Changes

Optical Heart Rate Monitoring detects the changes in blood filling the capillaries under your skin as your heart beats. Each time your heart beats the capillaries expand flow that occur with blood, and this ebb and flow can be used to estimate your heart rateeach heartbeat. Most optical heart rate monitors for use while exercising shine a green light into the skin and use a receptor to detect the changes in the reflected light. This approach has been used for decades, and I had an early version back in the 1980s. The latest optical heart rate monitors are vastly superior, but still have many accuracy issues. The most accurate form of heart rate monitoring is to use a chest strap that picks up the electrical signal from the heart. While a chest strap is not perfect, it works remarkably well as long as it has good contact with your skin, the battery is not flat, and the strap is not damaged. The accuracy of Optical Heart Rate Monitoring (OHRM) will depend on a number of factors:* The watch needs to fit just right. Because of the sensor is measuring the expansion of the capillaries changes in blood flow with each heartbeat, too much pressure will prevent this expansionpush the blood away from the sensor. However, to lose and the watch won't get a good reading due to lack of contact. Getting this tension just right can be tricky, especially if you're wrist expands or contracts over time.* Temperature seems to be a huge factor, and most systems work better in warmer conditions. If you're a little chilled, your body will restrict superficial blood flow to your capillaries to retain body heat, making it much harder for the optical HRM. In my experience, the issue is mostly around how warm you are when running, rather than at the absolute temperature. I've found that optical heart rate monitors can do better in colder conditions when I'm wrapped up warm and sweating a slightly than they will do in mild conditions where I'm running with the bear arms.

 Pulse oximeters are the small devices that are clipped to your fingertip to measure your pulse and oxygen saturation. Like optical heart rate monitors, pulse oximeters measure your pulse using a similar approach of looking for changes in capillary refillhow light is absorbed. While pulse oximeters provide a reliable and sufficiently accurate measure of your heart rate, they are extremely sensitive to movement, and your finger needs to be completely still. =How Optical Heart Rate Monitors Work=Optical Heart Rate Monitors for use when exercising have two sensors, one for detecting light, and the other for detecting movement. They shine a light into the skin, and then measure how much is reflected back to the light detector. Some of this variation is due to the beating of the heart, but some is due to a motion, especially motion related to each step. The optical heart rate monitors use the accelerometer to detect movement and will attempt to account for this. The difficulty of extracting the wanted signal from the noise is a primary reason why optical heart rate monitors have so many accuracy problems. The issue of step based noise is particularly problematic as the frequencies tend to be similar. It's been noted that optical heart rate monitoring may be less accurate when cycling than when running. One possible explanation that occurs to me is that the noise may be related to muscular contractions, rather than the impact of landing.