Key takeaways
In healthy people, VO2max is mostly limited by oxygen delivery: cardiac output and blood haemoglobin
Three weeks of bed rest cut VO2max by about a quarter, more than 30 years of ageing reduced it in the same men
VO2max is the ceiling; threshold and running economy decide who wins among runners with similar values
To be used, oxygen travels a chain: lungs → blood → heart → arteries → muscle capillaries → muscle cells → mitochondria. VO2max is set by the weakest links in that chain during whole-body exercise.
The usual limiter: oxygen delivery
In a classic 2000 review, Bassett and Howley concluded that in healthy people doing whole-body exercise such as running, VO2max is mostly limited by how much oxygen the cardiovascular system can deliver — that is, by cardiac output (heart rate × stroke volume) and the oxygen-carrying capacity of the blood (haemoglobin).
Evidence for this:
- Raising haemoglobin (by altitude adaptation, or illegally by blood doping/EPO) raises VO2max. Losing blood lowers it.
- Breathing extra oxygen raises VO2max; breathing thin air at altitude lowers it.
- Maximum heart rate barely changes with training, but stroke volume — how much blood each beat pushes out — rises a lot. Elite endurance athletes have large, compliant hearts.
Muscle mitochondria matter more for endurance (how long you can hold a high fraction of VO2max) than for VO2max itself. The lungs are rarely the limiter in healthy people at sea level, with the exception of some elite athletes whose blood oxygen drops at maximal effort.
The Dallas Bed Rest and Training Study
In 1966, five healthy young men spent three weeks in bed and were then trained for eight weeks. After bed rest their VO2max had fallen by about a quarter, mostly because stroke volume dropped; training brought it back above baseline.
Thirty years later, the same men were retested. On average, three weeks of bed rest had reduced their fitness more than three decades of ageing had. A six-month walking, jogging and cycling programme then restored much of their age-related loss. More on this in how fast you lose VO2max.
VO2max is the ceiling, not the race
Endurance performance depends on three things:
- VO2max — the size of the engine.
- Fractional utilisation — what percentage of it you can sustain (your lactate threshold).
- Economy — how much oxygen you need at a given speed.
Among runners with similar VO2max, threshold and economy decide who wins. That is why training includes threshold work and intervals, and why strength training improves economy.
Frequently asked questions
What determines your VO2max?
In healthy people doing whole-body exercise, mainly how much oxygen the cardiovascular system can deliver: cardiac output, meaning heart rate times stroke volume, and the oxygen-carrying capacity of the blood.
Is VO2max limited by the heart or the lungs?
Usually the heart and blood. The lungs are rarely the limiter in healthy people at sea level, except in some elite athletes whose blood oxygen drops at maximal effort.
Does a higher VO2max mean you are faster?
Not always. Endurance performance also depends on the fraction of VO2max you can sustain, your threshold, and your economy, and among runners with similar VO2max those decide who wins.
Sources
- Bassett DR, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc, 2000.
- Saltin B, et al. Response to exercise after bed rest and after training. Circulation, 1968.
- McGuire DK, et al. A 30-year follow-up of the Dallas Bedrest and Training Study. Circulation, 2001.