RMSSD

RMSSD stands for the root mean square of successive differences between adjacent normal RR intervals. It is a time-domain measure of heart rate variability (HRV) that primarily reflects short-term fluctuations in autonomic nervous system activity, with a particular emphasis on parasympathetic (vagal) tone. RMSSD is commonly computed from a sequence of consecutive RR intervals, typically derived from an ECG or other heart-rate monitor.

Calculation involves taking the differences between successive RR intervals, squaring them, averaging these squared differences, and then taking the square root. If RR intervals are in milliseconds and there are N intervals, RMSSD = sqrt( (1/(N-1)) * sum_{i=1}^{N-1} (RR_{i+1} - RR_i)^2 ). The metric emphasizes beat-to-beat variability rather than average heart rate.

Data quality is important for reliable RMSSD values. Records should be artifact-free, with ectopic or erroneous beats removed or corrected. RMSSD can be computed from relatively short recordings (for example, 1–5 minutes) and is a common metric used in both research and consumer wearables due to its robustness to some recording conditions.

Interpretation and uses: higher RMSSD indicates greater short-term HRV and is generally associated with higher parasympathetic activity and better vagal regulation, while lower RMSSD suggests reduced parasympathetic influence, stress, fatigue, or overtraining. It is widely used in sports science for recovery and training status, in sleep research, and in mental health studies. RMSSD is often examined alongside other HRV measures to provide a fuller view of autonomic function.

Limitations: RMSSD is influenced by age, breathing rate, posture, circadian phase, and recording duration. Comparisons are most meaningful when made against individual baselines and with standardized measurement protocols.