A very good triathlon coach I knew in Florida sent me an email shortly after the Swimwatch story discussing aerobic fitness was published. I had argued that the build-up training required to develop a good aerobic base could vary between 60 miles a week for David Rudisha and 150 miles a week for Mo Farah. I have coached swimmers like Eyad Masoud who thrive on 70 kilometres a week and Rhi Jeffrey who could handle 100 kilometres. Distances run or swum in an aerobic build-up had to be far enough to achieve the required aerobic goal but varied greatly from athlete to athlete.

Dana, the triathlon coach, said, “Good article, David. Thank you. I’m curious, what metrics are you using to determine the sweet spot of training distance for each athlete?”

That’s a good question. It is asking how does a coach know that Eyad achieves his best aerobic results at 70 kilometers while a swimmers like Rhi and Joe Skuba got better aerobic results swimming 100 kilometers a week. It is a good question because it is difficult to answer. There is certainly no mathematical formula that tells a coach exactly what distance is right. I wish it was possible to put sex, height, weight, age, PB for 1500 meters into a computer and a weekly aerobic distance would flash up on the screen. Sadly we know that would never work – but, to repeat Dana’s email, “What does work?”

In my view several ingredients go into the answer. The first is time. It takes time for a coach to determine what amount of aerobic training best suits a swimmer. For example with Eyad it took 18 months to settle on 70 kilometers. In that time we tried 100 kilometers. It was clearly not working. He was too tired. His swims were too slow. Distance conditioning is not a sprint but it is not a slow plod either. In Eyad’s case 100 was painfully slow. He was not getting aerobically fit. We tried 50 kilometers. That was done at a good speed. In fact he was bouncing off the walls. He was not nearly tired enough. It was too easy. Again he was not getting aerobically fit. 70 kilometers, however, worked. Eyad was tired but could manage the distance day after day at a good firm aerobic threshold speed. He was improving. He was getting aerobically fitter. So I guess the first “metric” of time could also be called “trial and error”.

The second factor is speed. Good aerobic conditioning needs to be run or swum at the fastest speed possible while not going anaerobic. In simple terms how far can the athlete run or swim and maintain a heart rate of around 150 to 160 BPM. Can they repeat that speed and effort day after day? If they can’t the distance is probably too far to be of much benefit. Done well and over several seasons the best aerobic speed will improve. As the athlete gets fitter the speed they achieve while maintaining the same heart rate effort should improve quite dramatically. After all that’s what it means to get aerobically fitter. In Jane Copland’s case an aerobic set of 100×100 on 1.30 at the beginning of her career would average 1.18 per 100. Four years later, at the same heart rate, she could swim the same set averaging 1.08 per 100. In the same period, at the same heart rate, a set of 10×400 went from 5.24 to 4.46 per 400. That is not exactly slow. Jane was getting aerobically fitter. The distance of 90 kilometers a week in her case was working. Do not, though, slavishly time every aerobic set. Of course there will be slower days depending on how the athlete feels. When it comes to speed you are looking for a “general trend”.

The third factor is recovery. The distance should not be so great that the athlete is not recovering between sessions. I was told once that what that meant was the athlete still had an interest in activities apart from running, swimming and sleeping. Mowing the lawns, washing the dishes, attending university and a trip to the movies were all possible in addition to the aerobic training. Failure to recover means the distance is too far. There are other signs such a constant colds and minor illnesses, inability to sleep, unusually bad skin and shortness of temper. A high heart rate when waking up in the morning can also indicate that the distance is too far. Of course the runner or swimmer is going to be tired but not so tired that they do not recover in time to do the next run or swim.

That then is what I look for – trial and error, speed and recovery. The goal is to find the maximum distance possible while still maintaining a good speed and achieving sufficient recovery. As we have said the correct answer will vary from person to person.

So Dana, that is my best answer to your metrics question. I know it does not sound hugely scientific, but then it wasn’t invented by a scientist. I was taught this by two New Zealanders, one who delivered milk and the other a principal of a large New Zealand school. One was called Arthur Lydiard and the other is Arch Jelley. They knew a thing or two about aerobic conditioning. And certainly the runners they coached could run a bit thanks in part to their aerobic conditioning. Four of those runners part way around the Waitakere Ranges are shown in the photograph at the top of this story.