Wrist-worn heart rate sensors have improved significantly, but understanding their known limitations prevents over-trusting specific numbers.
Optical wrist sensors generally perform reasonably well during steady-state activity like walking or moderate cardio, but accuracy noticeably decreases during high-intensity interval training and activities involving a lot of wrist movement, like weightlifting or boxing, where motion artifacts interfere with the optical reading.
Skin tone, wrist tattoos, and how tightly the device is worn can all affect optical sensor accuracy, a limitation that chest-strap sensors using electrical signals instead of optical light are generally less susceptible to.
For casual tracking and trend awareness, wrist sensors are generally good enough. For precise training zone work during high-intensity intervals, a chest strap remains meaningfully more reliable.