By Frederic Sabater Pastor, PhD
CTS Expert Coach,
Associate Researcher and Teacher at University of Perpignan Via Domitia
Many athletes we talk to have access to more data and information than they know what to do with, including the oxygen saturation data coming from their Oura Ring, Apple Watch, Whoop strap, or Garmin watch. Most people understand oxygen saturation from a medical standpoint – the little clip they put on your finger in the doctor’s office. But with the ability to track O2 saturation throughout the day, overnight, and over a period of weeks and months, how can or should athletes utilize O2 saturation data to guide training and recovery?
What is Oxygen Saturation?
Oxygen saturation measures how much oxygen the blood is carrying, relative to its maximal oxygen carrying capacity. Oxygen binds to hemoglobin in red blood cells, so oxygen saturation is the percentage of total hemoglobin that is carrying oxygen. Therefore, if oxygen saturation is 98, it means that 98% of your hemoglobin is carrying oxygen at that time.
Oxygen is critical to produce energy in our mitochondria. Athletes take oxygen from the air, via the lungs, and transfer it to the blood, which transports it to the working muscles and other organs. Oxygen moves down the “oxygen cascade”, following a pressure gradient from the lungs to the muscles. This means oxygen always travels from a region of higher pressure to a region of lower pressure. The greater the pressure differential, the harder oxygen is “pushed” from one region to the other. Conceptually, then, for oxygen to travel from the air to your muscles, pressure must be higher in the lungs than in the blood, and higher in the blood than within the muscle cells.
Conversely, CO2 works the same way. Carbon dioxide is produced in muscle cells during metabolism. The partial pressure of CO2 is higher in muscle cells than in the blood passing by, so CO2 moves into the bloodstream. When that venous blood returns to the lungs, CO2 again moves down the pressure gradient from the blood into the alveoli, and you exhale it. And for another quirk of human physiology that many people find counterintuitive, breathing rate is largely governed by the amount of CO2 in the bloodstream rather than your demand for oxygen.
Oxygen Saturation and Air Pressure
Since a pressure gradient is needed to transport oxygen, air pressure is the main factor that affects oxygen saturation. The composition of air in the earth’s atmosphere always contains 20.93% oxygen. However, when air pressure changes, the total number of gas molecules – including oxygen molecules – in any given volume (such a lungful of air) changes. For example, air pressure decreases at higher elevations, so molecules of oxygen and other gasses move farther apart. As a result, although the composition of air at higher elevations is the same (i.e. 20.93% oxygen), there are fewer oxygen molecules in a lungful (given volume) of that air.
The partial pressure of oxygen in the lungs is important because it affects the pressure gradient with the blood. At higher elevations, where there are fewer molecules of oxygen in the air within your lungs, the pressure gradient is lower than it would be at sea level. This leads to lower oxygen saturations as we travel to higher elevations.
Other factors affecting oxygen saturation
Besides air pressure, two other factors affect oxygen saturation: temperature and acidity. Higher temperatures and higher acidities (i.e., lower pH) decrease hemoglobin’s ability to bind with oxygen. Nature has been very smart here, and we benefit from this effect during exercise. During exercise, our muscles become warmer and the environment becomes more acidic. This warms and acidifies the blood in the capillaries right next to the muscle fibers, which prompts the hemoglobin to release oxygen. This promotes the transfer of oxygen from the blood into the muscle fibers and the mitochondria.
As blood travels back from the muscles toward the lungs, it gets mixed with other blood coming from other parts of the body. This blood is cooler and less acidic. As a result, the affinity between hemoglobin and oxygen increases by the time blood is in the lungs, where we want as much oxygen as possible to bind with that hemoglobin.
Figure: Hemoglobin dissociation curve. The solid yellow line shows percentage saturation of hemoglobin against partial pressure of arterial oxygen. The purple and green lines in the inset graphs show how temperature and acidity affect the curve. From McArdle, Katch and Katch (2006).
Using Wearable Devices to Monitor Oxygen Saturation
Benefits of measuring oxygen saturation for endurance athletes
Several companies have introduced oxygen saturation sensors in their wearable devices. 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 can be done at the base of the finger, using a ring (e.g., Oura Ring), or at the wrist (e.g., Garmin Fenix6, Whoop, Suunto, Apple Watch). These methods are good enough to measure saturation at rest, but they don’t deal well with movement and shaking, so they cannot measure oxygen saturation during exercise. There are two main use cases for this metric: tracking adaptation to altitude and sleep quality.
