Your leg muscles automatically act to stop you falling when you trip

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Miss a step when walking down the stairs, and your legs will attempt to recover your balance after the unexpected fall – but how? The key to remaining upright seems to be in the way our calf and foot muscles are activated.

“One of the things we know about human locomotion is our ability to stay on our feet, upright is pretty remarkable, but we don’t understand a lot about how we achieve this,” says Taylor Dick at the University of Queensland in Australia.

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To find out more, she and her colleagues conducted an experiment that involved attempting to make people fall over. The team had ten people jump in place on top of platforms sitting on a device that measured the forces exerted by each foot individually. They then removed the platforms without warning.

As participants tried to retain their balance, the team used electromyography and ultrasound sensors on their legs to track muscle activity and changes in muscle length.

They determined that experiencing an unexpected drop automatically increases the timing between when our muscles first activate and when they reach their shortest length.

This in turn enables the foot muscles to absorb and dissipate energy more effectively, allowing us to recover from the drop.

The team also found that while opposing muscles normally contract in turn when walking, both groups of muscles contract at the same time during an unexpected drop.

In cases where you aren’t able to successfully recover and end up falling, she says it may be because a different strategy is used, one that relies on signals travelling from your leg muscles to your brain and then back to your leg muscles, which may take more time than it does to travel the distance to the new “lower” ground.

Dick hopes that this research can inform the design of lower limb assistive devices such as prostheses and exoskeletons that can help people navigate staircases and move over uneven terrain.

Homayoon Kazerooni at the University of California, Berkeley says that the insights into the timing changes of human muscle activation could lead to better exoskeleton designs, including control algorithms that offer better stability over unpredictable terrain or at least help with recovering from a fall.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2021.0201

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