Building your engine: Zone 2 vs HIIT – What works for CrossFit?

Ever wondered why some workouts feel like a slow burn while others leave you gasping for air? The answer lies in how we train our aerobic system, and the science behind it is fascinating. In this post, we’ll explore:

• What aerobic capacity really means

• The difference between Zone 2 training and high-intensity intervals

• Why men and women respond differently

• How blood lactate thresholds shape performance

• Practical tips for CrossFit athletes

Even if "zone 2" training may not be your ideal training zone, if it's your "soul food", do it!

Understanding heart rate zones

Before we dive in, let’s define the zones:

• Zone 1: 50–60% of max HR (easy recovery pace)

• Zone 2: 60–70% (steady, conversational pace: our aerobic sweet spot)

• Zone 3: 70–80% (tempo work, starting to feel challenging)

• Zone 4: 80–90% (threshold: hard effort, near your limit)

• Zone 5: 90–100% (all-out sprint, max intensity)

  
  

Think of it like building a car: Zone 2 builds a bigger engine, while HIIT adds the turbocharger.

What is aerobic capacity?

Aerobic capacity is your body’s ability to use oxygen efficiently to produce energy over time. Essentially, it is the ability to take in (lungs), transport (heart and circulatory system) and utilise (skeletal muscle) oxygen.

It is what allows you to sustain effort, recover between sets, and handle volume without burning out. Even short CrossFit workout's rely on a strong aerobic base for repeatability.

Zone 2 training: The Aerobic foundation

Zone 2 training, or long slow distance (LSD) "steady-state work" at 60–70% of your max heart rate, feels almost too easy. But here’s why it matters:

• Improves mitochondrial density, fat oxidation, and capillary growth

• Enhances insulin sensitivity and long-term endurance

• Sits below your first lactate threshold (LT1), where lactate starts to rise above resting levels. Training here teaches your body to clear lactate efficiently and rely on fat for fuel.

• Durations of ~60 minutes in length are recommended

  
  

Gender differences:

• Men often gain more from Zone 2 training because they rely heavily on fat oxidation pathways.

• Women are physiologically more efficient with fat metabolism thanks to oestrogen, so while Zone 2 helps, their performance will generally improve more from higher-intensity work, especially peri and post menopausal women.

  
  

HIIT: Adding the turbocharger

High-Intensity Interval Training (HIIT) involves short bursts of near-maximal effort with rest, think 30-second bike sprints with 2 minutes recovery, or working "every minute, on the minute".

Benefits include:

• Boosts VO₂ max, lactate clearance, and neuromuscular power

• Pushes you near or above your second lactate threshold (LT2), also known as the lactate turnpoint, the point where lactate accumulates faster than your body can clear it, leading to rapid fatigue. Training at this intensity improves your ability to buffer and clear lactate, which is critical for CrossFit-style workouts.

  
  

Gender differences:

• Women often show strong adaptations to HIIT, including improved power output and endurance crossover.

• Men benefit too, but without a solid aerobic base, repeated high-intensity efforts can lead to burnout.

  
  

How it transfers to CrossFit

CrossFit demands both:

• Zone 2 as a solid base, so you can handle volume and recover.

• HIIT for the peak "top end" fitness, so you can hit those intense metcons hard.

  
  

Practical takeaway:

• Men: If you haven't already, prioritize Zone 2 to build a strong foundation.

• Women: Include HIIT regularly for a performance edge, but maintain some aerobic work, too.

  
  

The bottom line

Zone 2 builds your engine. HIIT adds the turbocharger. Lactate thresholds tell us where the magic happens. Both methods are just as important as each other, but how you balance them might depend on you.

Don't forget, it's just as important to remember what you enjoy (we term this "soul food!") as ultimately, if you enjoy any form of activity, you should do it!

Question for you:

Do you lean into long-zone training or short intense efforts? Or a mix of both? Share your thoughts below, or reach out with any questions on 07588286277, or better still, book a free introduction with us here where we can talk about all things aerobic capacity!