Heart Rate Zones

Heart Rate Zones: what are they?

Prescribing "run easy" fails because easy means something different to every client. Heart-rate zones fix that by giving the effort a number the client can see on their watch mid-session. The system is only as good as its anchor, though: a zone chart built on an age-predicted maximum inherits all the error of that estimate. Get the anchor right and zones become the most practical intensity language a coach has. Get it wrong and every session in the plan is quietly mis-prescribed.

The five-zone model

The most common model expresses each zone as a percentage of maximum heart rate. The boundaries are conventions, not physiological constants, so different platforms shift them by a few percent. What matters is that you use one model consistently for a client rather than mixing charts.

Percentage of max versus the Karvonen method

The simple method takes a percentage of maximum heart rate. The Karvonen method takes a percentage of heart rate reserve instead - the gap between resting and maximum heart rate - and adds resting heart rate back on. The formula is: target = ((HRmax - HRrest) x intensity) + HRrest.

Karvonen produces higher target numbers for the same nominal intensity, and it accounts for the fact that a client with a resting heart rate of 45 and a client with a resting heart rate of 75 do not have the same working range. It needs an accurate resting heart rate, taken lying down before getting out of bed, ideally averaged over several mornings. If you use Karvonen for one client, use it for all of them, and label the plan so nobody compares a Karvonen zone 2 to a percentage-of-max zone 2.

Anchoring zones to something real

The best anchor is a measured maximum heart rate from a maximal test, or a threshold heart rate from a 30-minute time trial. Both are specific to the client and to the activity. The worst anchor is an age formula, which describes a population average and can be off by a wide margin for any individual.

Heart rate also lies in predictable ways. It drifts upward over a long session at constant pace (cardiac drift), it rises in heat and at altitude, it is blunted by some medications, and it lags behind changes in effort by a minute or more, which makes it useless for short intervals. For anything under two minutes, prescribe pace or power and let heart rate be a record rather than a target.

• Anchor to a measured max or threshold heart rate whenever you can.
• Re-check the anchor after a big fitness change, not every month.
• Expect zone drift in heat, at altitude and when the client is under-slept.
• Do not use heart-rate zones to steer intervals shorter than about two minutes.

How much time in each zone

Most endurance programmes put the large majority of total time in zones 1 and 2 and a small slice in zones 4 and 5, with relatively little in zone 3. The reason is not that zone 3 is useless; it is that it is fatiguing enough to compromise the genuinely hard sessions while being less specific than either the easy or the hard end.

For a general fitness client training three times a week, the split is less important than the floor: one session that is genuinely easy and long enough to matter, and one that goes hard enough to be uncomfortable. The mistake is not the ratio, it is running every session at the same medium effort.