So I’ve been diving into heart rate and respiratory sensors in wearables lately, and I think it’s worth breaking down how these actually work — and how much we should trust them.

1) Heart Rate Sensors (PPG-based systems)

Most consumer wearables — like the Apple Watch, Garmin devices, and Fitbit trackers — use photoplethysmography (PPG).

Basically:

• LEDs (usually green) shine light into your skin

• Blood absorbs light differently depending on volume changes

• A photodiode measures the reflected light

• The device calculates pulse rate from that signal

It works surprisingly well at rest. During workouts? Motion artifacts, sweat, skin tone variation, and loose straps can all mess with readings.

Chest straps (like ECG-based monitors) are still more accurate because they measure the heart’s electrical activity directly — but they’re less convenient.

2) Respiratory Sensors (Breathing Rate Tracking)

This is where it gets more interesting.

Most wearables don’t have a dedicated breathing sensor. Instead, they estimate respiratory rate using:

• Heart rate variability (HRV) patterns

• Tiny chest movements detected by accelerometers

• Changes in blood flow amplitude (derived from PPG signals)

Some medical-grade wearables use impedance pneumography or dedicated strain sensors, but mainstream devices usually infer breathing indirectly.

Accuracy tends to be:

• Pretty solid during sleep

• Less reliable during high-intensity movement

3) The Big Question

Are these sensors medical-grade? No.
Are they useful? Absolutely — especially for trends.

Personally, I think the real value isn’t in spot accuracy, but in long-term pattern tracking:

• Resting heart rate trends

• HRV fluctuations

• Sleep breathing irregularities

Curious what everyone here thinks:

• Have you compared your wearable with a chest strap?

• Anyone using respiratory rate tracking for stress monitoring?

• Do you actually trust the numbers?

Would love to hear your experiences.