RMSSD (Root Mean Square of Successive Differences)

RMSSD stands for Root Mean Square of Successive Differences and is a widely used time-domain metric of heart rate variability (HRV). It is calculated by taking the square root of the mean of the squared differences between successive R-R intervals (the time between consecutive heartbeats). RMSSD primarily reflects parasympathetic (vagal) nervous system activity.

RMSSD is considered one of the most reliable short-term HRV metrics because it is relatively resistant to the influence of breathing patterns and can be meaningfully measured from recordings as short as one minute. Higher RMSSD values generally indicate stronger parasympathetic tone and better cardiovascular health, while lower values may suggest autonomic dysfunction, chronic stress, or poor recovery.

In contactless vitals measurement, RMSSD can be derived from rPPG-extracted inter-beat intervals. While rPPG-based RMSSD is best suited for trend monitoring and relative comparisons rather than absolute clinical values, it provides valuable data for stress assessment, recovery tracking, and fitness evaluation in applications ranging from telehealth to workplace wellness programs.