Using Oxygen Saturation to Monitor Adaptation to Altitude
Atmospheric pressure decreases as altitude increases and, with it, oxygen pressure. For example, oxygen pressure in Colorado Springs, Colorado (~6,000 ft), is just 80% of sea level pressure. It’s only 59% of sea level pressure at the summit of Pikes Peak (above 14,000 ft). This drop in air pressure also comes with a drop in oxygen saturation. When athletes travel to altitude, they start acclimatizing, first through an increase in plasma volume and respiration rate, and long-term by increasing the production of hemoglobin and red blood cells.
Over several days, before red blood cell production increases, resting levels of oxygen saturation increase, and then stabilize, usually below sea-level saturations.
Changes in oxygen saturation during altitude acclimatization at 4300m and 3810m. From Dünwald et al. (2021).
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Measuring oxygen saturation can be useful for endurance athletes participating in altitude training camps or racing at higher elevations than their usual altitude. It could also be useful for athletes using altitude tents to acclimatize at home for higher altitudes or to simply obtain benefits from sleeping at altitude. Athletes using altitude tents may obtain better results by tracking oxygen saturation to make sure the altitude “setting” of the tent is providing the desired physiological stimulus. If appropriate, athletes could subsequently increase the simulated altitude according to their physiological response, instead of using an arbitrary altitude and a pre-set progression in the altitude they simulate.
Best Practices for Guiding Endurance Training with Oxygen Saturation Data
As with so much data from wearable devices, the big question for athletes is how to use it to guide training and recovery decisions. Here’s a step by step guide to use oxygen saturation for endurance athletes:
- Establish a baseline. Collect data during a couple of weeks to know what your usual oxygen saturation is. This could be overnight data (during sleep) or spot checks at prescribed times during the day (i.e. first thing in the morning or just before bed).
- During a trip to altitude (or use of an altitude tent), track saturation in the same way, noting how much it drops relative to the baseline.
- Over time, observe whether you adapt to the altitude. Oxygen saturation will increase until it reaches a plateau.
- In the case of altitude training camps, it may be a good idea to wait until oxygen saturation starts to plateau before increasing the training load.
- In the case of sleeping in an altitude tent, once you adapt to the set altitude and your oxygen saturation increases above approximately 92%, you can increase the altitude such that oxygen saturation remains between 85% and 92%, which seems to be the optimal zone for adaptation.
- If you are doing an altitude camp several weeks before your main race at altitude, you will be able to get an idea of how many days it takes you to adapt to that change in altitude. This could be helpful for planning your trip before the race, so you can be sure that you get to the race altitude with enough time to adapt. Or, if you must travel to altitude just before the race, you’ll at least know how your body will respond so you can adjust racing and pacing strategies accordingly.
Tracking Sleep Quality and Sleep Apnea with Wearable Oxygen Saturation Devices
Some wearable device companies are focusing the oxygen saturation feature in their devices to measure sleep quality. The premise is that changes in oxygen saturation may indicate sleep problems, specifically sleep apnea, a condition in which breathing repeatedly stops while sleeping. The devices aim to detect sleep apnea early by noticing patterns of reduced oxygen saturation during sleep.
Although most endurance athletes are at low risk for sleep apnea, tracking oxygen saturation during sleep may still be beneficial. Athletes could check against their baseline. If there are any consistent changes, they should consult with their doctor, who may prescribe a more detailed sleep study. More commonly, athletes may use oxygen saturation during sleep as a flagging mechanism. It may indicate when they have lower sleep quality, perhaps because they are getting a respiratory tract infection or worse sleep quality due to fatigue, alcohol, or the use of certain medications. Athletes can then use it as a system to check what “fixes” their issues, improving their sleep quality, and simply use it as a tool to develop better sleep habits.
About Coach Frederic
Frederic Sabater Pastor @fredericspast is a multitalented and multilingual (English 🇺🇸, Spanish 🇪🇸, Catalan ) Spanish coach based out of France. He has published multiple acclaimed articles in the International Journal of Sports Physiology and Performance in the past few years such as “VO2max and Velocity at VO2max Play a Role in Ultradistance Trail-Running Performance”. In addition to his success in academia, Frederic has had coaching success with athletes in a variety of disciplines including triathlons, track and road races, obstacle course events, and ultra/trail races.
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