The Method N° 10 · Your goal

Conditioning

Cardio barely touches your muscle. Build the engine mostly easy, set against your own line.

Every study checked against the paper, . Each section carries its grade. Run the numbers in the app.

Fitness goes with a longer life

Settled

Cardiorespiratory fitness is how much oxygen your body can put to work, counted in METs, multiples of what you use sitting still. Across 199 cohorts, the fittest had about half the risk of dying of anything that the least fit had, and each one-MET step up went with 11 to 17% less.1 In 122,007 people, below-average fitness carried risk like smoking or diabetes, and the fittest group had the lowest risk of all.2 These are associations, not experiments, but they are large and they point one way.

Mostly easy, a little hard

Strong

Endurance athletes who do well keep about 80% of their sessions easy and the rest hard, and pushing well-trained athletes to train harder has not shown a convincing long-term gain.3 When trials pit that polarized mix against a pyramidal one, with more moderate work in the middle, the gains come out about even.4 The rule they share is the useful one: most of it easy.

Your line: critical power

Strong

Every engine has a critical power, or on foot a critical speed: the hardest effort your body can hold in a steady state. Above it you are spending a fixed reserve, and the time you last is set by how much is left.5 Just below it, the working muscle settled within three minutes and held for twenty; just above it, people were exhausted in about fifteen.6 It is a line, not a magic number: work right at it lasts about 20 to 30 minutes, and the estimate depends on the test.7

Set by your line, not a percentage

Still open

A session set against your own line lands where it is meant to. In one study, heavy sessions set by a share of VO2max put 70% of people above their critical power, and only 30% finished; set by thresholds, everyone did.8 Ten people, so read it as a strong hint.

Intervals: a small edge, if long enough

Strong

Intervals raise VO2max a little more than steady work, about 1.2 mL/kg/min across 28 studies,9 and the edge shows mainly when the hard pieces last two minutes or more.10 Inside the muscle, steady work and intervals build the aerobic machinery about equally, and training more often builds more.11

It costs your muscle very little

Strong

Across 43 studies, adding conditioning to lifting cost muscle growth and maximal strength next to nothing, and explosive power a little, more when both shared a session.12 A 2024 analysis found lower-body strength gains blunted in men, not women,13 and running costs more than cycling, as do more and longer sessions.14 A 2026 review of 17 analyses found combined training matched lifting alone for strength and size.15 So count it, and keep it.

How the Vanguard app runs this

Conditioning in the app is set as a share of your own critical power or speed, per machine, and an easy day has a ceiling it never crosses. Time in each zone is counted from what you log, and the week's conditioning counts against each muscle's recovery ceiling, so the sets it plans stay under it.

The research

The research

The 15 sources this chapter stands on, with what each one actually found.

  1. Lang, Prince, Merucci et al., 2024 · Br J Sports Med

    Overview of meta-analyses · 26 reviews · 199 cohorts

    High against low fitness: about half the risk of death from any cause (hazard ratio 0.47); each one-MET step up, 11 to 17% lower. Observational, and the authors rate the certainty very low to moderate.

    Read the paper by Lang, Prince, Merucci et al., 2024 (opens in a new tab)

  2. Mandsager, Harb, Cremer et al., 2018 · JAMA Netw Open

    Retrospective cohort · n = 122,007 · 8.4 years

    Below-average against above-average fitness carried risk like smoking (1.41) or diabetes (1.40); the elite group had the lowest risk. Patients referred for treadmill tests at one centre.

    Read the paper by Mandsager, Harb, Cremer et al., 2018 (opens in a new tab)

  3. Seiler, 2010 · Int J Sports Physiol Perform

    Review of descriptive studies

    About 80% of elite endurance athletes' sessions were easy and about 20% hard. Making well-trained athletes train harder showed no convincing long-term gain. The 80/20 counts sessions, not minutes.

    Read the paper by Seiler, 2010 (opens in a new tab)

  4. Rosenblat, Watt, Arnold et al., 2025 · Sports Med

    Network meta-analysis of individual data · 13 studies · 348 people

    Polarized against pyramidal: no difference in VO2max or time trial. Competitive athletes may gain a little more from polarized, recreational athletes from pyramidal.

