Should you train swimming by time or by distance?

Researchers followed 12 well-trained swimmers through 20 training sessions. After each session, the swimmers said how hard it had felt out of ten. The researchers multiplied that number by the minutes. They also measured the work from heart rate. The simple number, minutes times effort, agreed with the heart-rate measure in every swimmer.

Time is also fair to every swimmer. A 45-minute swim asks about as much of a slow swimmer as of a fast one. The fast one just covers more water. Write the session as 2,000 metres instead. Then it is half an hour for a fast swimmer and an hour for a slow one.

And minutes are how you plan your week: forty before work, an hour on Saturday. A swim by the clock fits straight into that.

Pool length alone changes how far you get. Eleven swimmers swam 400 metres at the same effort in a 25-metre pool and in a 50-metre pool. Their oxygen use, heart rate and how hard it felt were the same in both. In the short pool they were about 5 percent faster, because they turned and pushed off a wall twice as often. So the same minutes at the same effort were the same work in both pools. Only the metres changed.

A wetsuit changes it too. Eleven experienced triathletes swam 200-metre repeats at a set speed, once in a wetsuit and once without. In the wetsuit their hips rode higher and they took fewer strokes. Their heart rate was lower, and the swim felt easier. The same metres cost them less. Open water has no walls to push off at all, and waves and wind change it again.

So 1,500 metres is one swim in a short pool, another in a long pool and another in a lake. Thirty minutes at an easy effort is the same swim in all of them. That is why a swim by the clock works wherever you get in the water.

A study tested ten smartwatches in a 50-metre pool. On 400 metres of steady front crawl, the best of them counted the distance almost perfectly. When the swimmers changed stroke every 25 metres, most watches got worse, and the worst was out by more than 90 percent. So on a steady swim, the metres on your wrist are fine. On a swim with drills and a few lengths of breaststroke to catch your breath, trust the minutes.

Set out to swim 2,000 metres and the last length is a finish line. It is easy to speed up to reach it, and an easy swim stops being easy. Set out to swim 40 minutes and going faster does not end it sooner. So you can stay at the effort you meant.

Both ways build fitness. Time keeps easy swims easy, and it works in any water. Watching the clock can feel longer than counting lengths at first. That passes once you trust the effort.

Train by time and you give up the pace clock. You do not need it. In the water your breathing follows the stroke. So it cannot tell you the effort the way it does on land. Heart rate reads differently there too. One question still works: how long could I keep this up? An easy swim is one you could hold for hours. A threshold effort is one you could hold for about twenty minutes.

How hard it feels out of ten works too. Six elite open-water swimmers from a national Olympic team swam six 500-metre efforts, each a little harder than the last. At an easy effort they rated it 3 or less. That was before lactate started to build in their blood. At threshold and above, they rated it 7 or more. An easy swim sits at about 3, and a threshold swim at about 7.

A threshold swim needs no pace clock and no wall to push off. It is blocks of five minutes at about 7, each followed by a minute of easy swimming. If your club swims sets of 100 metres, those still work. What counts is the minutes you spend at the effort the set asks for.

On your next easy swim, set a time goal in your watch's Workout app instead of a distance. Swim 30 minutes at about a 3, calm enough that you could go on for hours.

On race day you swim a distance, and the clock tells you how long it took. What gets you ready for it is not one long swim. It is weeks of swimming, with the minutes at each effort adding up. As your stroke improves, the same minutes carry you further.

Why the time you could hold an effort tells you which effort you are in is in How long could you hold this?