HR	Heart Rate
HRV	Heart Rate Variability
RR Interval	The time between the peak R-wave of one QRS complex on an ECG to the next
SS	Stress Score
SPS	Sympathetic–Parasympathetic Ratio
MSS	Modified Stress Score
MSPS	Modified Sympathetic–Parasympathetic Ratio
RMSSD	Root Mean Squared of the Successive Differences of R-R Intervals
SDNN	Mean of Standard Deviations of all Normal-to-Normal R-R Intervals
SD1	Poincaré plot standard deviation perpendicular to the line of identity
SD2	Poincaré plot standard deviation along the line of identity
ECG	Electrocardiography
GXT	Graded Exercise Test(s)
PRE	5 min pre-GXT ECG recording (minutes 5–10 of a 10 min recording
POST1	5 min post-GXT ECG recording (minutes 5–10 of a 30 min recording)
POST2	5 min post-GXT ECG recording (minutes 25–30 of a 30 min recording)
RMANOVA	Repeated-Measures Analysis of Variance
ICC	Intraclass Correlation Coefficient
CI	Confidence Interval