    Read the paper by Rosenblat, Watt, Arnold et al., 2025 (opens in a new tab)

  5. Poole, Burnley, Vanhatalo et al., 2016 · Med Sci Sports Exerc

    Review

    Across species and exercise types, power and duration follow the same curve. Critical power is its floor, and above it the time you last is set by a fixed reserve of work.

    Read the paper by Poole, Burnley, Vanhatalo et al., 2016 (opens in a new tab)

  6. Jones, Wilkerson, DiMenna et al., 2008 · Am J Physiol Regul Integr Comp Physiol

    Lab study · n = 6

    10% below critical power, the muscle's fuel and acidity settled within 3 minutes and held for 20. 10% above it, exhaustion came at about 15 minutes.

    Read the paper by Jones, Wilkerson, DiMenna et al., 2008 (opens in a new tab)

  7. Jones, Burnley, Black et al., 2019 · Physiol Rep

    Review

    Work right at critical power lasts about 20 to 30 minutes, the maximal lactate steady state usually sits a little below it, and the estimate depends on the test.

    Read the paper by Jones, Burnley, Black et al., 2019 (opens in a new tab)

  8. Meyler, Bottoms, Wellsted et al., 2023 · Exp Physiol

    Lab study · n = 10

    Heavy sessions set by a share of VO2max put 70% of people above critical power and 30% finished; set by thresholds, all finished. For intervals, 20% against 100%.

    Read the paper by Meyler, Bottoms, Wellsted et al., 2023 (opens in a new tab)

  9. Milanović, Sporiš, Weston, 2015 · Sports Med

    Meta-analysis · 28 studies · 723 people

    Against no exercise, steady training added 4.9 and intervals 5.5 mL/kg/min of VO2max; intervals against steady, about 1.2, a small edge.

    Read the paper by Milanović, Sporiš, Weston, 2015 (opens in a new tab)

  10. Wen, Utesch, Wu et al., 2019 · J Sci Med Sport

    Meta-analysis · 53 trials

    Against steady training, intervals helped only with work pieces of 2 minutes or more, 15 minutes or more of hard work a session, and programs of 4 to 12 weeks or longer.

    Read the paper by Wen, Utesch, Wu et al., 2019 (opens in a new tab)

  11. Mølmen, Almquist, Skattebo, 2025 · Sports Med

    Systematic review and meta-regression · 5,973 people

    Mitochondria rose 23% with steady training and 27% with intervals, no real difference; capillaries rose more with steady work. Six sessions a week beat four, and four beat two.

    Read the paper by Mølmen, Almquist, Skattebo, 2025 (opens in a new tab)

  12. Schumann, Feuerbacher, Sünkeler et al., 2022 · Sports Med

    Meta-analysis · 43 studies

    Adding aerobic training changed muscle growth (−0.01) and maximal strength (−0.06) by effectively nothing. Explosive strength dipped (−0.28), more when both shared a session.

    Read the paper by Schumann, Feuerbacher, Sünkeler et al., 2022 (opens in a new tab)

  13. Huiberts, Wüst, van der Zwaard, 2024 · Sports Med

    Meta-analysis · 59 studies · 1,346 people

    Lower-body strength gains were blunted in men (−0.43), not in women; VO2max gains were blunted only in untrained people.

    Read the paper by Huiberts, Wüst, van der Zwaard, 2024 (opens in a new tab)

  14. Wilson, Marin, Rhea et al., 2012 · J Strength Cond Res

    Meta-analysis · 21 studies

    Running reduced muscle and strength gains; cycling did not. More frequent and longer endurance sessions went with smaller gains.

    Read the paper by Wilson, Marin, Rhea et al., 2012 (opens in a new tab)

  15. Held, Wolf, Rappelt et al., 2026 · Sports Med

    Umbrella review · 17 meta-analyses · 144 studies

    Combined training matched lifting alone for strength, power and muscle size, and endurance alone for aerobic capacity.

    Read the paper by Held, Wolf, Rappelt et al., 2026 (opens in a new tab)

Research describes groups of people. Your coach will tell you how it applies to you.

Questions this chapter answers