Multivariable Dynamic Ankle Mechanical Impedance With Active Muscles

Abstract

Multivariable dynamic ankle mechanical impedance in two coupled degrees-of-freedom (DOFs) was quantified when muscles were active. Measurements were performed at five different target activation levels of tibialis anterior and soleus, from 10% to 30% of maximum voluntary contraction (MVC) with increments of 5% MVC. Interestingly, several ankle behaviors characterized in our previous study of the relaxed ankle were observed with muscles active: ankle mechanical impedance in joint coordinates showed responses largely consistent with a second-order system consisting of inertia, viscosity, and stiffness; stiffness was greater in the sagittal plane than in the frontal plane at all activation conditions for all subjects; and the coupling between dorsiflexion–plantarflexion and inversion–eversion was small—the two DOF measurements were well explained by a strictly diagonal impedance matrix. In general, ankle stiffness increased linearly with muscle activation in all directions in the 2-D space formed by the sagittal and frontal planes, but more in the sagittal than in the frontal plane, resulting in an accentuated “peanut shape.” This characterization of young healthy subjects’ ankle mechanical impedance with active muscles will serve as a baseline to investigate pathophysiological ankle behaviors of biomechanically and/or neurologically impaired patients.

Energetic Passivity of the Human Ankle Joint

“…The transformation was performed because estimates of the off-diagonal components of the transfer matrix H in the original joint coordinates were not reliable due to negligible coupling between 2 DOFs of the ankle [35,44]. In fact, the choice of u and y is coordinate independent, since passivity analysis deals with power exchange (u The impedance matrix was estimated using torques at the ankle and corresponding angular displacements [44].…”

“…Details of the experimental setup have been provided previously [35]. In summary, a wearable ankle robot (Anklebot, Interactive Motion Technologies Inc., Watertown, MA) was used in both experiments.…”

“…Measurements were made at different target levels of TA and SOL activity, from 10% to 30% MVC with increments of 5% MVC. A simple impedance controller with a proportional gain 2000 N/m (in joint coordinates, this corresponds to 37.2 and 76.0 Nm/rad for IE and DP, respectively) was implemented for each actuator to hold the ankle near its initial position against active ankle torques and combined with the random perturbations [35].…”

Predictability and Robustness in the Manipulation of Dynamically Complex Objects

“…This is difficult as variability and noise in the human system is high, with an approximate precision in timing of 9 ms (Faisal et al 2008; Cohen and Sternad 2012). Instead, intrinsic musculo-skeletal properties augmented by spinal reflexes deliver essentially instantaneous reaction and can provide stabilization to counteract noise or instability (Colgate and Hogan 1988; Burdet et al 2001; Franklin et al 2003; Selen et al 2009; Lee et al 2014). While mechanical impedance is essential, dexterous control in the presence of delays nevertheless requires one to anticipate, preempt, and exploit the forces and motions of an object.…”

An estimation of strength ratios of antagonistic muscle groups based on variable moment arm

“…Characterization of limbs is useful to understand physical interaction between human and environment, because human interacts with environment by using upper and lower limbs [1]. Strength of limb muscles is evaluated by estimation except for invasive ways.…”