As any beachgoers will tell you, running and walking on sand leaves you much more tired than covering the same distance on a hard surface.
This isn’t because beach cricket is more competitive than backyard cricket – it’s because people use more energy walking across sand.
Sand is a “compliant” surface: it deforms when weight or pressure is put on it.
There are a number of different biomechanical explanations for the extra energy use – like changes in gait or interruptions to the pendulum-like motion of legs. But it’s hard to test consistently – shoes, different surfaces, and different instructions can all lead to different data.
In fact, most human movement research is done on hard surfaces, meaning researchers don’t have a clear understanding of how people move on compliant surfaces like sand.
Now, a study published in Journal of the Royal Society Interface has brought us more data.
Researchers based at the University of Liverpool in the UK, asked 30 healthy volunteers to walk across three different types of terrain.
These included a hard floor, a thin foam, and a thicker foam, representing different levels of compliant surfaces.
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Participants spent seven minutes walking across each surface, while their motions and oxygen intake were recorded, as well as the forces under their feet.
This data was used to build computer simulations of humans walking on each surface, allowing researchers to study how different muscles worked as people moved.
They found that the softer surfaces provoked longer strides, making the muscles in hip and knee joints work harder. This is what uses up more energy, theorise the researchers.
Unlike some previous studies had suggested, the researchers didn’t find that pendulum motions, or the mechanics of the lower leg, had much to do with the extra energy consumption.
Instead, they write in their paper, they have a “more intricate explanatory model for increased energetic costs of walking on compliant terrains”.
The researchers are interested in using their simulation technique to investigate the problem further, and possibly test it out on other animals to see if they move differently.