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Sandcastles: A mental model of endurance training

Training endurance is like building a sandcastle. You need a minimum volume of "sand", intensity to provide some "water", and the expertise not to overdo it. If you get the mix wrong, your castle will collapse.

— Scott Semple

An illustration of an equation: a pile of dry sand + a water droplet + "expertise" = a sandcastle
Illustrations by Scott Semple

My first version of Sandcastles was published at Uphill Athlete on May 11th, 2018. This version is better.

The "sand" of endurance training is aerobic capacity, which primarily comes from high-volume, low-intensity training. Aerobic capacity is a gratifying "more is better" component, but it eventually runs into lifestyle constraints.

The "water" is intensity, which has to be applied in the right amount at the right time. It's a Goldilocks component; you need it just right—not too much, not too little.

The necessary expertise to combine the sand and water comes from experience, good coaching, or both.

Basic endurance is dry sand.

Basic endurance is like piling perfectly dry sand. Without any moisture content, the grains won't stick together. The sand is always piled in a cone.

With only a cone of sand, the returns diminish as the pile gets higher. Each additional inch of height requires a greater volume of sand to cover the previous one. Eventually—due to genetics or life constraints—you can't add any more.

An illustration of "cones" of sand on top of each other forming a pile to show that each unit of height requires a bigger cone.
Each additional inch for the pile demands a greater and greater amount of total sand.

Intensity is water.

Adding intensity to a training program is like adding water to a pile of sand. With some moisture, you can start to shape the sand.

Shaping the sand allows for taller piles and interesting shapes. You aren't limited to dry cones. It lets you steepen and sculpt the pile and increase the height without needing as much volume.

Adding some intensity is exciting and gratifying because:

  • You can shape and sculpt faster;
  • You can make the pile steeper—higher with less volume; and
  • It gives the illusion of a shortcut—changes happen quickly.
An illustration of an equation: dry sand + water = steeper pile
Adding water increases the angle of repose, reducing the additional volume required for an additional unit of height.

If a little water is good, is a lot of water better?

With sandcastles, obviously not. But what's an obvious mistake with a sandcastle is just a little too tempting in training.

With sandcastles, no one says:

It's falling over! Add more water!

or

If water is better than sand, let's just use water!

But self-limited defined intensity is also fun—true high-intensity is not—so the temptation in training is hard to see for what it is: an illusion.

Too much water is destructive, not productive. (...unless your selling short-term-focused exercise fads. Then, gratification via intensity is The Money.)

An illustration of a shower of water onto an oozing pile of sand.
"More water!" is a great way to wreck a sandcastle.

A Stronger, Smarter Approach

  • Basic endurance is the foundation—it must continually grow.
  • Intensity refines the structure—but it can't stand alone.
  • Great training is about balance—knowing when to add water and when to add more sand.