Stability is more important than energy expenditure when we walk, a study using robotics has found.
For most of us walking is a fundamental physical activity, yet many elements of the biomechanics of the process remain poorly understood.
Investigating the precise dynamics of walking has lately become the subject of intense attention, as the growing use of robotic exoskeletons in spinal injury rehabilitation throws up urgent questions about what pressures the robots should apply to patients’ legs to optimise recovery.
In research led by Paolo Bonato of Harvard University’s Wyss Institute for Biologically Inspired Engineering, healthy volunteers strapped into exoskeletons and ran on treadmills to test how externally applied forces affected their gait.
The researchers found that participants adjusted their gait when the robots forced a change to the length of their step. A change to step height produced no alteration to gait – even if height and length were adjusted simultaneously.{%recommended 3637%}
The result was surprising, but the scientists suggest it can be explained by the way in which a person’s central nervous system (CNS) responds to threats to stability.
“Lifting your foot higher mid-stride doesn’t really make you that much less stable, whereas placing your foot closer or further away from your centre of mass can really throw off your balance,” explains Giacomo Severini, co-author of the paper published in the journal Science Robotics. “So the body adjusts much more readily to that disturbance.”
Preserving stability is prioritised by the brain, the paper says, even when doing so results in increased energy use and disruption to walking pattern.
The authors suggest this is because the potential consequences of stability loss are severe, including sprains and fractures.
The findings have implications for the use of exoskeletons in rehabilitation. Programming them to improve step height during recovery might be of little therapeutic benefit, because changes do not prompt the CNS to modify gait.
Instead, the researchers note, exoskeletons may need to be re-engineered so that they can combine height adjustments with threats to stability.
Says Severini: “To modify step height you’d need to design forces so that the change in height, which the brain normally interprets as neutral, becomes challenging to the patient’s balance.”
