Zone 2 has become the unofficial language of recreational endurance training over the last five years, sold to runners as the magic, conversational-pace effort that builds aerobic engines without burning out joints or central nervous systems. A new body of physiology research published through 2025 and into 2026 is now pushing back, not on the principle of low-intensity work, but on the heart-rate maths that most runners use to define it.
The simplest popular definitions of zone 2, typically a fixed 60-70 per cent of maximum heart rate or a generic blood-lactate band, hide an uncomfortable amount of variation. A peer-reviewed comparison of common zone 2 markers reported coefficients of variation between roughly 6 and 29 per cent across athletes, depending on which submaximal threshold was used to anchor the zone. In practical terms, two runners with identical maximum heart rates can have ventilatory or lactate thresholds that differ by 15 to 20 beats per minute, which is enough to put one of them in genuine aerobic territory while the other is straying into a tempo effort.
The disagreement is not whether low-intensity volume matters. Polarised and pyramidal models, both heavy on easy mileage, continue to dominate elite and sub-elite endurance training, and the evidence that easy running drives mitochondrial adaptation, fat-oxidation capacity and tendinous resilience is broadly settled. The argument is over which physiological signal best identifies the upper edge of that easy zone for a given runner on a given day, and how big the consequences are when the signal is set too generously.
The alternatives most often cited in the new literature are ventilatory threshold one, identified through gas-exchange testing, and individual lactate threshold, identified by a pin-prick lactate ramp protocol. Both move the zone with the runner's actual physiology rather than a population-averaged percentage of maximum heart rate. The more accessible proxies, talk test or rating of perceived exertion, also stand up reasonably well in the data, and the research consistently finds them more reliable than off-the-peg heart-rate band charts on a wrist-worn device.
For everyday runners, the practical takeaway is not to abandon zone 2 but to stop trusting an arbitrary band of beats per minute as gospel. Periodic threshold testing, a sharper read on perceived effort, and a willingness to sit a little lower than the chart recommends are the moves the new research supports, particularly for athletes layering in long aerobic blocks ahead of a marathon or ultramarathon build. The zone is real. The fixed numbers around it are not.