A time-tested approach popular among top endurance athletes is to spend 80 per cent of your weekly exercise time doing long, slow efforts, and 20 per cent going short and fast.skynesher/Getty Images
In the pursuit of a better cardio workout, you have two main levers: You can push harder or you can go longer.
It’s far from clear how these levers interact, though. Is it more challenging to jog for 5K or run all-out for 1K? Which will make you fitter? Which will take you longer to recover from?
Most modern sports watches and fitness trackers attempt to answer these questions by assigning a single number that combines duration and intensity to approximate the “training load” of each workout. But the science behind such calculations remains highly speculative, as a recent study by researchers in Finland reveals.
In the International Journal of Sports Physiology and Performance, Pekka Matomaki and his colleagues at the University of Jyvaskyla address a seemingly simple question: How many minutes of easy running are equivalent to one minute of hard running? The answers turn out to be all over the map depending on how you calculate them, but they all point to an important conclusion: Intensity is a much more sensitive lever than duration.
The concept of training load first emerged in the 1970s. The simplest version simply involves multiplying intensity (speed for a runner or power for a cyclist, for example) by duration (time in minutes). A 10K run at 10 kilometres an hour, for example, would produce a training load of 100.
There’s a problem, though. If you decide to run 5K instead but want to produce the same training load of 100, you would need to run it at 20 km/h. That’s significantly – and unrealistically – harder than the 10K run. Clearly, intensity and duration don’t scale in the same way.
Over the years, researchers have come up with numerous approaches to calculate training load in a way that more accurately reflects the stress imposed on the body. You can base those calculations on the total number of calories burned, on how much your performance drops from start to finish, on markers of fatigue such as heart rate or lactate levels in the blood, or you can simply estimate your subjective level of effort.
Each of these approaches produces different results, Matomaki and his colleagues found. For the purposes of comparison, they defined “easy running” as a heart rate between 70 and 80 per cent of maximum and “hard running” as 95 to 97.5 per cent of max. With those assumptions, a minute of hard running was equivalent to anywhere from 1.5 to 50 minutes of easy running, depending on the method used.
One reason for the wide range of results is that there isn’t a single “correct” answer. A short, hard run and a long, easy one aren’t interchangeable. They produce different stresses on the body and elicit different adaptations. The former might stress your heart and circulatory system more, the latter your muscles.
Still, there are some common threads that recur in the different approaches. First, duration causes a roughly linear increase in training load. If you run twice as far, you’ve incurred twice the training stress.
That relationship eventually starts to break down. Most people would say that a marathon is more than twice as hard as a half-marathon at the same pace. But as a first approximation, a linear relationship between duration and training load makes sense.
The same is not true for intensity, however, which seems to produce an exponential increase in training load – that is, a relationship that curves upward with increasing steepness. Double your speed, and your workout will get more than twice as hard.
This insight is a reminder that, when it comes to putting together a sustainable workout routine, getting the intensity wrong can be a killer. Going a little too hard will leave you more exhausted than going a little too long, so err on the side of caution with your pacing.
More generally, the lack of a universal relationship between intensity and duration is a good argument that you should include both in your exercise plan, since they have different fitness benefits. A time-tested approach popular among top endurance athletes is to spend 80 per cent of your weekly exercise time doing long, slow efforts, and 20 per cent going short and fast.
As for how to measure these efforts, the simplest approach that Matomaki studied may well be the best: At the end of each workout, rate your overall exertion on a scale from one to 10. Easy sessions should rate five or lower, hard ones eight or more. Whatever the details of the workout, your body and brain know hard they’ve been pushed – no math required.
Alex Hutchinson is the author of the forthcoming book The Explorer’s Gene: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map.
