Altered Neural Control Reduces Shear Forces and Ankle Impedance on a Slippery Surface

Objective: This study investigated the consequences of reduced ankle muscle activity on slippery surfaces. We hypothesized that reduced activation would reduce shear forces and ankle impedance to improve contact and reduce slip potential. Methods: Data were collected from unimpaired adults walking across non-slippery and slippery walkways. Set within the walkway was a robotic platform with an embedded force plate for collecting shear forces and estimating the mechanical impedance of the ankle; impedance was characterized by a model with stiffness, damping, and inertia. Results: We found a significant reduction in shear force due to reduced muscle activity in late mid-stance. We found no significant difference in stiffness between the surfaces. However, the muscle activation changes that contributed to shear force modulation occurred in late mid-stance, where reliable impedance estimates could not be made due to the foot leaving the measurement platform. When impedance could be measured, we found that a change in muscle activity predicted a change in stiffness, providing indirect estimates that stiffness was likely reduced later in stance. Conclusion: These results suggest that reduced muscle activity on slippery surfaces serves to reduce shear forces, and possibly also stiffness, during late mid-stance. Significance: These results have implications for identifying and training likely fallers, and possibly for designing prosthetic systems that help prevent falls when walking across different terrains.