Asphalt isn't what will injure you

“Run on dirt, asphalt wrecks your knees” is one of the most repeated pieces of advice. And part of it is measured and true: put sensors on, and impact forces are higher on concrete than asphalt, higher on asphalt than dirt.

And yet

When you count injuries instead of measuring forces, the relationship collapses. Reviews of risk factors in runners don’t find that running on hard surfaces is associated with more injuries.

The reason is that the leg changes its stiffness according to what it lands on. On hard ground, knee and ankle flex a little more and absorb it themselves; on soft ground, the opposite. The system self-regulates, and much of the difference the sensor measures never reaches the bone.

What the surface does change

• Uneven ground reduces repetitive-strain injuries but raises acute ones: rolled ankles and falls.
• A treadmill is more even and a bit softer, but it removes variation: the same movement every stride, the same landing every time.
• Always running on the same side of a cambered road does load one leg differently from the other. That isn’t hardness: it’s slope.

What actually predicts an injury

The same things as always, and none of them is the surface: how sharply you raised your load, how many weeks you’ve been running, and whether you’ve been injured before. Swapping asphalt for dirt doesn’t offset a forty per cent jump in weekly mileage.

What to do tomorrow

• Run wherever you feel like and wherever it’s safe. That isn’t the decision that matters.
• If you like trails, mix it up: varied ground spreads the work across more tissues.
• If you’re new to uneven ground, go slower the first few times. The risk there is your ankle, not your knee.