Wearable health sensors turn a small device on the wrist into a continuous window into the body. They matter because they can track trends in heart rate, motion, sleep, oxygen level, and sometimes electrical heart signals outside a clinic. These measurements help users notice patterns and can support doctors with longer term data.
A wearable is not a full medical lab, but it can provide useful signals when its limits are understood.
Most wrist wearables combine optical, electrical, and motion sensors with software that filters noisy data. Green, red, and infrared LEDs shine light into the skin, while photodiodes measure the light that returns after interacting with blood and tissue. Accelerometers and gyroscopes detect movement so the device can estimate steps, activity, and sleep stages, and can also help remove motion artifacts from heart signals.
Some devices include electrodes for electrocardiogram measurements, which record voltage differences caused by the heart's electrical activity.
Key Facts
- Heart rate from optical sensing uses photoplethysmography, or PPG, which tracks small changes in reflected light caused by blood volume pulses.
- Pulse rate can be estimated from timing: heart rate in beats per minute = 60 / pulse period in seconds.
- Blood oxygen saturation is estimated by comparing red and infrared light absorption: SpO2 depends on the ratio of red absorption to infrared absorption.
- Electrocardiogram sensors measure voltage differences at the skin, often in millivolts, to detect the heart's electrical timing.
- Motion sensors measure acceleration and rotation, commonly using accelerometers for m/s^2 and gyroscopes for degrees per second.
- Signal quality depends on good skin contact, stable fit, correct sensor placement, and algorithms that reduce noise and motion artifacts.
Vocabulary
- Photoplethysmography
- Photoplethysmography is an optical method that estimates blood volume changes by shining light into the skin and measuring reflected or transmitted light.
- Photodiode
- A photodiode is a light sensitive electronic component that converts incoming light into an electrical signal.
- Accelerometer
- An accelerometer is a sensor that measures changes in velocity over time, such as wrist movement or steps.
- Electrocardiogram
- An electrocardiogram is a recording of the heart's electrical activity measured as voltage changes over time.
- Motion artifact
- A motion artifact is unwanted signal distortion caused by movement of the body, device, or sensor contact.
Common Mistakes to Avoid
- Treating a wearable reading as a guaranteed diagnosis is wrong because most wrist devices estimate signals and may require confirmation with clinical equipment.
- Wearing the device too loose is wrong because poor skin contact lets outside light and motion interfere with optical and electrical measurements.
- Ignoring motion during a heart rate reading is wrong because exercise, shaking, or sliding on the skin can create artifacts that look like real pulses.
- Assuming all sensors measure the same thing is wrong because PPG, ECG, accelerometers, and temperature sensors detect different physical signals and need different interpretations.
Practice Questions
- 1 A wearable detects one pulse every 0.80 s while the user is resting. Calculate the heart rate in beats per minute.
- 2 During a 2 minute walk, a sensor counts 240 steps. What is the step rate in steps per minute, and what is the average time per step?
- 3 A student's smartwatch shows a sudden high heart rate while the watch is loose and the student is waving their arm. Explain why the reading may be unreliable and name two ways to improve the measurement.