Imagine yourself walking on a nice, sun-drenched beach. You hear seagulls, you feel the sun and you are chatting with friends. Now, for a second, think about how you move. How do you walk over the loose sand? And on the packed sand closer to the water? Well, you probably hadn't thought about it as most of our movements occur unconsciously and, even if you did think about it, I am here to tell you that we know remarkably little about how we move on surfaces that are not solid, because most research on movement occurs in laboratory settings on solid surfaces. Chuchu Zhu and colleagues from Rutgers University, USA, realized the same, so they decided to bring the outside world into the lab to compare how people walk on solid and sandy surfaces.
To test how humans adjust their walking strategy, the team used a 3-segmented trackway that had a solid section, followed by a sandy section, with a solid surface at the end, mimicking some of the conditions that we experience in the real world. Then they asked 20 participants to walk along the trackway while they videoed the participants’ movements to identify posture differences when walking on the solid surface and sand. In addition, the researchers used force platforms in the trackway to measure the forces exerted by the participants as they moved across the different surfaces. The team then analyzed the force measurements, to identify when the walkers’ heals hit the ground and when their toes pushed off as the foot left the ground, and the video recordings, to reconstruct how the walkers’ joints were moving, to identify which leg joints power the movement. Additionally, the team tracked the position and velocity of each individual's center of mass, to see how much it wobbled about as the walker's movements became more unstable.
Zhu and colleagues found that the participants that were walking on sand kept their feet in contact with surface longer than on the solid surface, potentially compensating for walking on the more unstable ground. In addition, the walkers moved with a higher average walking speed on sand, which the researchers suggest is due to the fact that people want to minimize the time they spend on harder-to-walk-on sandy surfaces. The center of mass of the body also wobbled around more while walking on the sand. Additionally, the time the feet were on the ground and the width of each stride (i.e. how far apart the feet are) increased when the participants were walking on sand. The researchers suggest this is because of the trade-off between minimizing the amount of energy used during walking and maintaining stability. The team also noticed that the impact forces when the foot hit the ground increased on sand and the way the ankle moved changed, suggesting that the ankle joint plays a key role in maintaining stability while walking on sand.
Scientists have studied animal movement for centuries, going back all the way to Aristotle, who wrote in his book ‘Movement of Animals’ about the general principles of locomotion, yet we are barely beginning to scratch the surface of understanding how we move over more complex terrains, such as loose sandy surfaces. Therefore, studies like this, where we bring real-world conditions to the laboratory, are fundamental to our understanding of natural maneuvers, as most of our daily movements occur either on uneven – think broken sidewalks or stairs – or loose surfaces. Insights into movement and compensation strategies on uneven or loose surfaces will also provide key knowledge to improve assistive devices and might spark novel robotic designs that can assist people in complex terrains.
