Exercise Scientist Shares Myths About Zone 2 and HIIT Cardio

Christy’s recommendation emerges from the entire paper’s logic: since the benefits claimed for zone 2 (mitochondrial content, fat oxidative capacity, VO2max) are actually better achieved at higher intensities, the focus should be on accumulating as much volume as possible at those higher intensities without compromising recovery. She draws an analogy to resistance training: close proximity to failure is what drives adaptation, not light weights. In endurance metabolism, pushing beyond current capacity (above lactate threshold) forces the body to adapt so that the same workload becomes less metabolically stressful. She does not prescribe a fixed number because individual recovery capacity varies widely, but the guiding principle is to prioritize intensity over arbitrary heart-rate zone targets.

Christy uses the metaphor of pulling the threshold up from above rather than trying to push it from below. She draws on systematic review evidence showing that in trained individuals, intensity is necessary to improve lactate threshold. The 2018 review indicated that even for mitochondrial improvements, you need to cross a certain intensity threshold. She argues that just like resistance training requires proximity to failure, metabolic training requires pushing beyond current capacity to force adaptation. This doesn’t mean all-out sprints; it can be sustained work slightly above threshold that you can maintain for, say, 20-40 minutes, accumulating volume where the metabolic stress is meaningful.

The host Siim mentions that he has seen meta-analyses showing longer intervals (over 2 minutes) are superior to shorter ones, likely because they accumulate more volume at high intensity. Christy agrees with the volume logic. She notes that doing 4-minute intervals in the lab was eye-opening for her own tolerance. The progression from 1 to 4 minutes is important because jumping straight into 4-minute max-sustainable efforts is brutal. The protocol is not about sprinting flat-out; it's about sustaining a pace that keeps you at 80-90% of max heart rate for the full 4 minutes, which trains the body to buffer and clear lactate and improves aerobic power.

This protocol was more discussed by the host Siim, but Christy agrees. The host explained that he does zone 2 because adding a lot of HIIT would impede his recovery from resistance training. Zone 2's low intensity and low sympathetic drive make it a tool to increase total training volume without the cost of additional high-intensity stress. This aligns with Christy's broader point that elite athletes use zone 2 for low-cost volume, but the general public rarely needs it for that purpose because their volume isn't high enough to require it. However, for someone lifting heavy often, zone 2 can serve a recovery-friendly aerobic slot.

Christy emphasizes that the amount of intensity someone can accumulate is largely dictated by what they can recover from. She gives an example: if you do a HIIT session in the evening, don't adequately consume carbohydrates, and then attempt another interval session the next morning after poor sleep, your performance and recovery will be compromised. Therefore, rather than a fixed number of HIIT sessions, people should monitor how they feel and whether they can sustain performance. This approach is less prescriptive but grounded in the reality that recovery is the limiting factor for training volume at higher intensities.

The paper was a narrative review, not systematic, because very few training studies actually prescribe exercise below the lactate threshold. The team struggled to find studies that fit true zone 2 criteria. Consequently, they had to interpolate using percentages of work rate max, heart rate zones, or gas exchange thresholds. Their conclusion: the mechanistic theory behind zone 2 doesn’t match because AMPK activation requires metabolic stress (accumulation of AMP/ADP) that occurs when glycolysis kicks in at higher intensities. Zone 2, by definition, stays below that threshold, so it does not robustly activate the key signaling for mitochondrial adaptations. They also found mixed evidence at best for mitochondrial outcomes from zone 2 training, and much stronger evidence for intensity-driven adaptations. The paper also notes that most ‘moderate intensity continuous training’ in comparator studies is actually above zone 2 (zone 3/4), further muddying the online discourse.

When comparing HIIT vs. MICT (moderate intensity continuous training), the MICT arms often use intensities that fall in zone 3 or even zone 4, not zone 2. This matters because zone 2’s specific claim is that staying below lactate threshold produces unique fat-burning and mitochondrial benefits. If the studies showing benefits all use higher intensities, they don’t validate zone 2, they validate continuous training at higher intensities. Christy also dissected a specific study that some used to argue endurance training yields equal VO2max gains after weeks, showing it conflated long-duration continuous training (which is usually zone 3+) with true zone 2. She emphasized that the ‘endurance training’ group in those analyses almost certainly includes mostly zone 3+ continuous work, so the takeaway that zone 2 specifically leads to those adaptations is unsupported.

Christy shared that a recreationally active, healthy 20-year-old university student often has a lactate threshold around 60-70 watts on a bike, which is very light. As soon as they start a light jog, they would be above zone 2. Therefore, many people who believe they are doing zone 2 training based on heart rate or perceived exertion are likely actually exercising in zone 3 or higher. This is not necessarily bad; in fact, it may be beneficial because it means they are automatically working at intensity levels that better stimulate mitochondrial adaptations. It also suggests that strict adherence to zone 2 prescriptions might not be possible for most without laboratory testing.

Christy referenced a 2018 systematic review covering 56 training studies, concluding that improvements in mitochondrial content or respiratory capacity would not be expected below 60% of work rate max, which is above zone 2 for most. She also discussed a more recent meta-analysis that split populations by training status and showed that in trained individuals, endurance training produced no increase in mitochondrial content, whereas HIIT and sprint interval training did. Additionally, an older systematic review on lactate threshold (a marker of fat oxidative capacity) found that in trained people, intensity is a key driver of improvements. This aligns with the principle that as people become more aerobically fit, they need greater metabolic stress to trigger further adaptations — analogous to resistance training where proximity to failure matters more than absolute load.

Christy is careful to say zone 2 is better than nothing and she’s happy if someone sedentary starts there. However, for someone already doing 30-minute runs, bike sessions, or HIIT and feeling good, the messaging that they need zone 2 could be a disservice: if they replace their intensity days with zone 2, they may lose the stronger mitochondrial and cardiorespiratory stimulus. She emphasizes that the goal isn’t to demonize zone 2 but to correct the evidence-free claim that it’s uniquely necessary. The real danger is that people who are already active might adopt a suboptimal volume of intensity under the false belief that zone 2 is optimal.

In her department, they have done long-term HIIT interventions in clinical populations (cardiac rehab, insulin resistance, type 2 diabetes) multiple times per week following activity guidelines without issues. Additionally, a specific study attempted to overtrain participants by having them do a HIIT session in the morning and another in the evening, 5 days a week, for 3 weeks. They tested weekly for overreaching symptoms and never had to stop the protocol; participants tolerated it well. This suggests that with proper recovery and nutrition, many people can accumulate substantial intensity volume without overtraining. She also acknowledged that total recoverable volume depends on sleep, glycogen replenishment, and individual factors, so the upper limit is personal, not fixed.