Medically reviewed by Ioana Bina, MD, PhD
This article includes a preprint of new medical research that has not been peer-reviewed yet. However, these findings are so significant that they will help guide additional studies into sports science and the future development of objective biomarkers for athletic performance and recovery.
In addition to heart rate monitoring, elite athletes and biohackers of the future will be rigorously testing their cortisol. Cortisol, a catabolic steroid hormone, is emerging as the best biomarker for athletic performance and recovery. Research has shown limited correlations between performance and heart rate variability (HRV), testosterone, and resting heart rate (RHR). However, new evidence indicates that cortisol levels over time are very strongly correlated with performance.
Since the early 1980s, heart rate monitors have been the gold standard for measuring athletic performance. It started in 1977 when Seppo Säynäjäkangas founded his company, Polar, in Finland, to monitor cross-country skiers. Today, heart rate monitors have gone from being a tool strapped around the chest for elite athletes to the wristwatches of weekend warriors. Wearable devices can improve performance, but they are unreliable indicators of overtraining and injury. And monitoring mental health is just as important as monitoring physical health, as evidenced by the cortisol levels of Olympians we measured.
Sports scientist Jukka Huovinen has been working on HRV, cortisol, and testosterone measurement his whole career. His interests stretch to endocrine function, neuromechanics, and adaptogens. His latest study indicates that cortisol measurement may be an even better way to track an athlete's stress levels and optimize their training. The findings could have staggering implications for commonly held knowledge about performance and the overriding power of cortisol which is a mirror into our brain. (see HPA axis)
Image 1 – Source: Huovinen et al. 2023
Harnessing individual hormonal responses and heart rate variability for developing military performance and athletic training – case example from larger study
Cortisol has a significant inverse correlation with performance <Image 1>. Increased cortisol is the most significant individual predictor of performance in the following week. When cortisol returns to its optimal range, performance goes up.
Elevated cortisol inhibits testosterone and when cortisol returns to normal levels, testosterone production is catching up. Testosterone doesn’t tell the story as well because it’s downstream from the Master Hormone Cortisol and doesn’t change that much as cortisol over days and weeks.
Image 2 – Performance Correlation with Cortisol and Testosterone in elite athletes
Source: Huovinen et al. 2023 – Harnessing individual hormonal responses and heart rate variability for developing military performance and athletic training – case example from larger study.
These findings confirm a well-known Nature study of track and field athletes at the 2015 European Games in Baku, Azerbaijan. An increase in cortisol before a competition could be detrimental to performance in track and field athletes. Athletes with a less pronounced endocrine response in the 24 hours before an important competition fared better than those exhibiting greater endocrine changes.
Olympic Legend Sifan Hassan
Sifan Hassan’s legendary performance at the Tokyo Olympics tells another cortisol story. Sifan stated: “I felt like I had 20 cups of coffee.” According to Professor Maria Hopman, an expert in physiology, this can be attributed to a combination of adrenaline and cortisol. The adrenaline wears off after a few minutes, but cortisol can stay elevated for hours, providing more focus and energy when at optimal levels. It is plausible that optimal cortisol levels contributed to her gold in the evening."
All of that and more is why Sifan started testing her cortisol levels with Pardigm.com, a startup in beta in Silicon Valley that will soon allow saliva testing at the kitchen table. Last year Sifan said, “Cortisol testing will help me win more gold in Paris 2024.” And this year, she epically won her maiden marathon, obliterating the all-star field.
Another proof point is Shadrack winning the Nationals after cortisol testing which gives athletes a unique look into their body’s stress levels. And they can tell when it’s fine to push harder. And when it’s time to rest up. That’s the true path to victory.
As we showed in our book, research to date does not show a significant correlation between heart-rate variability (HRV), for example, and an athlete’s physical recovery post-exertion. While more data need to be collected, cortisol remains the most relevant biomarker for assessing whether it’s safe for an athlete to train and compete.
We put the question to Dr. Maria Hopman, professor of integrative physiology at Radboud University Medical Center in Nijmegen, The Netherlands. Based on the research available today about recovery and readiness to train, do you think cortisol is a better biomarker for athletes than HRV?
True to her scientific integrity, Dr. Hopman is guarded but optimistic. “We don’t yet have enough studies involving cortisol and hundreds of athletes,” she says. “Before I come down in favor of one biomarker over another, I want to see more data. I need to see many measurement points before I reach any hard and fast conclusion. But I will give you a qualified yes. Cortisol is a better biomarker than HRV because it can change over a relatively short period of time. HRV doesn’t. Testosterone doesn’t. Vo2 max doesn’t. If something doesn’t change, how can you use it as a metric for anything?” Some biomarkers change intraday but the most important trend to consider is day-to-day variation.
The study found that athletes with higher cortisol levels were more likely to experience fatigue, injury, and burnout. Apart from physical training, researchers also found that athletes who were able to reduce their cortisol levels through stress management techniques, such as mindset training, were able to improve their performance.
"Cortisol is a stress hormone that can harm athletic performance," said lead scientist Jukka Huovinen. "By measuring cortisol levels, we can identify athletes at risk of overtraining and help them to make changes to their training or lifestyle that can improve their performance."
The study's findings have important implications for the training of Olympians and other elite athletes. In the past, coaches have relied on heart rate monitors to track an athlete's fitness level. However, heart rate monitors only measure physical exertion, not cortisol levels. This means that an athlete could be training relatively little but still be at risk of overtraining if their cortisol levels are high.
Here are some tips for athletes who want to use cortisol measurement to improve their performance:
Talk to your doctor or a sports psychologist about how to measure your cortisol levels
Track your cortisol levels over time to see how they respond to different training loads and stressors
Make changes to your training or lifestyle to reduce your cortisol levels if they are too high
"Cortisol measurement is a powerful tool that can help athletes to achieve their full potential," said Huovinen. "Once tests are more widely available I encourage all athletes of all levels to consider using this tool to improve their performance."