Why Running Tracks Are Spongy,  Not Rock Hard

Concrete feels fast. Grass feels slow. So the obvious conclusion is that harder surfaces make you quicker and softer ones slow you down, which makes the springy rubber surface under a modern running track look like a strange design choice.

It isn’t. Concrete and grass represent two different problems, for opposite reasons.

Two different properties: resilience and damping

A surface’s stiffness isn’t the whole story. What matters is what happens to the energy once you load the surface with your body weight.

There are two possible outcomes:

Resilience: the surface compresses under load, then gives that energy back as it springs back to shape, similar to a trampoline or a rubber ball.

Damping: the surface absorbs the energy and mostly doesn’t give it back, similar to memory foam or wet sand.

Grass and sand are highly damped. Push into them and much of that energy goes into deforming the ground rather than returning to you.

Concrete, by contrast, barely compresses at all. There is essentially no surface energy exchange, so it is not damped in the same way. Instead, its rigidity means your legs have to deal with much of the impact themselves.

What the Harvard experiments found

In the late 1970s, Thomas McMahon and Peter Greene at Harvard set out to investigate this question by building test tracks with adjustable stiffness, ranging from rigid wooden boards to soft foam-rubber blocks.

As they softened the surface from fully rigid, foot-ground contact time and peak impact force both dropped, while running performance improved.

But this only held up to a point. Push the surface too soft, and contact time started increasing again rather than continuing to fall.

The best surface wasn’t the hardest available, or the softest. It was one roughly two to four times stiffer than a runner’s own leg, creating a useful middle ground between the two extremes.

Tracks built to this tuned stiffness at Harvard, Yale and Madison Square Garden delivered measurable results: running times improved by around two to three percent, while injury rates were roughly half those seen on rigid surfaces such as asphalt.

Why softer, within limits, can lower effort

A separate study measured this directly by having runners cross surfaces of varying stiffness on an adjustable platform.

As the surface became more compliant across the range tested, runners’ metabolic cost dropped by 12%, while their leg stiffness increased by 29%.

The researchers attributed the improved running economy to increased energy rebound from the more compliant surface. Some of the effort put into the ground on impact was returned on the next push-off, rather than needing to be generated again by the muscles alone.

This is the same elastic energy-return concept covered in our guide to how shoe shape changes running form, but applied to the ground beneath you rather than the shoe on your foot.

Why concrete and grass fail in opposite directions

Concrete gives very little back. Because it barely deforms, there is little surface spring effect to return energy, leaving your legs to deal with much of the impact themselves.

Grass and sand give plenty, but lose most of it. Because they are highly damped rather than highly resilient, much of the energy that goes into compressing them is lost to ground deformation instead of being returned as the surface springs back.

You can feel this directly: run on sand and notice how much more effort each stride takes despite the surface being soft.

A tuned track sits between these extremes. It is firm enough to be resilient rather than highly damped, so it can return some of the energy put into it, but compliant enough to reduce the peak impact your legs would otherwise have to absorb.

That combination, relatively low impact alongside useful energy return, is difficult to achieve at either extreme.

Related reading

• How shoe shape changes your running form
• What is heel drop in running shoes?

This is general information about sports surface science, not personalised training or medical advice.