Why a 8.7 km tempo run counts as 10 km

In short

• On balance a fast kilometre therefore costs your tissue more than an easy one, even though the same figure appears on your screen.
• A load kilometre is a kilometre that counts for what it costs you rather than for how long it is.
• Over 8.7 kilometres that lands around ten load kilometres.
• The heavier of those two counts, because a run-walk build-up lives in the minutes and a tempo week lives in the kilometres.

1.A kilometre is not a fixed measure

On your watch a kilometre is distance, and every kilometre is as long as the one before it. To your tendons, bones and muscles it is something else: a series of landings, and every landing carries a force.

Run faster and your stride lengthens, so you land less often per kilometre. But you push off harder and spend longer off the ground, so each landing hits harder. Research into running mechanics shows that the force per step rises faster than the number of steps falls. On balance a fast kilometre therefore costs your tissue more than an easy one, even though the same figure appears on your screen.

That explains something you probably recognise. Two weeks with exactly the same distance can feel very different, and the week with the tempo run in it is the heavy one.

2.What a load kilometre is

A load kilometre is a kilometre that counts for what it costs you rather than for how long it is. The reference for that is your own easy running pace: at that pace a kilometre weighs exactly one load kilometre. Run faster and the same kilometre weighs more; run easier and it weighs less.

That reference is called your reference pace, and it comes from your own easy runs of recent weeks. So it is not a norm and not a target, but a description of how you run when you run easy. Get faster and it shifts with you, so the comparison between this week and your habit keeps holding.

Gradient counts in the same way. Uphill the same kilometre costs more, so it counts as a longer flat kilometre. That is the same conversion you know from your grade-adjusted pace, and it looks at the metres you climbed.

With those three pieces you have the whole measure: your kilometres, your pace relative to your own easy pace, and the climbing along the way.

3.Back to those 8.7 kilometres

Take a run with roughly half of it at tempo and the rest easy. It counts about a seventh heavier than the same distance done entirely easy. Over 8.7 kilometres that lands around ten load kilometres.

Turn it around and you see why that is fairer. Someone running ten kilometres easy and someone running 8.7 with a solid block in it have not done the same thing. On distance alone they look like equals, and the second run is the heavier of the two.

4.What your plan does with it

Your running build-up puts your last seven days next to what you carried in recent weeks, and attaches one word to it: helping, neutral, build-up or holding you back. What those words mean is set out in the article on build-up in your running build-up.

Now that the measurement reads your weighted kilometres, it does so next to your time on your feet. The heavier of those two counts, because a run-walk build-up lives in the minutes and a tempo week lives in the kilometres. A week with a lot of fast work therefore reaches build-up sooner than it would on distance alone.

You see it back in the Running build-up sheet. It shows your base, your week and the band you are in, and it says that pace and gradient have been weighed in. If too few of your runs carry a pace, the sheet says the reading rests on your kilometres and your time on your feet for now.

Frequently asked questions

In the app

Further reading

Trimetra weighs your running kilometres by pace and gradient, so a week with fast work in it weighs as much in your plan as it did in your legs.