A number of articles and videos have recently questioned the whole Zone 2 training phenomenon, often from the same media outlets that were presenting Zone 2 as a silver bullet only a short while ago. This conversion on the road to Damascus appears to have been encouraged by a recent paper entitled Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population.
So is Zone 2 training dead, probably not. But some of the mythology surrounding it needs to be buried.
The first problem is terminology. In exercise physiology, a simple three zone model is often based on two physiological thresholds:
LT1 is the point at which blood lactate begins to rise systematically above its low intensity baseline. VT1 is the corresponding respiratory change, where ventilation begins increasing disproportionately to oxygen consumption. Increasing metabolic acid is buffered by bicarbonate, producing additional carbon dioxide which must be exhaled. LT1 and VT1 are closely related but are not necessarily identical. Below this threshold the athlete is working in what exercise physiologists call the moderate intensity domain. Lactate is still being produced, but production and utilisation remain sufficiently well matched for blood lactate to remain relatively stable and low.
This is approximately what the paper means by Zone 2.
The original case for low intensity training was never that professional athletes should avoid harder exercise. An endurance athlete training for 10, 15 or 20 hours per week simply cannot perform most of that work near threshold or at VO₂max intensity. The resulting fatigue would make the programme unsustainable. Low intensity training allows very large volumes to be accumulated while still recovering sufficiently to train again the next day.
That volume produces important adaptations. Endurance trained muscle develops greater oxidative capacity, increased capillarisation, improved fat oxidation and a greater ability to use lactate as fuel. The lactate that is produced by higher intensity efforts. As a result, large volumes of relatively easy training can improve performance at much higher workloads.
Professional athletes therefore combine large amounts of low intensity work with smaller quantities of harder training, producing either pyramidal or polarized training distributions.
This is one of the most important points in the recent review. A professional cyclist riding below LT1 may still be producing a very high percentage of maximum work rate. An untrained cyclist below LT1 may be working at a fraction of that power. The paper notes that the upper boundary of the moderate intensity domain can occur across a very wide range, approximately 24% to 80% of VO₂max depending on fitness and training status. It gives examples ranging from cycling around or below 100 W in sedentary people to around 300 W in endurance athletes.
This is important when discussing adaptation. Two people can both be below LT1 while imposing very different demands on skeletal muscle. For the professional cyclist, low intensity training may still involve a substantial absolute workload, large oxygen turnover and thousands of powerful muscle contractions over several hours and a lot of calories consumed.
For an untrained rider, an equivalent physiological zone may represent a much smaller training stimulus. This makes it dangerous to copy elite training percentages without considering absolute workload and training status.
There is another reason athletes are often told to keep easy days genuinely easy. It is very easy to leave home intending to perform a low intensity ride and gradually drift above LT1. The intensity does not necessarily feel particularly difficult. It can feel pleasantly brisk, much faster than an easy ride but nowhere near the discomfort of a hard interval session. The problem is that repeatedly riding in this region can produce more fatigue and glycogen depletion than staying below LT1, without providing the same stimulus as genuinely hard training.
Easy sessions become too hard and hard sessions become too easy. This is where the idea of junk miles comes from. The middle intensity itself is not junk. Training between LT1 and LT2 can be extremely useful. The problem is accumulating large amounts of it unintentionally and compromising the rest of the programme.
The authors state that they failed to find substantive evidence that Zone 2 is superior to higher exercise intensities for improving mitochondrial capacity or fat oxidative capacity. They also acknowledge an important limitation. Very few studies have explicitly examined Zone 2 as it is currently described in popular media. The argument is therefore not that low intensity exercise produces no adaptation what the paper does is challenge the claim that there is something uniquely beneficial about exercising just below LT1.
The review finds that low intensity exercise often produces relatively small or inconsistent changes in cellular energy status. Higher intensities tend to create larger disturbances in AMP, ADP, phosphocreatine, lactate and pH, with stronger signalling associated with mitochondrial bio-genesis. This isn't really surprising. A harder training stimulus generally produces a larger acute physiological disturbance. The question is how much of that harder stimulus an athlete can repeatedly recover from.
The paper points out that Zone 2 has sometimes been presented as something close to a metabolic holy grail, with suggestions that exercising above it somehow prevents the special mitochondrial benefits from occurring. That is a much stronger claim than the one usually made in endurance coaching. Stephen Seiler, whose work helped popularise the study of polarized training, has never argued that high intensity exercise should be avoided. Polarized training explicitly contains high intensity training.
The original observation was much simpler. Successful endurance athletes tended to perform very large amounts of training below the first threshold, relatively little in the middle, and a small but important quantity at high intensity. That observation came largely from examining how successful athletes actually trained.
Another problem is that Zone 2 means different things in different training systems. Some athletes simply accept the five zone calculation produced by their watch or cycle computer. A generic system may place Zone 2 around 60 to 70% of maximum heart rate. That may have little relationship to the athlete's actual LT1. The recent review makes this point indirectly by showing just how widely the first threshold varies between individuals.
The problem becomes even greater if maximum heart rate itself has been estimated from age rather than measured. An estimated HRmax is being used to calculate another estimated threshold. Where possible, a measured LT1 or VT1 is much more useful.
One of the most striking passages in the review refers to a previous meta analysis suggesting that exercise below 60% of maximum work rate would not be expected to improve mitochondrial content or mitochondrial respiratory capacity. The authors note that this intensity would probably be equivalent to, or above, Zone 2 in many people who are not endurance trained. At first sight, 60% sounds surprisingly low. However, 60% of maximum work rate is not the same as 60% of maximum heart rate or 60% of VO₂max. The maximum work rate is derived from a ramp test and is the maximum power output usually over a period of a couple of minutes. It can be very different from maximum heart rate.
The paper primarily challenges the claim that Zone 2 is uniquely optimal for mitochondrial adaptation and that for untrained subjects Zone 2 might even be too low an effort to produce those adaptations. However for previously sedentary people predominantly low intensity exercise remains a good idea. For an untrained individual, even a modest workload can provide a meaningful stimulus. Low intensity exercise is also easier to tolerate, easier to recover from and generally easier to repeat consistently. There are also obvious health and safety reasons not to take a completely sedentary person and immediately prescribe repeated maximal interval sessions. Once a basic level of fitness and exercise tolerance has been established, higher intensities can progressively be introduced. A sedentary beginner exercising for 150 minutes per week and an experienced cyclist limited to the same 150 minutes should not use the same programme.
The paper challenges some of the exaggerated claims that have grown around zone 2 training. There is little evidence that exercising immediately below LT1 produces a unique mitochondrial adaptation that disappears if the athlete works harder. Higher exercise intensities often produce stronger mitochondrial signalling and greater improvements in cardio-respiratory fitness, particularly when training time is limited.
The important question is how much hard training can be productively absorbed, and how much low intensity volume can be built around it. At two or three hours per week, a relatively large proportion of work may sensibly occur above LT1, even in the dreaded "junk miles" twilight zone.
At twenty hours per week, more than 80% may naturally fall below it. Polarized, pyramidal and high intensity approaches may therefore be less contradictory than they first appear. They may simply represent different solutions to the same physiological problem of different training volumes and different levels of fitness.