Scroll through any fitness forum and you'll find two deeply entrenched camps: the HIIT devotees who swear that 20 minutes of intervals beats an hour of slow jogging, and the Zone 2 advocates who point to elite endurance athletes spending 80% of their time at conversational pace. Who's right?
The honest answer: both are, depending on what you're trying to achieve. This guide breaks down the physiology, the research, and the practical case for each — so you can stop arguing about it online and start using both strategically.
What is HIIT?
High-intensity interval training (HIIT) alternates short bursts of near-maximum effort with recovery periods. A classic structure might be 30 seconds all-out followed by 90 seconds of easy movement, repeated 8–10 times. The defining feature: you're working at 85–100% of your maximum heart rate during the effort intervals.
HIIT sessions are typically short — 20 to 35 minutes including warm-up — which is part of their appeal. The work-to-rest ratio varies depending on the goal: shorter rest for metabolic conditioning, longer rest for true power and speed development.
What is Zone 2?
Zone 2 is sustained aerobic exercise at 60–70% of your maximum heart rate — a pace where you can hold a conversation but wouldn't particularly want to. It's commonly called "easy cardio," though that undersells its physiological impact. Sessions typically run 45–90 minutes.
The defining feature of Zone 2 is that it's primarily fuelled by fat oxidation rather than glycogen. Your body is working, but within its aerobic capacity — no significant lactate accumulation, no meaningful oxygen debt.
What HIIT does to your body
HIIT is extraordinarily effective at improving VO2 max — the maximum rate at which your body can use oxygen — in a short time. A 2019 meta-analysis in the British Journal of Sports Medicine found HIIT superior to moderate-intensity continuous training for VO2 max improvement across most populations.
HIIT also improves insulin sensitivity, triggers excess post-exercise oxygen consumption (the "afterburn effect"), and is highly time-efficient. For people with limited training time, it delivers a lot of stimulus in a short window.
The downsides: HIIT is physiologically expensive. Recovery takes 36–72 hours, limiting how often you can do it. Done too frequently, it raises cortisol, suppresses immune function, and increases injury risk. Two to three HIIT sessions per week is the ceiling for most people.
What Zone 2 does to your body
Zone 2 specialises in something HIIT barely touches: mitochondrial biogenesis. Each Zone 2 session signals your muscle cells to grow more mitochondria — the energy factories that power aerobic activity. More mitochondria means greater fat-burning capacity, better metabolic health, and a higher aerobic ceiling.
Zone 2 also improves fat oxidation efficiency, cardiac stroke volume, and the ability of Type 1 muscle fibres to clear lactate. These are slow-building adaptations that compound over months and years.
Critically, Zone 2 is recoverable. You can do it four to five times per week without meaningful fatigue accumulation, adding enormous volume without stress.
The research verdict
Neither approach wins outright — they target different physiological systems and complement each other. The relevant question is: which is being underused in your current training?
Most recreational athletes who think they do a lot of cardio actually spend most of their time in Zone 3 — the intensity range sometimes called the "grey zone." It's too hard to recover from quickly and too easy to drive the adaptations of true high-intensity work. It's the worst of both worlds.
The 80/20 model used by elite endurance athletes — roughly 80% of volume in Zone 1–2 and 20% in Zone 4–5 — consistently outperforms the grey zone approach in long-term fitness development.
How to combine both
A well-structured week for most people:
- 2–3 Zone 2 sessions (45–90 min each) — building the aerobic base
- 1–2 HIIT sessions — targeting VO2 max and high-end capacity
- 1 rest or Zone 1 recovery day — letting adaptations consolidate
Start with Zone 2 as the foundation. Add HIIT on top once you can sustain 3+ hours of Zone 2 per week without fatigue. If you're currently doing lots of HIIT and your fitness feels stuck, Zone 2 volume is almost certainly what's missing.
How Underdog tracks both
Underdog scores every workout by time spent in each heart rate zone. Your HIIT sessions rack up points in Zone 4 and Zone 5 — the high-intensity tiers that earn the most per minute. Your Zone 2 runs and rides earn points in the aerobic base tier. Every workout type counts, and your weekly total determines your league ranking — whether you trained hard or trained long.