Importance of Aerobic and Anaerobic Thresholds

I recently listened to a few episodes of Extramilest, a podcast about marathon running hosted by Floris Gierman, featuring Scott Johnston, a climber, skier, and mountain runner who has coached many elite athletes. Johnston is also the co-author of Training for the Uphill Athlete, with Kilian Jornet, one of the greatest ultra runners in the world.

Across two episodes (here and here), Johnston offered one of the clearest explanations I’ve heard of two critical markers of endurance for distance runners: aerobic threshold and anaerobic threshold. These are essential not only for guiding training but also for tracking performance over time.

Aerobic Threshold

The aerobic threshold represents the top of Zone 2 in a five-zone heart rate model. For runners preparing for a half or full marathon, a few key takeaways stand out:

• Most training should happen at or below this threshold. Running at this intensity allows you to build more volume and duration without excessive fatigue, which is essential for building endurance.

• It drives key physiological adaptations. Training in this zone promotes mitochondrial development, improving the body’s ability to sustain effort at faster paces over longer periods.

• It forms the foundation of fitness. Johnston describes this as the base of a pyramid: everything else builds on top of it. If this foundation is weak, higher-intensity training becomes less effective and even counterproductive.

• Standard formulas can be misleading. Many runners rely on rules like 60-70% of max heart rate, with max heart rate estimated as 220 minus age. But individual variation can be significant, resulting in inaccurate heart rate zones.

• Individualized testing is more accurate. One method is a heart rate drift test: run at a steady pace for 45–60 minutes and monitor whether your heart rate increases by more than approximately 5 percent. The fastest pace you can sustain without significant drift is a good estimate of your aerobic threshold.

Anaerobic Threshold

The anaerobic threshold occurs at a much higher intensity, typically at the top of Zone 3 or bottom of Zone 4, and should be used less frequently in training.

• It reflects your sustainable hard effort. This is roughly the fastest pace you can maintain for about 30-45 minutes.

• It’s relatively easy to estimate. A common proxy is a 10K time trial, which closely aligns with anaerobic threshold effort for many runners.

• It requires careful use. Because this intensity is more taxing, overusing it can lead to fatigue, plateau, or injury. Only about 10-20 percent of training should be higher intensity, which includes anaerobic threshold work. It’s most effective when layered on top of a strong aerobic base.

Using Both Effectively

The key insight from Johnston’s framework is that endurance performance is built from the bottom up. Aerobic threshold training should make up the majority of your training, especially if your goal is long-distance racing, and takes months of consistent running to develop. Anaerobic threshold work still matters, but as a complement, not the focus or foundation.

A broader goal is to narrow the gap between your aerobic and anaerobic thresholds, primarily by improving your aerobic base. For most runners, this means spending around 80 percent of training time at or below aerobic threshold.

Many runners fall into the trap of training too hard too often, hovering in the middle that’s neither easy enough to build aerobic capacity nor hard enough to meaningfully improve speed. Training with these two thresholds in focus helps avoid that trap and leads to more consistent, sustainable progress with lower risk of injury.