In the realm of endurance sports training these days, there isn’t a much hotter (or controversial) topic than Zone 2 training. Zone 2 training has long been a training staple for many endurance athletes as it’s historically been touted as the best intensity to train at to induce specific physiological adaptations such as mitochondrial and capillary density.

However, recent specific research into Zone 2 training has largely debunked this long-standing narrative. More specifically, it turns the narrative on its head as higher intensities have been found to have a greater impact on mitochondrial and capillary density as compared to low intensities.

This new research will undoubtedly shift athletes’ perceptions of training intensity and therefore how to structure training programs.

In this episode of ‘All Things Endurance,’ host Rick Prince discusses the Zone 2 topic with Dr. Brendon Gurd, Professor of Muscle Physiology at Queen’s University.

Dr. Gurd discusses the science behind different exercise intensities with respect to physiological adaptations. In short, zone 2 as it pertains to being the ideal zone for enhancing physiological adaptations such as mitochondrial and capillary density is a myth.

Whether you’re an athlete or coach, consider this myth busting podcast a must!

Here are 2 resources related to this podcast we'd like you to have: Dr. Gurd's Zone 2 Presentation Slide deck (as a PDF), and a very new research article (April 2025) reviewing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiovascular fitness

Download Here: https://uesca.com/download/zone-2-podcast-resources/

Below are the areas that Rick and Dr. Gurd discuss:

1. Could you tell our listeners a bit about your background?
2. Zone 2 is a really hot topic these days, specifically in regard to increasing one’s aerobic fitness and overall health. How did this become such a focal point recently?
3. From an athletic performance standpoint, it seems that the main rationale for Z2 training is that it has the greatest effect on increasing mitochondrial and capillary density. Is there any truth to this?
   1. Is there any intensity which increases mitochondrial and capillary density the most?
4. As there is often a trickle down effect from what professional athletes do to what us mere mortals do, do you think that the fact that a lot of pros do high volume at relative low intensity has factored into the popularity of Z2?
5. Are there any overall aerobic training trends that you would recommend to most endurance athletes?
6. For a time-crunched endurance athlete, would you recommend an overall higher intensity level for a good portion of their training sessions (vs. a lot of time in Z2)?
7. As one of the basic aspects of adaptation is that you need to have stress to elicit gains, should athletes that predominately do only Z2 training expect to plateau without increasing bouts of intensity?
8. Aside from recovery days, is there such a thing as going too easy where little to no physiological adaptations occur? If so, what is what level/intensity?
9. Lastly, should all endurance athletes incorporate all levels of intensity into their training?