Key takeaways
Training raises VO2max mainly by increasing maximal cardiac output and the oxygen-carrying capacity of the blood
Plasma volume expands within days to weeks, red cell volume and stroke volume keep growing over months
Capillaries and mitochondria grow too, helping muscles extract oxygen and sustain harder efforts
VO2max is the product of two things: how much blood your heart pumps per minute (cardiac output) and how much oxygen your muscles pull out of each litre of that blood (the arteriovenous oxygen difference). Training can improve both. The question is which changes matter most, and how fast they happen.
The big picture: delivery first
In their 2000 review, Bassett and Howley concluded that the rise in VO2max with training comes primarily from an increase in maximal cardiac output, not from greater oxygen extraction. Lundby, Montero and Joyner reached a similar view in 2017: training-induced increases in VO2max are largely driven by an expansion of red blood cell volume and an associated rise in stroke volume, which can also improve on its own. Muscle changes still matter, just less for the VO2max number. For the full oxygen chain, see what limits VO2max.
Stroke volume: a bigger pump
Maximum heart rate barely changes with training; it mostly depends on age. So a larger cardiac output has to come from stroke volume, the amount of blood pushed out per beat. Endurance training increases it in two ways:
- More filling. A larger blood volume returns more blood to the heart, stretching it more before each beat.
- Heart remodelling. Over months, the left ventricle becomes larger and more compliant, holding more blood.
The reverse is just as striking. In the 1968 Dallas Bed Rest study led by Bengt Saltin, three weeks in bed caused a large fall in VO2max, driven mainly by a drop in stroke volume, and subsequent training brought it back up. See how fast you lose VO2max.
Plasma and red cell volume: more blood
Convertino's review of blood volume and endurance training described a two-stage pattern. Plasma volume expansion accounts for nearly all of the early rise in blood volume during the first 2 to 4 weeks. After that, the extra volume is split roughly equally between plasma and red cells. A larger blood volume gives more filling pressure for a bigger stroke volume and a lower heart rate during exercise.
The best evidence that blood drives VO2max gains comes from a clever 2015 study by Montero and colleagues. Sixteen untrained men trained for 6 weeks. VO2peak rose by 9% and peak cardiac output by 7%, while red cell volume rose 8%, plasma volume 4% and haemoglobin mass 6%. Muscle also changed: capillaries per fibre rose 18% and mitochondrial volume 43%. Then the researchers removed blood to bring red cell volume back to its pre-training level, and VO2peak fell by 7%. Cardiac output, red cell volume and haemoglobin mass were the independent predictors of VO2peak; the muscle changes were not.
This also explains why altitude training and heat training interest endurance athletes: both act on blood volume.
Capillaries and mitochondria: better extraction
In a classic study, Andersen and Henriksson trained five people on a bike for 8 weeks, four times a week at 80% of VO2max. VO2max rose 16%, capillary density rose 20%, and the activity of key mitochondrial enzymes rose about 40%. More capillaries slow the passage of blood through the muscle and shorten the distance oxygen has to travel, which helps extraction. More mitochondria let the muscle use that oxygen and, crucially, sustain a higher fraction of VO2max. See mitochondria and VO2max.
Timeline of adaptations
Murias, Kowalchuk and Paterson trained young and older men three times a week for 12 weeks. VO2max rose within 3 weeks, reaching about 18% higher in the young men and 31% higher in the older men by the end. In the first 3 weeks, roughly two thirds of the gain came from wider oxygen extraction; after that, further gains were explained entirely by greater maximal cardiac output.
| Timeframe | Main adaptations | What you notice |
|---|---|---|
| Days to 2 weeks | Plasma volume starts expanding | Heart rate slightly lower at the same pace |
| 2 to 4 weeks | Plasma volume up, early extraction gains, first capillary and mitochondrial changes | First measurable VO2max gain in beginners |
| 1 to 3 months | Red cell volume and stroke volume rising, more capillaries and mitochondria | Steady VO2max gains, easier threshold efforts |
| 3 months to years | Heart remodelling, continued blood volume and muscle adaptation | Slower gains; bigger steps need more volume or intensity |
Exact timing varies with your starting fitness, age and training dose.
Do older adults still adapt?
Yes. In the Murias study the older men, aged around 68, gained proportionally more VO2max than the young men over the same 12 weeks. The mix was slightly different: in the older group about 69% of the gain came from a higher maximal cardiac output and the rest from wider extraction, while in the young group the split was closer to 56 and 44. The heart and blood remain trainable well into later life. See VO2max and ageing.
How to train for these adaptations
- Accumulate easy volume. Several sessions a week of conversational effort expand blood volume and grow capillaries and mitochondria. See zone 2 training and use the heart rate zone calculator to set your easy range.
- Add intervals near VO2max once or twice a week to load the heart at its maximum stroke volume.
- Be patient. The fast early gains slow down; the heart and red cell adaptations that follow take months.
- Stay consistent. Plasma volume is the first gain to come and also among the first to go.
Lundby and colleagues also argued that every previously untrained person will improve VO2max if the stimulus exceeds a certain volume or intensity. More on individual differences in VO2max trainability. Recheck your estimate every 6 to 8 weeks with the VO2max calculator.
Frequently asked questions
How does training increase VO2max?
Mainly by raising how much oxygen the heart and blood can deliver. Stroke volume, plasma volume and red blood cell volume all increase, and muscles add capillaries and mitochondria that help extract and use the extra oxygen.
How long does it take for VO2max to increase?
In untrained people, VO2max can rise measurably within about 3 weeks. Further gains over the following months come mainly from a larger maximal cardiac output as the heart and blood volume keep adapting.
Does endurance training increase stroke volume?
Yes. Stroke volume, the amount of blood pumped per beat, rises with endurance training, partly because a larger blood volume fills the heart more and partly because the heart itself adapts. That is why resting and submaximal heart rate fall.
Does exercise increase blood volume?
Yes. Plasma volume expands within the first few weeks of training, and red blood cell volume follows over the next weeks to months. Together they raise oxygen delivery and allow a larger stroke volume.
Sources
- Saltin B, Blomqvist G, Mitchell JH, et al. Response to exercise after bed rest and after training. Circulation, 1968.
- Convertino VA. Blood volume: its adaptation to endurance training. Med Sci Sports Exerc, 1991.
- Bassett DR, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med Sci Sports Exerc, 2000.
- Lundby C, Montero D, Joyner M. Biology of VO2max: looking under the physiology lamp. Acta Physiol (Oxf), 2017.
- Montero D, Cathomen A, Jacobs RA, et al. Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training. J Physiol, 2015.
- Andersen P, Henriksson J. Capillary supply of the quadriceps femoris muscle of man: adaptive response to exercise. J Physiol, 1977.
- Murias JM, Kowalchuk JM, Paterson DH. Time course and mechanisms of adaptations in cardiorespiratory fitness with endurance training in older and young men. J Appl Physiol, 2010.