system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do cardio machines need two hands to monitor heart rate but smartwatches only need one wrist?

EDIT: I'm referring to gym machines like threadmill, spinning, elliptical machines.
gpt
A smart watch uses light and a sensor to 'see' the blood pulsing through your skin. 

A cardio machine runs a little bit of electricity through your body and because your heart uses electricity to beat it can see changes in how the electricity comes back. Because you need to complete a circuit that includes your heart it uses two points on your hands.