Multivariable dynamic ankle mechanical impedance with active muscles.

Area of Science:

• Biomechanics
• Human Motor Control
• Musculoskeletal System

Background:

• Understanding ankle mechanical impedance is crucial for diagnosing and treating biomechanical and neurological impairments.
• Previous research characterized the relaxed ankle's mechanical impedance, but active muscle contributions remain less understood.

Purpose of the Study:

• To quantify multivariable dynamic ankle mechanical impedance in two coupled degrees-of-freedom (DOFs) with active muscles.
• To establish a baseline for characterizing healthy ankle impedance to investigate pathological conditions.

Main Methods:

• Measurements of ankle mechanical impedance were taken at five target activation levels (10-30% MVC) for tibialis anterior and soleus muscles.
• A second-order system model (inertia, viscosity, stiffness) was used to analyze impedance in joint coordinates.
• Coupling between dorsiflexion-plantarflexion and inversion-eversion was assessed.

Main Results:

• Ankle impedance with active muscles exhibited behaviors consistent with a second-order system.
• Stiffness was consistently greater in the sagittal plane than the frontal plane across all activation levels.
• A small coupling between the two DOFs was observed, well-explained by a diagonal impedance matrix.
• Ankle stiffness increased linearly with muscle activation, more so in the sagittal plane, creating an accentuated "peanut shape."

Conclusions:

• Active muscle contraction significantly influences ankle mechanical impedance.
• The characterized impedance of healthy, active ankles provides a critical baseline for future research on patient populations.
• Findings highlight the anisotropic and direction-dependent nature of ankle stiffness during voluntary muscle activation.