Wrist heart rate monitors, commonly found in smartwatches and fitness trackers, primarily work by using a technique called photoplethysmography (PPG) to measure your heart rate through the skin. In essence, they shine a light (usually green) onto your wrist and measure the amount of light reflected back.
The Science Behind It: Green Light and Blood
Here's a more detailed breakdown:
-
Green Light: Most wrist-based monitors use a green LED light. The reason for this choice is that blood absorbs green light well. This is based on spectroscopy principles, where red and green are considered opposites on the color wheel, leading to a high absorption rate of green light by blood.
-
Light Reflection and Absorption: The monitor shines this green light into the skin. When your heart beats, more blood flows through the vessels in your wrist. Because blood absorbs green light, more light is absorbed when there's more blood flowing. The sensor then measures the amount of light that isn't absorbed, but rather reflected back to it.
-
The Sensor's Role: A photodiode (light sensor) detects the intensity of the reflected light. This sensor is very sensitive and can detect minute changes in light reflection caused by the pulsating blood flow.
-
Calculating Heart Rate: The monitor then analyzes the patterns in light reflection. Each peak in blood volume (when the heart contracts and pumps blood) corresponds to a minimum in reflected light (because more light is being absorbed). By counting the number of these peaks per minute, the monitor can calculate your heart rate (beats per minute, or BPM).
Factors Affecting Accuracy
While wrist-based heart rate monitors are convenient, their accuracy can be affected by several factors:
- Fit: A loose fit can allow ambient light to interfere with the sensor, leading to inaccurate readings. The monitor needs to be snug but not too tight.
- Movement: Significant movement during exercise, particularly activities involving wrist flexion, can disrupt the signal and introduce errors.
- Skin Pigmentation: Very dark skin can absorb more light, which might impact the sensor's ability to accurately detect changes in blood flow. However, manufacturers are constantly improving algorithms to compensate for this.
- Tattoos: Tattoos, especially those with dark inks, can interfere with the light signal.
- Cold Temperatures: Cold can cause blood vessels to constrict, reducing blood flow at the wrist and potentially leading to inaccurate readings.
Beyond Green Light: Other Technologies
While green light is the most common approach, some advanced monitors might incorporate other colors of light (like red or infrared) and sophisticated algorithms to improve accuracy. These approaches can help to compensate for variations in skin tone and other factors.
