Place a finger on your pulse, wait for the second hand on the clock to hit twelve, and count the pulses for one minute. Voila! That’s your heart rate. Easiest thing in the world to measure.
Or is it?
It turns out that measuring heart rate is much more complicated. It’s not just about the number of beats per minute, but also about the waves that your pulse generates. You may think: “Oh, my watch measures that!” but getting meaningful insights about your heart is far more complicated than simply strapping a device onto your wrist.
Just because your heart is in the right place, doesn’t mean your wearable is.
Challenges of Measuring Heart Rate
With each beat, an electrical wave travels through your heart. This wave causes your muscles to squeeze and pump blood, alternating between heart chambers. This electrical activity can be measured directly through an electrocardiogram (ECG/EKG), which uses electrodes attached to your body.
If you want to measure heart rate without wires or chest straps, photoplethysmography (PPG) is much less invasive. PPG measures the volumetric changes of your heart using light reflection. As the heart contracts, it sends a pulse of blood into your arteries. This pulse causes your arteries to alternate between swelling and contracting.
This pattern, known as your blood volume pulse (BVP), can be measured by shining a light on your skin and capturing the changes in light reflected back from the red blood cells in your arteries.
This sounds simple, but it’s harder than it initially appears. First, it can be difficult to reach the arteries you’re trying to read from. Second, once you’ve found a good spot to measure, it can be hard to keep out external light from influencing the reading. Third, both light and movement can create “noise” in the signal—false spikes in your data that risk skewing your results.
It’s not wise to build a house on shaky ground. If the signal you’re reading your heart rate from is off, at best your data will have limited usefulness to you. At worst, it could give you erroneous information that you act on, such as false data on your heart rate zone during a workout.
Good signal in, good insights out.
Best Practices for Measuring Heart Rate
Here are a few best practices to keep in mind if you use a wearable to track heart rate:
- Keep it close to your blood: Any device should be close to an artery that produces a strong blood volume pulse signal.
- Optimize Your Fit: Unnecessary movement during the recording can corrupt the light. For the most accurate data, minimize movement and maximize comfort by ensuring your device is not too loose.
- Draw the curtains: To help your device’s light travel unimpeded, minimize external light pollution. If you can see light leaking out of your device, there’s likely light leaking into your signal as well.
Oura’s Answer: A Ring
Have you ever wondered why Oura is a ring? Why not a watch, wristband, or earlobe sensor?
Because the benefits of a ring far outweigh other options. Here’s why:
- The finger is ideal for blood flow: It has abundant arteries and capillaries for reliable data measurement.
- Rings minimize movement: There are no moving parts between joints and rings don’t require adjusting.
- Rings seal off light: A well-fit ring prevents outside light from disrupting data collection. Some people find it uncomfortable to wear a heart-rate tracking watch as tightly as it should be to get a good reading, especially at night.
- Infrared offers an edge: Oura uses an infrared light PPG, which penetrates deeper into your tissue than the green light LEDs in most wearables. This allows signals to be read from large finger arteries instead of, for example, the smaller capillaries at the surface of your skin on the wrist.
If you want to improve your sleep or manage your stress, getting accurate heart rate data is crucial.
- Castaneda, Denisse, Aibhlin Esparza, Mohammad Ghamari, Cinna Soltanpur, and Homer Nazeran. “A review on wearable photoplethysmography sensors and their potential future applications in health care.” International journal of biosensors & bioelectronics 4, no. 4 (2018): 195. (link)