Apple Watch was supposed to be launched with a multitude of sensors for fitness and health monitoring, but the company Apple Lossless Audio CODEC (ALAC), decided this year that some of them cannot be used in the current version of the product, so that finally the Apple Watch has a sensor for monitoring heartbeats, and today Apple created a special page on its website to explain how in which he works.
Apple Watch it contains this sensor to monitor our heart activity during fitness activities, and based on the information provided by this sensor, the calories burned daily are calculated. Moreover, using a glance implemented in Watch OS, a user can check how many beats per minute his heart has at any time, and this was explained by the Apple company until the moment of the launch of this page.
According to the information published today, Apple Watch it measures the heartbeat every 10 minutes, the information recorded by it being saved in the Health application of iOS 8, we being able to view them, along with chronological graphs, whenever we want to find out if we need to visit a doctor or no, but these data are the same for third-party fitness applications.
Separate from the information regarding the operation of the sensor with Watch OS and iOS, the Apple company also describes in detail how the sensor records information about our heartbeats, based on the technology called photoplethysmography that you have described in detail below, it is the basis of all similar sensors used in other smartwatches or mobile phones.
The photoplethysmographic transducer has a monochromatic light source and a photoelectric cell. The light beam passes through the skin and is partially absorbed by the hemoglobin in the blood mass circulating under the skin. The unabsorbed fraction will reach the photoelectric cell through transmission or reflection, and depending on the amount of light received, the cell produces a current that will be amplified and recorded graphically (photoplethysmogram). The systolic-diastolic blood flow variations in the skin network determine equivalent variations in the current, and the shape of the graph will be similar to that of the arterial pulse. The normal pulse wave has a steep anacrotic slope, a sharp peak, and a dicrotic wave that is concave at the base. With the help of photoplethysmography, parameters necessary for the diagnosis of peripheral vascular diseases can be determined: the shape of the pulse wave, the amplitude and frequency of the component waves. Pulse waves are attenuated in the presence of arterial occlusive diseases. Arterial obstruction causes the depression of the wave with a smooth slope, a rounded peak and the loss of the dicrotic wave (the concave side becomes convex). Several peripheral pulse curves and a reference ECG are recorded simultaneously.
The interesting part is that Apple admits that the heart rate sensor does not provide 100% accurate information, and the placement Apple Watch very close to the skin gives the most accurate heart rate readings, while wearing the watch with a wider strap will give inaccurate information, but you probably already imagined this without these explanations.
A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you're exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading. Motion is another factor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.
Finally, the American company also talks about the extended testing period to choose the materials that are used in its watch, but this aspect will matter less for the millions of users who will buy the watch and will have extremely high expectations from it based on the promotion made by Apple.
