The Most Overlooked Cardio Zone You’re Probably Rushing Through

Stage 2 cardio is often used interchangeably with Zone 2: steady, conversational aerobic work that sits below your first major “gear change” in physiology. Technically, it’s the intensity below the first lactate/ventilatory threshold (LT1/VT1), where lactate production and clearance are still balanced and breathing remains controlled. In practical terms, it’s the pace you can hold for a long time without feeling like you’re “working hard.”

What’s happening in Stage 2?

At Stage 2 intensities, you’re predominantly using aerobic metabolism (oxygen-driven energy production). This is where the body is highly capable of using fat as a fuel source, and where the muscle cell machinery that supports endurance, mitochondria, capillaries, and oxidative enzymes, gets a strong training signal with relatively low recovery cost.

A key reason coaches love this zone: it tends to be repeatable. You can accumulate meaningful training volume without the mechanical stress and hormonal “spike” that comes with harder intervals. Research on endurance athletes consistently shows that high performers typically do a large portion of training at low intensity, often paired with smaller amounts of higher-intensity work (a pattern often described as “polarised”) (Seiler & Kjerland, 2006; Mann et al., 2013).

Evidence-based benefits

1) Aerobic base and mitochondrial adaptations
Stage 2 supports the cellular changes that improve how efficiently you produce energy aerobically. Over time, this can raise the speed/power you can sustain before “tipping” into heavier breathing and faster lactate accumulation.

2) Improved fat oxidation and metabolic flexibility
Moderate, steady exercise is closely linked to improving the ability to oxidise fat, especially when kept below the first threshold (Achten & Jeukendrup, 2003). This is not a promise of fat loss from a single session, it’s about upgrading the engine’s fuel options.

3) Threshold development and sustainable fitness
Due to LT1/VT1 being a foundational endurance marker, training near (but below) it can improve your ability to do more work aerobically before needing to rely heavily on glycolysis. Reviews of lactate threshold concepts highlight its importance for endurance performance and training prescription (Faude et al., 2009).

How this translates on wearables (and where people get stuck)

Most wearables translate Stage 2 into a heart-rate zone, but the accuracy depends on how your zones were set.

What to look for on your watch:

• Heart rate: Often roughly 70–80% of max HR for many people, but wide individual variation makes this a starting point, not a rule.
  

• Breathing/talk test: You can speak in full sentences; breathing is deeper but controlled.
  

• RPE (effort): About 3–4/10, you feel like you’re exercising, but you could keep going.

Why wearables can mislabel Stage 2

• Estimated max HR can be off by 10–20 bpm, shifting all your zones.
  

• Optical HR sensors can lag or spike (especially in cold weather or with arm movement).

• Cardiac drift: On longer sessions, HR rises even if pace/power stays constant - so you may need to slightly back off to stay in Stage 2.

How to dial it in 

• If your wearable offers threshold detection (some estimate lactate threshold or ventilatory thresholds from runs/cycling), use that as a better anchor than age-based max HR.
  

• For steady sessions, use pace/power + HR together: if pace is stable but HR climbs steadily, you’re drifting, cooling, hydration, or slightly reducing intensity can keep you honest.

• A simple field check: when you finish, you should feel like you could do more, your legs worked, but you’re not wrecked.

Stage 2 isn’t flashy, but it’s one of the most reliable levers for building a bigger aerobic “ceiling” while keeping recovery manageable. Let your wearable guide you, but let your breathing, talk test, and consistency confirm you’re truly in the zone.

References 

Achten, J., & Jeukendrup, A. E. (2003). Heart rate monitoring: Applications and limitations. Sports Medicine, 33(7), 517–538.

Faude, O., Kindermann, W., & Meyer, T. (2009). Lactate threshold concepts: How valid are they? Sports Medicine, 39(6), 469–490.

Mann, T. N., Lamberts, R. P., & Lambert, M. I. (2013). High responders and low responders: Factors associated with individual variation in response to endurance training. Sports Medicine, 43(8), 645–657.

Seiler, S., & Kjerland, G. Ø. (2006). Quantifying training intensity distribution in elite endurance athletes: Is there evidence for an “optimal” distribution? Scandinavian Journal of Medicine & Science in Sports, 16(1), 49–56.