Wikipedia: Heart rate variability

Measurement concepts (time vs frequency domain), autonomic physiology, and limitations—background for HRV biofeedback and breathing protocols.

Why it helps

Clarifies RMSSD, HF power, LF/HF misuse and device heterogeneity problems readers hit when interpreting consumer wearables.

Pair with resonance-frequency biofeedback trials and slow-breathing reviews already sourced on the HRV and breathing protocol pages.

Heart Rate Variability
Educational tertiary content; intervention outcomes must come from linked PubMed training studies