What's Cool In Road Cycling

Fitness Improvement: How Much Does Blood Volume Matter?

Best of TOOLBOX: Yes, training does make you fitter, but how does it actually improve your fitness? How much comes down to increasing haemoglobin and blood volume to improve oxygen delivery? How might we test this?

blood boom

My last Toolbox article talked about detraining, so it’s time to balance it out with a story about what training actually does to your body.

Cardiovascular Training

As aerobic fitness and measures such as max aerobic capacity (VO2max) is a key marker of improved fitness, one of the dominant theories for how training improves fitness is via an improved capacity by the body to deliver oxygen to the muscles. This improvement can be broken down into a further range of potential mechanisms:

  • A greater amount of red blood cells and specifically haemoglobin, the molecule that actually binds to oxygen and carries it within blood.
  • Cardiovascular changes that improves the pathway for delivering blood throughout the body.
  • Improved capacity of the muscles to extract oxygen from the delivered blood. Remember, even under extreme exercise, the “blue” blood within veins still has a significant amount of oxygen, showing that muscles do not fully extract all available oxygen.

Of course, decades of blood doping and EPO use have shown us very clearly the “benefits” to be gained from extra red blood cells and hemoglobin. One of the first systematic studies done to examine the impact of blood doping was done in Canada (Buick et al. 1980). In it, 11 trained runners had 900 mL of autologous (i.e., their own blood) reinfusion. Haemoglobin, VO2max, and running time to exhaustion increased both 24 h and a week following the reinfusion.

Skattebo et al. 2020

So more red blood cells and haemoglobin certain helps, even in highly trained athletes. But is this the main cause of performance improvement when we start training? That was the purpose of a new study from Norwegian researchers published recently in the European Journal of Applied Physiology (Skattebo et al. 2020).

Here’s the experimental design:

  • Twelve untrained (VO2peak 44 mL/kg/min) performed 10 weeks of 3x/week endurance training.
  • Before and after the ten weeks, echocardiography of the heart, muscle biopsies, blood volume, and haemoglobin mass were measured.
  • Before and after the ten weeks, VO2peak and max cardiac output (how much blood the heart can pump in a minute) were measured during BOTH upright and supine cycling. This was a really interesting and nice design, as the supine cycling removed the challenges to the body from gravity, where the body has to force blood back to the heart from the feet against gravity.
  • And the most interesting part of the study was that, after ten weeks, the upright and supine max tests were repeated after the training-induced increase in hemoglobin was removed by removing the equivalent amount of red blood cells. This meant that we had post-training testing where the body was trained, but had the same lower amount of blood as it had before training.

The Heart of the Matter

I found the overall research question interesting, and the design was quite innovative and thorough. What were the major findings?

  • In keeping with other studies, endurance training increased blood volume (~3.7%) and also haemoglobin mass (~3.3%).
  • Also in keeping with expectations, VO2peak improved post-training during both upright (11%) and supine cycling (10%).
  • Here’s the interesting thing. After phlebotomy to remove ~3% of blood volume, upright and supine VO2peak remained the same (11% and 11% increase compared to pre-training values).
  • Maximal cardiac output also remained elevated at the same level, and was found to account for about 70% of the overall VO2peak improvement. This was likely contributed to by a larger and stronger heart, as left ventricular mass increased.
  • The remaining 30% of the overall VO2peak improvement was mostly attributed to the muscle’s improved ability to extract oxygen from the delivered blood. This included an increase in mitochondrial activity markers in the muscle samples.


Donkeys into Race Horses

An old adage or excuse by confessed EPO/blood dopers was that “You can’t turn donkeys into race horses,” implying that all the drugs in the world means nothing without still doing the hard training.

A study like the one from Skattebo et al. gives some truth to that adage. The main finding is not that blood volume or haemoglobin doesn’t matter. Buick et al. clearly showed that it does, especially at the elite level once you have put all the training in.

Instead, what it reminds us is that training induces holistic changes throughout our body, each of which interact to make us fitter and stronger. The changes in the heart and greater amount of mitochondrial activity shown in this study is indeed just the very tip of the iceberg of what happens.

blood sieberg


Studies like this are funny in the sense that, in attempting to isolate one variable, they end up instead showing that so much else also matters. It reminds me of the story of the Dutch boy plugging a hole in the dike with a finger, only to see 5 more holes spring up.

This holistic view is also good for us to keep in mind with our training. There is no workout that trains only one thing. Even a pure sprint workout impacts multiple metabolic systems.

On an even more holistic scale, everything counts to making you a better athlete, from your training to your off-bike core work and strength program, to your recovery habits, sleep, nutrition, and mental training. So don’t obsess over one thing and ignore the whole athlete!

Ride fast and have fun!


Buick FJ, Gledhill N, Froese AB, et al (1980) Effect of induced erythrocythemia on aerobic work capacity. J Appl Physiol 48:636–642. https://doi.org/10.1152/jappl.1980.48.4.636

Skattebo Ø, Bjerring AW, Auensen M, et al (2020) Blood volume expansion does not explain the increase in peak oxygen uptake induced by 10 weeks of endurance training. Eur J Appl Physiol 120:985–999. https://doi.org/10.1007/s00421-020-04336-2


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