Cardiovascular Training

Most people who "do cardio" are training in a zone that is too intense to produce optimal mitochondrial adaptations and not intense enough to drive VO2 max improvements — a metabolic grey zone that burns calories in the moment but builds very little long-term fitness.

The Two Adaptations That Matter

The first adaptation is mitochondrial density: the number, size, and efficiency of mitochondria within muscle cells. Greater mitochondrial density means your muscles can produce more energy aerobically, sustain effort longer before accumulating lactate, and recover faster between intense efforts. This adaptation is built primarily by low-intensity sustained training.

The second adaptation is cardiac output and VO2 max: the ceiling of your aerobic capacity, primarily determined by how much blood the heart can pump per beat (stroke volume) and how efficiently working muscles can extract oxygen from that blood. VO2 max is built primarily by high-intensity training that pushes cardiac output to near-maximal levels for sustained periods.

These two adaptations are complementary and partially independent. High mitochondrial density without an adequate VO2 max ceiling limits how much energy you can produce aerobically per unit time. A high VO2 max without mitochondrial density means you can reach a high intensity briefly but cannot sustain effort efficiently. The classic analogy is a car: mitochondrial density is the engine's fuel efficiency; VO2 max is the engine's maximum power output. You want both.

Zone 2 Training

Zone 2 is a precise physiological concept that is often imprecisely defined. Formally, it is the highest intensity at which the body can clear lactate as fast as it produces it — the maximal lactate steady state. Practically, Zone 2 corresponds to an intensity at which you can hold a conversation but would prefer not to. Heart rate will typically be 60–70 per cent of maximum, though this is highly individual — and the 60–70% formula is population-derived and can be significantly wrong for any given person.

Finding your actual Zone 2 heart rate. The most accessible method is the talk test: you should be able to speak in full sentences, but would not want to sustain a conversation. If you are gasping between words, you are above Zone 2; if speaking feels completely effortless, you are likely below it. A rough mathematical starting point is the Maffetone formula: 180 minus your age, adjusted ±5 bpm for training history (subtract 5 if you are new to training; add 5 if you have trained consistently for two or more years without injury). This figure is indicative, not prescriptive. Lab-based lactate testing is the gold standard for identifying your precise Zone 2 threshold, but it is not required — the talk test is sufficient to train productively in this zone.

Any sustained low-intensity effort counts: cycling, rowing, incline walking, swimming.

Zone 2 training is the centrepiece of a framework developed by Iñigo San Millán, a physiologist at the University of Colorado who has worked with elite endurance athletes and proposes that this specific intensity is uniquely effective at driving mitochondrial biogenesis and fat oxidation. This is a practitioner framework popularised through public talks and interviews rather than a single peer-reviewed paper, and it is a mechanistic argument rather than a settled experimental finding — narrative reviews of the broader literature have found a smaller and more contested evidence base for Zone 2 specifically than the framework's popularity would suggest, with some studies indicating that higher-intensity training produces equal or greater mitochondrial adaptation at matched training volumes. Greater fat oxidation at a given intensity, where it does occur, is associated with better metabolic health, improved endurance performance, and improvements in insulin sensitivity. The WHO's 150-minute weekly recommendation for moderate-intensity aerobic activity is the most defensible figure for the general population and maps broadly onto Zone 2 work. Targets above this threshold reflect longevity-optimisation goals supported^[2] more by mechanistic reasoning and athlete data than by controlled trials in non-athlete adults.

VO2 Max Training

The mortality data here is worth restating briefly — the Cleveland Clinic analysis covered in Section 1 found individuals in the top fitness category had approximately five-fold lower all-cause mortality risk than those at the bottom. VO2 max improvement is not an athletic goal — it is a medical one.

VO2 max is improved by training that pushes cardiac output to 90–100 per cent of maximum for sustained durations of 3–8 minutes. The most studied protocol is the Norwegian 4x4 interval: four rounds of four minutes at 90–95 per cent of maximum heart rate, separated by three minutes of active recovery at low intensity.^[12] The protocol was developed by Jan Helgerud and colleagues at the Norwegian University of Science and Technology.

One to two VO2 max sessions per week is sufficient to drive meaningful improvements; more produces diminishing returns relative to accumulated fatigue. Each session requires a 10–15 minute warm-up and 10-minute cool-down. Total session time: approximately 45–50 minutes. The work intervals should feel genuinely maximal.

What equipment or modality? VO2 max training can be done on any cardio equipment that allows you to reach 90–95% of maximum heart rate and sustain it for 4 minutes. Each option has practical tradeoffs:

If in doubt, start with the bike. It has the lowest barrier to entry, the lowest injury risk, and produces equivalent VO2 max adaptations to running when the work intensity is correct.

The 80/20 Rule

Elite endurance athletes across cycling, running, cross-country skiing, and swimming consistently show a training distribution of approximately 80 per cent low intensity and 20 per cent high intensity.^[13] This pattern — sometimes called polarised training — consistently outperforms training structures where most time is spent at moderate intensity (Zone 3 — the "grey zone" or "black hole" of endurance training).

The grey zone is where most recreational athletes spend most of their time. It is hard enough to feel productive, not hard enough to drive meaningful adaptation, and fatiguing enough to impair recovery for subsequent sessions. If you do five hours of cardiovascular training per week, four of those hours should be Zone 2 and one hour should be high-intensity intervals.

Combining Cardio and Resistance Training

The interference effect — the hypothesis that concurrent aerobic and resistance training impairs strength and hypertrophy adaptations — was identified in the 1980s. The more recent evidence is more nuanced: interference effects appear primarily in contexts of very high aerobic volume or when sessions are poorly structured.^[14]

For most trainees doing the volumes recommended in this guide, interference effects are minimal if training is organised sensibly:

Note for female trainees: Zone 2 heart rate estimates derived from age-based formulas are less reliable for women — see Section 6 for the full explanation and a more accurate approach.