
When you’re grinding through burpees or thrusters, your respiratory system works hard to manage gas exchange and remove carbon dioxide efficiently.
Well-trained CrossFit athletes have 30–40% higher maximal inspiratory and expiratory pressures than sedentary individuals, meaning they can take in more oxygen while minimizing respiratory fatigue. (6)
How to Improve Breathing Mechanics
One of the easiest ways to improve breathing efficiency is through breath-pacing drills.
These techniques help synchronize your breathing with movement, preventing shallow breaths that limit aerobic efficiency.
You can also use various devices to strengthen your breathing muscles.
Scientific research supports a strong correlation between maximal inspiratory pressure (MIP) and forced vital capacity (FVC) in CrossFit athletes. (6)
In other words, the stronger your respiratory muscles, the better your lungs can handle oxygen demands at higher intensity levels.
To enhance breathing control, focus on diaphragmatic breathing during both steady-state workouts and max-effort lifts.
Final Thoughts on The Three Trainable Systems
Understanding and training your aerobic threshold is key to enhancing endurance, improving performance, and delaying fatigue during high-intensity CrossFit workouts.
By focusing on the balance between aerobic and anaerobic energy systems, you can push your limits and train more efficiently.
Whether through targeted heart rate methods, lactate threshold testing, or structured training strategies, consistently working on your aerobic base will lead to better endurance and faster recovery.
Keep monitoring your progress, stay consistent, and use these strategies to maximize your potential in training and performance.
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References:
1. Gastin P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports medicine (Auckland, N.Z.), 31(10), 725–741. https://doi.org/10.2165/00007256-200131100-00003
2. Gastin P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports medicine (Auckland, N.Z.), 31(10), 725–741. https://doi.org/10.2165/00007256-200131100-00003
3. Archacki, D., Zieliński, J., Pospieszna, B., Włodarczyk, M., & Kusy, K. (2024). The contribution of energy systems during 15-second sprint exercise in athletes of different sports specializations. PeerJ, 12, e17863. https://doi.org/10.7717/peerj.17863
4. Pittaras, A., Faselis, C., Doumas, M., Grassos, C., & Kokkinos, P. (2023). Physical Activity and Cardiac Morphologic Adaptations. Reviews in cardiovascular medicine, 24(5), 142. https://doi.org/10.31083/j.rcm2405142
5. Hendrickse, P., & Degens, H. (2019). The role of the microcirculation in muscle function and plasticity. Journal of muscle research and cell motility, 40(2), 127–140. https://doi.org/10.1007/s10974-019-09520-2
6. Fabrin, S. C. V., Palinkas, M., Fioco, E. M., Gomes, G. G. C., Regueiro, E. M. G., da Silva, G. P., Siéssere, S., Verri, E. D., & Regalo, S. C. H. (2023). Functional assessment of respiratory muscles and lung capacity of CrossFit athletes. Journal of exercise rehabilitation, 19(1), 67–74. https://doi.org/10.12965/jer.2244594.297
