Heart rate variability (HRV)

Heart rate variability (HRV) is the variation in time between heartbeats.^{1Viada, The Ultimate Hybrid Athlete, p. 301.} Even at a stable heart rate, the heart doesn’t beat like a metronome because many systems signal it to speed up or slow down.^{1Viada, The Ultimate Hybrid Athlete, p. 301.} Alex Viada’s rule of thumb is that a less-stressed heart responds to more inputs and shows more variability, so lower variability generally signals higher stress.^{1Viada, The Ultimate Hybrid Athlete, p. 301.} ISSA describes these interbeat intervals as easily affected by physical and emotional stress.^{2International Sports Sciences Association, Strength and Conditioning, p. 179.}

In practice

Read the pattern over time: HRV changes through the day with hormonal and activity cycles, and assessing the larger pattern requires precise measurement over long periods.^{1Viada, The Ultimate Hybrid Athlete, p. 301.} ISSA says a high reading can also reflect an illness-related immune response, increased stress or cardiac abnormalities.^{2International Sports Sciences Association, Strength and Conditioning, p. 179.} It gives an average healthy HRV of 55 to 105 milliseconds at ages 20 to 25 and a typical range of 25 to 45 milliseconds over 65.^{2International Sports Sciences Association, Strength and Conditioning, p. 179.}

Viada cautions that data gaps from charging or removing a wearable distort readings, and that most wearables can’t capture the full pattern.^{1Viada, The Ultimate Hybrid Athlete, p. 301.} A run after a leg session may produce a higher heart rate because of lower glycogen and muscular fatigue; a wearable may label that run unproductive because of poor recovery when the strength session explains the response.^{3Viada, The Ultimate Hybrid Athlete, p. 299.}

Pair HRV with resting heart rate measured on waking, which ISSA says can indicate how well a client is recovering and whether they need more sleep.^{4International Sports Sciences Association, Strength and Conditioning, p. 178.}