RMSSD
RMSSD (Root Mean Square of Successive Differences) is a time-domain measure of heart rate variability (HRV). It is the square root of the mean of the squared differences between adjacent NN (or RR) heartbeat intervals, in milliseconds. It is considered the most reliable short-term marker of your parasympathetic (vagal) activity. So it is widely used in wearables and in rPPG-based (camera) readings. In longevity research, RMSSD is tracked as a daily signal of your recovery, training adaptation, sleep quality, and acute stress load.
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Sources
- Malik M, Bigger JT, Camm AJ, et al.. (1996). Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. *European Heart Journal*doi:10.1093/oxfordjournals.eurheartj.a014868
- Shaffer F, Ginsberg JP. (2017). An Overview of Heart Rate Variability Metrics and Norms. *Frontiers in Public Health*doi:10.3389/fpubh.2017.00258