Cardio and energy systems

Zones, VO2 max, HIIT vs. LISS and how to combine cardio with strength without losing performance.

The three energy systems

Your body produces ATP through three overlapping pathways depending on duration and intensity: the phosphagen system (ATP-PC, dominant for the first ~10 sec, no lactate), the glycolytic system (10 sec–2 min, generating lactate) and the oxidative system (>2 min, using oxygen with almost unlimited capacity). They are not sealed compartments: even in a sprint the oxidative system is contributing from the first second.

How to apply it

• Think in terms of 'which system predominates' rather than 'which system is used': they always overlap
• A 10 s sprint and a long run train different energy systems: do not expect the same adaptations from both

VO2 max: the marker that best predicts your longevity

VO2 max measures how much oxygen your body consumes at maximum effort and is one of the most robust predictors of mortality there is: each 1 MET increase in your cardiorespiratory fitness is associated with 13–17% lower all-cause mortality. It depends on a central component (how much blood your heart pumps) and a peripheral one (how much oxygen your muscle extracts), and both are trainable although they respond at different rates.

How to apply it

• You do not need to be an athlete: going from 'low' to 'moderate' VO2 max already gives the biggest health benefit
• With consistent aerobic training, improving 15–25% in 8–12 weeks is normal

Metabolic thresholds: why 'going at your own pace' is not so simple

The aerobic threshold (VT1, lactate ~2 mmol/L) marks the limit of effort you can sustain almost indefinitely; the anaerobic threshold (VT2, lactate >4 mmol/L) marks the point where lactate accumulates very fast and the effort only lasts 30–60 min. Threshold predicts endurance performance better than VO2 max alone: with a lower VO2 max but a higher threshold you can outperform someone with a higher VO2 max and a low threshold.

How to apply it

• If you want endurance performance, train to raise your threshold, not just your VO2 max
• The classic 'zone 2' sits right at or below your aerobic threshold

Training zones: heart rate, power and RPE

Zones divide intensity into bands (usually 5) based on % of max heart rate, % of FTP (cycling power) or perceived effort (RPE/Borg). Age-based max heart rate formulas have an error of up to ±11 bpm, so they serve as a starting point, not absolute truth; combining them with RPE is more reliable in practice.

How to apply it

• Use RPE as a corrective: if your HR says 'zone 2' but it feels like zone 4, trust the feeling
• Heart rate is distorted by heat, caffeine, stress and accumulated fatigue: do not treat it as a fixed number

LISS/MICT vs. HIIT vs. SIT: what each one does

LISS/MICT (60–80% of max HR, long sessions) builds your aerobic base with low stress and should be the bulk of your volume. HIIT (>85% of max HR in intervals) gives the biggest VO2 max increase per session: a meta-analysis of 723 people found improvements ~9% greater than moderate continuous training. SIT (supramaximal sprints of 10–30 s) achieves adaptations similar to continuous training in a fraction of the time, but it is very demanding and has worse long-term adherence.

How to apply it

• If you can only do one type, make it LISS/zone 2: the best benefit-risk-adherence ratio
• Add 1–2 HIIT sessions/week if you want to raise VO2 max faster, do not use it exclusively
• Do not start with SIT if you are coming from being sedentary: it requires a minimum aerobic base first

How much cardio you need

International guidelines recommend 150–300 min/week of moderate aerobic activity or 75–150 min of vigorous activity (or the equivalent combination), plus muscle strengthening on ≥2 days/week. Progressing without getting injured means not increasing volume by more than ~10% per week: the rule is boring but it is the one that prevents setbacks.

How to apply it

• If you are starting from scratch, aim first for the minimum (150 min/week) before chasing more
• Increase volume before intensity if you are a beginner: it is the safest order of progression

Periodising cardio: polarised vs. pyramidal

The polarised model (≈80% of volume in easy zones, little in the middle zone, 15–20% in hard zones) consistently outperforms the threshold model (everything concentrated in the middle zone) in 12–16 week studies, and it is the standard among elite endurance athletes. Putting too much volume in the intermediate 'grey zone' — that 'comfortably hard' pace — is the most common mistake among people who do cardio without a plan.

How to apply it

• Spread your cardio out: mostly easy, a small portion hard, little in the middle
• If you notice you always train 'half-hard', you are probably in the least productive zone

What happens if you stop: detraining

Aerobic adaptations are lost at different rates: plasma volume falls within days, VO2 max by 6–8% in 4 weeks, oxidative enzymes by up to 50% in 12 weeks. The good news is that keeping just 1–2 sessions/week at the same intensity (even if you cut volume drastically) preserves most of the adaptations for months.

How to apply it

• In impossible periods, do not drop it entirely: 1–2 short but intense sessions maintain almost everything
• Prioritise maintaining intensity over volume if you have to cut back

Cardio and strength together: the interference effect

Combining strength and cardio in the same plan (concurrent training) can somewhat blunt gains in maximal strength and especially power, but the effect on hypertrophy is small (<10%) and depends heavily on your cardio volume: with 2–3 short sessions a week, interference is minimal for most people. It is not an impassable wall, it is a question of dose.

How to apply it

• Do not avoid cardio for fear of 'losing your gains': at a moderate dose the effect is small
• If strength/hypertrophy is your priority, limit cardio to 2–3 short sessions (~20–30 min) a week

How to minimise interference if you do both

If you combine strength and cardio, order and separation matter: train strength before cardio if strength/hypertrophy is your priority, separate the two sessions by at least 6 hours when you can, and prefer low-muscle-damage modalities (bike, rowing, swimming) over running if they share muscle groups. Protein and carbohydrates around training help sustain both adaptations.

How to apply it

• Strength first if muscle is your priority; cardio first if endurance is your priority
• Separate the sessions by ≥6 h when you can, or train legs and cardio on different days
• Prefer cycling, rowing or swimming over running if you want to minimise interference with your lower body