What is oxygen saturation?
Pulse oximetry is a method of measuring the oxygen saturation of the blood. But what is that? Oxygen saturation tells you how much oxygen is your blood carrying compared to its maximal capacity. Oxygen binds to haemoglobin in red blood cells, so oxygen saturation is the percentage of total haemoglobin that is carrying oxygen. For example, a normal oxygen saturation at sea level is 95% or higher. That means that 95% of your total haemoglobin is currently carrying oxygen.
Three factors affect oxygen saturation
Air pressure is the main factor that affects oxygen saturation. The air always contains about 21% oxygen, even at high elevations where air pressure is much lower. But lower air pressure means that there are fewer air molecules and therefore also fewer oxygen molecules available in each inhale. This results in less oxygen available in your lungs to bind to haemoglobin and therefore lower oxygen saturations.
The other two factors are temperature and acidity. Higher temperatures and higher acidities decrease haemoglobin’s ability to bind with oxygen. We benefit from this effect during exercise. That’s because our muscles get warmer and the environment becomes more acidic due to lactic acid. This warms and acidifies also the blood in the capillaries right next to the muscle fibres, which makes the haemoglobin release oxygen into the muscle. Conversely, as the blood gets cooler and away from the acidic environment on its way back to the lungs, the new oxygen binds with haemoglobin more easily again.
Why measure oxygen saturation as a cyclist?
Several brands like Garmin, Whoop, Apple, Polar, Sunnto or Samsung have introduced oxygen saturation sensors in their wearable devices allowing them to do pulse oximetry. These sensors work by shining infrared light into the tissue and measuring how much of that light is absorbed or reflected by the tissue, which allows them to calculate saturation. This method is good enough to measure saturation at rest, but movement is a problem, so they can’t measure oxygen saturation during exercise. This means that you can use your smart watch’s pulse oximetry for two things: tracking altitude adaptation and sleep quality.
Using pulse oximetry for monitoring altitude adaptations
Training at altitude is no longer reserved only for pros. Many cycling enthusiasts plan a cycling vacation or straight-up training camp at elevation to gain an advantage for racing. When talking about marginal gains, it can make a big difference knowing how quickly and how well you’re adapting to altitude to get the most out of an altitude camp.
For this reason, measuring oxygen saturation can be useful for endurance athletes during altitude training. It can also be useful when using an altitude tent. Tracking oxygen saturation can help you find the right setting of the tent to provide the desired stimulus. If you see you’re adapting well, you can ramp up the intensity, instead of using a pre-set progression the tent manufacturer suggests.
Tips for guiding endurance training with pulse oximetry
When you travel to altitude as a cyclist, the process of acclimatizing starts immediately and continues over days and weeks. In the short term, your respiration rate increases and an increase in plasma volume quickly follows. Over the long term, your body starts producing more haemoglobin and red blood cells – this is where the performance benefits come from.
For tracking oxygen saturation, it’s important to understand that over several days, before red blood cell production increases, resting levels of oxygen saturation increase, and then stabilise. Here is how to best use oxygen saturation to optimise your training in practice:
- First, you have to establish a baseline by collecting data at your normal altitude. Let your watch measure overnight, first thing in the morning or just before bed.
- Keep tracking your oxygen saturation the same way after arriving at your altitude camp. Your oxygen saturation will first drop but then start to increase over a few days until it reaches a plateau.
- Make sure to wait until your oxygen saturation gets to this plateau before increasing the training load.
- Now you know how many days in advance you have to travel to your race at elevation to give your body enough time to adapt.
Spotting sleep issues early with pulse oximetry
The second use case of pulse oximetry is less about improving performance and more about maintaining health. Wearable devices that are able to measure oxygen saturation overnight can give you an early warning that you may be experiencing sleep issues. A drop in saturation compared to your norm can signal you’re getting a respiratory infection or poor sleep as a result of accumulated fatigue. But it could also be a sign of sleep apnoea. Sleep apnoea is a condition in which breathing repeatedly stops while sleeping. This can result in loud snoring, poor recovery or even more serious problems like heart trouble or high blood pressure.
It’s something that can happen to anybody, although cyclists with a normal body weight are at a low risk. So, if you see unusual drops in oxygen saturation on your device compared to your baseline, it could be an early sign of sleep apnoea. Talk to your doctor to properly diagnose and address the issue.
