Summary
Simon Hill and exercise physiologist Drew Harrisberg unpack practical takeaways from Hill's previous deep-dive with Dr. Inigo San Millan on zone 2 training. The conversation covers how to determine your zone 2 heart rate using the Karvonen formula and heart rate reserve, why nasal breathing is an effective intensity regulator, and how heart rate drift during longer sessions requires adjusting power output downward. Harrisberg presents a practical 3-day cardio program: a zone 2 session with a 4-minute HIIT finisher, a zone 3-4 freestyle workout, and a dedicated 4x4 HIIT protocol (4 minutes work, 3 minutes active rest, repeated 4 times). He explains that the 4x4 protocol is the most studied evidence-based HIIT format for improving VO2 max. A key theme is that the 80/20 rule (80% zone 2, 20% high intensity) works for high-volume athletes but should shift toward more high-intensity work when total training volume is low. They discuss a study comparing evidence-based HIIT protocols against social media workouts, finding that shorter intervals like Tabata fail to achieve sufficient time above 85% of max heart rate.
Key Points
- Use the Karvonen formula with heart rate reserve to calculate individualized zone 2 targets, as the crude 60-70% of max heart rate range has massive variability
- Nasal breathing during zone 2 sessions is an effective intensity regulator -- if you cannot maintain nose-only breathing, you have likely drifted above zone 2
- The 80/20 rule works for high-volume athletes, but people with limited training time should shift toward a higher proportion of high-intensity work
- The evidence-based 4x4 HIIT protocol (4 minutes at 85%+ max heart rate, 3 minutes active rest, repeated 4 times) is the most studied format for improving VO2 max
- Minimum effective HIIT dose can be as little as one 4-minute interval added to the end of a zone 2 session
- Social media HIIT protocols like Tabata rank lowest for time spent above 85% max heart rate; interval duration is the key factor
- The 150-300 minute weekly guidelines refer to general moderate-intensity physical activity, not specifically zone 2 cardiovascular training
- Consider balancing 150 minutes of zone 2 with resistance training and HIIT rather than dedicating all 300 minutes to zone 2
Key Moments
Fat oxidation variability spans 60-80% of max heart rate
Drew Harrisberg explains that studies on maximum fat oxidation show massive individual variability, with some people peaking at 60% and others at 80% of max heart rate. He recommends a broader moderate intensity bracket of 60-75%.
"And if you look at the studies looking at max fat oxidation, the variability is massive. Some people maximize their fat oxidation at 60% of the heart rate max, other people at 80%."
Nasal breathing as a zone 2 intensity regulator
Harrisberg describes using nasal-only breathing during runs as an effective way to regulate intensity and stay in zone 2. As soon as you push higher, you naturally need to breathe through your mouth.
"From Bondi to brontium back, I will not breathe through my mouth. Because what that does is, as soon as you push into a higher zone, like zone three and four, naturally you're going to try to breathe through your mouth."
The 80/20 rule scales with training volume
The hosts discuss how the 80/20 ratio makes sense for elite athletes doing massive volumes, but for the general population with limited time, the proportion of high-intensity work should increase.
"If you're not doing much volume over the week, I would say push towards higher intensities. If you're doing huge volume, definitely lean on the zone too."
Evidence-based 3-day cardio program structure
Harrisberg outlines his 3-day cardio program: Day 1 is zone 2 with a 4-minute HIIT finisher, Day 2 is a zone 3-4 freestyle session, and Day 3 is a dedicated 4x4 HIIT protocol for VO2 max improvement.
"So I've got three cardio workouts a week. The first one is a zone two workout for the vast majority of the time. And right on the end of it, we tag on a four-minute hit finisher."
