Beyond Kilometers and Hours: The Evolving Metrics of Endurance Training

No matter how many times the boffins in lab coats tell us that volume is not as critical as it was once thought when it comes to endurance sport training, we all still tally up at the end of our weeks, months and even years. As an example, popular training app, Strava sends us our year in review every December and there is very little in there that mentions intensity. It is all about the numbers and specifically, the totals. There is division amongst the endurance community though. There are two camps made up of those that add the kilometres covered and those that focus on time. I would hazard a guess that the division is somewhere down the middle but I would wager that there are some general differences and trends in each group.

I suspect that the distance group’s average age is older than the time-followers. Athletes who have been doing this for longer probably developed the habit of tallying kilometres in a time before the multitude of feedback data that we have available today. A time when the number of kilometres you covered and the speed at which you covered them, was the most significant achievement of your training week. Younger athletes who know nothing other than the data-rich training apps of today are more focused on time spent in different training zones than how many kilometres they have covered. So, does that mean that adding up the time spent training is the better way to do things given how training knowledge and techniques has progressed over the last 20-30 years?

The answer is probably yes. For one, distance covered in different geographical areas will require different effort levels. For example, a 400km week on the bike in The Netherlands will be achieved at much higher speeds and a lower effort than the same distance would be covered in the mountainous Swiss Alps. Distance is even less significant for those who spend a lot of time training on platforms like Zwift where the speeds and mileage covered as a result, do not resemble what is achievable in real life. However, an hour at threshold is still an hour at threshold, whether on the roads of your local suburb or those in Watopia.

Secondly, we are now aware that endurance sport racing, whether it be cycling, running, or triathlon is not purely about endurance, surprisingly. Training has become considerably more specific and targeted. Sessions have become focused on training at precise effort levels that target different energy systems and sometimes this is done at low speeds resulting in less distance covered. Those with a focus on adding up the kilometres run the risk of doing too much in the week, just to reach a specific total.

Another thing that the modern endurance athlete does is include training sessions that are not the core discipline of their sport. Supplementary workouts focused on strength and mobility have become significant components in every athlete’s week. There is no mileage to measure there but these sessions certainly contribute to the cost and output of the week and should be included when tallying up. The only way to do that is using time as the variable.

Triathletes should definitely be using time to calculate their weekly volume because each of the three disciplines are done at vastly different speeds so, simply adding kilometres together will provide no indication of the impact of the week’s training. For example, 10km is a significant kick up in a triathlete’s swimming mileage but a completely insignificant number as a bike ride. I know of an iron-distance professional athlete who at pre-race press conferences answered the inevitable, but annoying, ‘weekly mileage question’ with the biggest weeks he had ever done in the swim, the bike and the run but which he had never done in the same week. If the reporters had just added up the time it would have taken to cover those distances, in each discipline in a week, they would quickly realise there was just not enough hours in a day, let alone the energy to spend that much time training.

So, yes. I don’t think there is much more debate that time is definitely a better variable than kilometres when tallying up our weekly training but… is it the best variable there is? I present to you TSS (Training Stress Score)…

TSS is a variable that is calculated using power or heart rate and time to calculate the impact that a training session has on our overall fitness and accumulated fatigue. Simply put, a short hard session will have a higher TSS than a short easy one but possibly a similar or even higher impact than a long, easy session. So, TSS does not only take into account how long we have been training but also at what level of intensity that time was spent. The formulas are still being refined and they are not perfect yet but they are already good enough to provide a decent idea of how hard our training week was, irrespective of how many hours or kilometres we covered.