Table 2: Descriptions of the HRV measures analyzed [¹⁴] and their trend under stress [¹⁵].

Domain	Measure	Description	Features Trend Under Stress
Time-domain	AVNN	Average of NN intervals (ms)	↓
–	RMSSD	Root mean square of differences of successive NN intervals (ms)	↓
–	pNN20	NN variations above 20 ms (%)	↓
Frequency-domain	LF/HF	Ratio of Low Frequency and High Frequency power bands.	↑