Mechanisms for maintaining balance during locomotion
Final Report Abstract
The goal of this project was to understand how the human central nervous system uses sensory information from different sensory organs to maintain balance during walking. The human body is dynamically unstable during walking, meaning that to prevent falling, the central nervous system must constantly regulate the movements of the body by modulating the force with which the feet push against the ground at each step. For instance, when the body starts to lean towards the left, the ground reaction force has to be changed to push it back towards the right. One way to do that is to shift the foot placement at the next step more towards the left, which results in gravity pulling the body more to the right. This control process, called foot placement mechanism, is well understood mechanically and has been applied successfully in walking robots, but the details of how humans apply it are unclear. Furthermore, the foot placement mechanism is limited to only act when a new step is made, and it was unknown what humans do to regulate balance during periods of single or double stance between the discrete events of placing or lifting a foot. To study these questions, we asked volunteers to walk on a treadmill in a virtual reality environment. We perturbed the participants’ sense of balance by either rotating the virtual world around them as if they were falling sideways, or by sending a mild electrical current between two electrodes behind the ears, affecting the vestibular system in the inner ear that senses body orientation and movement, which also induces the sensation of falling sideways. We then measured how participants changed their muscle activation the resulting ground reaction force on the treadmill, and movements of the body in response to the stimuli. As expected, when participants perceived a fall to the right they tended to shift the foot placement at the next foot to the right. Surprisingly, this movement of the foot during swing was not generated by a simple outward shift of the swing leg at the hip, but rather by a coordinated rotation of the stance leg knee and swing leg hip, involving a rotation of the pelvis. During single stance, between taking steps, participants changed the activation of their lateral ankle musculature to adjust the degree at which they actively counter gravity. When on their left leg and perceiving a fall to the right, subjects would increase the leftward pull with their lateral ankle muscles to counter the perceived fall. We also found evidence that participants shift weight between their two legs during double stance. When perceiving a fall to the right, participants increased the push-off force of their right, trailing leg, pushing the body to the front and left more strongly than usual. This finding was surprising, because push-off force is generally understood to be important for propulsion rather than balance. In one experiment where we delivered the fall stimuli at different points during the gait cycle, we observed that subjects flexibly coordinate these three mechanisms to generate a single, functional response. When the stimulus arrived long enough before the next step, subjects shifted their foot placement in the direction of the perceived fall. When the stimulus arrived so close before a step that there was not enough time to still shift the foot placement, subjects used other mechanisms. In both cases, the resulting force with which they pushed against the ground to move their body against the perceived fall was the same.
Publications
- Complementary mechanisms for uprightbalance during walking. PLoS ONE, pages 1–16, 2017
Hendrik Reimann, Tyler D. Fettrow, Elizabeth D. Thompson, Peter Agada,B. J. McFadyen, and John J. Jeka
(See online at https://doi.org/10.1371/journal.pone.0172215.g001) - Neural Control of Balance During Walking. Frontiers in Physiology, 9 (September):1271, 2018
Hendrik Reimann, Tyler Fettrow, Elizabeth D. Thompson, and John J. Jeka
(See online at https://doi.org/10.3389/fphys.2018.01271) - Strategies for the Control of Balance During Locomotion. Kinesiology Review, 7(1):18–25, 2018
Hendrik Reimann, Tyler Fettrow, and John J. Jeka
(See online at https://doi.org/10.1123/kr.2017-0053)
