Five training zones from your maximum and resting heart rate, worked out two ways so you can see where they disagree.
Heart rate zones exist to stop you from doing everything at the same middling effort. Easy running should build aerobic capacity without costing you recovery, and hard running should be hard enough to force adaptation. The zone that ruins most training weeks is the one in between: fast enough to accumulate fatigue, slow enough to improve very little. Zones give you a number that tells you when you have drifted into it.
Percent of maximum divides your maximum heart rate into bands. Karvonen works from heart rate reserve, the range between resting and maximum, and adds your resting rate back at the bottom. For a runner with a resting heart rate in the forties the two methods can differ by ten beats in the easy zones, which is the difference between a run that builds fitness and one that feels pointless. Karvonen is generally the better guide for easy running.
Running here adds heat strain on top of the work your legs are doing, and your heart carries both. The same pace that sat comfortably in zone 2 at six in the morning will read zone 3 by half past seven. This is not a fitness problem and it is not a reason to override the zone. If you hold heart rate steady and let pace fall, you get the training effect you were after. If you hold pace and let heart rate climb, you get an easy run that costs you like a hard one.
Heart rate lags. At the start of an interval your heart has not caught up with your legs, so chasing zone 5 in the first minute of a three minute repetition will only make you start too fast. Use pace or effort for short hard work and keep heart rate for the long, steady, easy majority of your week. Also treat a single strange reading with suspicion before you treat it as data, because chest straps drift, wrist sensors lose the signal in the rain, and both lie in ways that look convincing on a graph.
Karvonen, if you know your true resting heart rate. It accounts for the fact that your working range starts at rest, not at zero, so it places the easy zones where they belong for a trained runner. Percent of maximum is simpler and is what most watches default to. If the two disagree, they are not both wrong so much as measuring slightly different things.
A formula will not tell you. Any age estimate carries an error of roughly ten to twelve beats either way, which is more than the width of a zone. To measure it, warm up thoroughly, then run three to four hard hill repeats of two to three minutes with short recoveries, going all out on the last one. The highest number you see is close to your maximum. Do this only if you are healthy and used to hard training, and never as your first hard session.
No. Heat and humidity add roughly five to ten beats per minute at the same pace, sometimes more, because your body is also pumping blood to the skin to cool you. This is called cardiac drift and it gets worse the longer you run. It means that in Malaysian conditions, running to heart rate will correctly slow you down. The pace drops but the training effect is the same.
Cardiac drift again, driven mostly by heat and fluid loss. On a long run in the tropics it is normal to finish ten to fifteen beats higher than you started at identical pace. Do not chase the zone by slowing to a walk, and do not ignore it entirely. Judge the first half by heart rate and let the second half drift a little.
Most of it. The common guidance is that around eighty percent of weekly running should sit in the easy aerobic zones and twenty percent should be genuinely hard. The failure mode for most amateur runners is not too little intensity, it is a whole week of moderate running that is too hard to recover from and too easy to improve anything.
Heart rate for easy running, pace for hard sessions. Easy runs are about physiological load, and heart rate reads that correctly on a hot day when pace lies. Intervals are about speed at a target effort, and heart rate lags too far behind at the start of each repetition to be useful. The VDOT Calculator will give you the pace side of that.
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