Comment by PaulHoule

Note they put a Holter monitor on it

https://www.pnas.org/doi/10.1073/pnas.1914273116

to get an ECG which is one of several strategies they could use. (e.g. lately I've been interested in Heart Rate Variability which has gotten me looking at reading heart rate with cameras, radars, pressure gauges, ultrasound, etc.)