HRV should really be measured at rest eg check it for 4-5 mins when you first get up and before any food or drink. If measured during exercise, it really doesn’t mean very much as it will vary so much. HRV is heart rate variability – whereby, even at rest your heart will not beat completely in rhythm from second to second. The ANS autonomic nervous system support your heart and keep it balanced between the parasympathetic (rest and digest) and sympathetic (flight or fight) – and even when relaxed, from heart beat to heartbeat there will be slight difference in timing eg beat 1, and then beat 2 – 950 msecs later, then beat 3 – 967 msecs later – the HRV between beat 2 and 3 is 967-950 ie 17 msecs. HRV declines with age and the middle 50% of 25 year old males have a HRV of between 55-95 msecs. A 65 year old (me) can expect a HRV of between 20-50 msecs (mine is about 28). Ideally your body need to be able to switch from parasympathetic to sympathetic if the situation demands it eg a race, thus enabling your heart rate to increase, and when the parasympathetic branch is activated eg after a gym session, your heart rate / breathing etc can slow down.