Table 1.

Overview of cardiac vagal activity (CVA) measures included in the systematic review and meta-analysis.

CVA Measure	Definition
RMSSD	Root mean square of successive differences between adjacent R–R intervals in milliseconds or log-transformed values
HF	High frequency component in the power spectrum range between 0.15 and 0.4 Hz in milliseconds squared, seconds squared, normalized units, or log-transformed values to the basis of 10 (log) or e (ln)
RSA	Respiratory sinus arrhythmia, defined as heart rate variability (HRV) in synchrony with respiration, by which the R–R interval is shortened during inspiration and prolonged during expiration [49].
BS	Baroreflex slope, defined as the resulting changes in R–R intervals from modulated carotid baroreceptor (sequential neck pressure and suction; [50]).
CBS	Cardiovagal baroreflex sensitivity, defined as the slope relating R–R interval and systolic blood pressure [51].
CVI	Cardiac vagal index, defined as the ratio of the longest and shortest R–R interval during a 4 s exercise test [52].
HRR	Heart rate (HR) reactivity, defined as the difference between peak HR during 3 min exercise and HR at the first minute post exercise (i.e., recovery; [40]).
DBT	Deep breathing test, defined as the difference between the longest and shortest R–R interval while breathing at resonance frequency (6 breath/min; inhale and exhale phase lasting each for 5 s). This can be also described as expiration to inspiration ratio at resonance frequency [53].
Valsalva	Valsalva ratio, defined as the ratio of the maximum R–R interval after strain and the shortest R–R interval during strain. Under the Valsalva maneuver, participants’ nostrils were closed by nose clip and participants were asked to blow by doing forceful expiration into the rubber tube of a mercury sphygmomanometer, raise the mercury column to 40 mm Hg and maintain that level for at least 15 s. HR was recorded continuously during the whole procedure [54].