Fitness is a chain,
not a single number.

What are we trying to sustain?

Walk briskly uphill, cycle across a city or run for several minutes and the working muscles need a continuous supply of energy. For longer efforts, much of that energy is produced through aerobic metabolism, where oxygen plays a central role.

Cardiovascular fitness describes the capacity of several systems to support that work. The lungs, blood, heart, vessels and muscle cells are connected. A limitation anywhere along the chain can influence the whole performance.

VO₂ and VO₂max.

VO₂ is the rate at which the body consumes oxygen. It normally rises as physical work becomes harder. VO₂max is the highest rate measured during a progressively demanding test.

The value is commonly expressed as litres per minute or relative to body mass in millilitres per kilogram per minute. Relative values can make comparison easier, but body composition and the activity used for testing can affect the result.

TERMS USED THROUGH THIS ARTICLE

Cardiac output and stroke volume.

The heart moves blood around the body. Cardiac output is the volume pumped each minute:

Cardiac output = heart rate × stroke volume

Stroke volume is the amount of blood ejected with each beat. During exercise, heart rate rises and stroke volume can increase. Endurance training often improves the heart’s ability to fill and eject blood, allowing more blood — and therefore more oxygen — to reach active tissue.

This is a central adaptation. It helps explain why a trained person may perform the same submaximal work with a lower heart rate than before: each beat can contribute more.

How delivery and use meet.

The Fick principle connects circulation with tissue. In simplified form, whole-body oxygen consumption equals cardiac output multiplied by the difference between oxygen in arterial blood and oxygen returning in venous blood.

A rise in VO₂ can therefore come from greater blood flow, greater oxygen extraction or both. This is why an endurance response cannot be reduced to “a stronger heart”. The working muscle must also receive and use what the circulation provides.

What changes inside muscle?

Delivering oxygen is only useful if the muscle can receive and use it. Endurance training can increase capillarisation, the network of very small blood vessels around muscle fibres. A greater exchange surface can support the movement of oxygen and nutrients between blood and tissue.

Training can also increase mitochondrial content and function. Mitochondria are structures involved in aerobic energy production. More and better-functioning mitochondria can improve the muscle’s capacity to produce energy with oxygen.

CENTRAL + PERIPHERAL

Central adaptations.

Heart and circulation: stroke volume, cardiac output and the delivery of oxygenated blood.

Peripheral adaptations.

Working tissue: capillaries, mitochondria and the muscle’s ability to extract and use oxygen.

How VO₂max is measured.

A laboratory test usually increases speed, incline or power in stages or as a continuous ramp while expired air is analysed. Oxygen and carbon dioxide concentrations, breathing volume, heart rate and workload are tracked. The highest valid oxygen uptake reached is reported as VO₂peak or VO₂max, depending on the protocol and criteria.

Absolute VO₂ is reported in litres per minute. Relative VO₂ divides the value by body mass and is reported as millilitres per kilogram per minute. Relative values are useful when moving body mass matters, but they can change when body mass changes even if absolute oxygen use does not.

Intensity changes the question.

An easy walk, a steady run and a hard interval session all involve aerobic metabolism, but the relative demand is different. Training intensity influences which parts of the system are most strongly challenged.

Longer work at a manageable intensity can accumulate volume and develop the ability to sustain activity. Higher-intensity intervals can create a strong stimulus for VO₂max in less continuous time, though they also produce more fatigue and are not appropriate for every session or every person.

The choice should follow the purpose. A programme built entirely from very hard work may be difficult to recover from. A programme that never creates a sufficient challenge may stop producing adaptation.

ILLUSTRATION / INTENSITY CONTINUUM

Intensity changes breathing, sustainable duration and recovery cost.

Easy / conversation comfortable Moderate / phrases possible Hard / few words

These speech cues are practical approximations. Heart-rate zones and thresholds can add precision, but fixed percentages still vary between people and activities.

Why VO₂max is not the whole story.

Two people with a similar VO₂max may perform differently. Endurance also depends on how much of maximum capacity can be sustained, movement economy, fatigue resistance, technique, heat, fuelling and the specific task.

Movement economy describes how much energy or oxygen is required at a given pace. If the same pace costs less, it can feel easier even when VO₂max has not changed dramatically.

This matters outside sport. Improving fitness may mean climbing the same stairs with less strain or walking through a long day with more reserve. The benefit does not have to appear as a faster race time.

PERSPECTIVE

A larger reserve changes the relative cost.

If everyday work uses less of your maximum capacity, the task can become easier to sustain. The useful outcome is often the distance between what the day demands and what your body can provide.

Fitness and health.

Regular physical activity is associated with lower risk of several non-communicable diseases and with benefits for mental health, sleep and physical function. Cardiorespiratory fitness is also strongly associated with health outcomes in observational research.

Associations do not mean that one fitness score determines an individual’s future. Health is shaped by many factors. Fitness is one modifiable capacity within that larger picture.

How do you begin?

Begin with the activity you can repeat. Walking, cycling, running, rowing and many other forms can train the aerobic system when the duration and intensity create an appropriate demand.

1. Choose the task. What activity is available and acceptable to you?
2. Choose the purpose. General activity, longer endurance, a faster pace or improved VO₂max are related but different goals.
3. Choose a repeatable dose. Make room for recovery and for the rest of your life.
4. Review the response. Pace, perceived effort, heart rate and consistency can all provide information.

A fitness test can be useful when the result changes a decision. It is not required before every person begins moving more.

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Sources & references.

1. World Health Organization. Physical activity. 2024.
2. Rosenblat MA, Granata C, Thomas SG. Effect of interval training on factors influencing maximal oxygen consumption. Sports Medicine. 2022. doi:10.1007/s40279-021-01624-5
3. Mølmen KS, Almquist NW, Skattebo Ø. Exercise training, mitochondrial and capillary growth in human skeletal muscle. Sports Medicine. 2025. doi:10.1007/s40279-024-02120-2
4. Bassett DR, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise. 2000. doi:10.1097/00005768-200001000-00012
5. Ashcroft SP et al. Exercise induces tissue-specific adaptations to enhance cardiometabolic health. Cell Metabolism. 2024. doi:10.1016/j.cmet.2023.12.008

Educational information, not individual treatment advice. Zacharias Razvi is a final-semester physiotherapy student and is not yet an authorised physiotherapist.