Time-Varying Ankle Mechanical Impedance During Human Locomotion

Area of Science:

• Biomechanics
• Human Locomotion
• Neuroscience

Background:

• Human locomotion requires continuous modulation of joint mechanical impedance for stability.
• Ankle mechanical impedance is critical at the neuro-mechanical system's interface with the environment.
• Previous studies characterized steady-state ankle impedance, but dynamic variations during walking remained largely uncharacterized.

Purpose of the Study:

• To characterize human ankle mechanical impedance in two degrees-of-freedom simultaneously over time during walking.
• To investigate dynamic changes in ankle viscosity and stiffness during key gait events like heel-strike and toe-off.
• To explore the role of the central nervous system in modulating ankle impedance during locomotion.

Main Methods:

• Utilized a wearable ankle robot, Anklebot, for data collection.
• Employed ensemble-based linear time-varying system identification methods.
• Estimated time-varying ankle mechanical impedance from pre-swing through early stance phases, including heel-strike and toe-off.

Main Results:

• Ankle mechanical impedance (viscosity and stiffness) was accurately modeled as a second-order system in both inversion-eversion and dorsiflexion-plantarflexion.
• Viscosity and stiffness significantly decreased before toe-off, remained constant during swing, and increased around heel-strike.
• Increased viscosity and stiffness preceding heel-strike provide evidence of central nervous system "pretuning".

Conclusions:

• Human ankle mechanical impedance is time-varying throughout the gait cycle.
• The central nervous system actively modulates ankle impedance, particularly demonstrating anticipatory "pretuning" before heel-strike.
• Findings have implications for understanding neurologically impaired locomotion and developing clinical interventions.