Impedance of the Human Ankle During Standing for Posture Control

Abstract

BACKGROUNDThe stiffness and damping of anatomical joints can be modulated by muscle co-contraction, where antagonistic muscles contract simultaneously, increasing both the joint's stiffness and damping. In a second order system, the mechanical impedance, or simply impedance, is a function of the system's inertia, damping, and stiffness. The ankle impedance can be defined as the resultant force due to an external motion perturbation. The impedance modulation of the human ankle is required for stable walking. The estimation of the time-varying impedance modulation of the human ankle is the focus of research by different groups [1,2].The human body can be modeled as an inverted pendulum with a rocker shaped based [3]. In this model, the body is unstable if the radius of the rocker base is smaller than the height of the center of the mass. This means that the body is unstable during walking and standing, unless external forces are applied [4]. The ankle quasi-stiffness and the rocker radius are directly related, meaning a larger quasi-stiffness will result in a larger rocker radius. The quasi-stiffness of a joint is defined as the slope of the torque angle curve of a joint during an activity, and it is not necessarily the same as the stiffness. It has been shown that for stability, the rocker shape radius and quasi-stiffness in the sagittal plane are larger during stance than during walking. The required quasi-stiffness are 458 N.m/rad for stable walking and 1432 N.m/rad for standing in a 71 kg person and a leg length of 1 m [4].A previous study has shown that, different from stable walking, ankle stiffness on the sagittal plane during quiet standing is not neurally controlled to maintain postural stability [5]. Instead, the main mechanism for balancing is the modulation of the ankle torque by the neural system, with a very small stiffness variation. This paper studies the ankle properties during standing, analyzing whether the stiffness and damping

Frontal plane ankle stiffness increases with axial load independent of muscle activity

“…Our measures of ankle stiffness at axial loading equivalent to 50% body weight (0.055 ± 0.004 Nm/rad/N) are within the range of other studies that quantified frontal plane ankle stiffness at 50% BW [.044 -0.112 Nm/rad/N] (A. Ribeiro et al, 2018;Matos et al, 2021;Nalam et al, 2021). Our results also agree with previous studies showing that frontal-plane ankle stiffness increased with increasing weight on the ankle during standing (Matos et al, 2021;Nalam et al, 2021).…”

Frontal plane ankle stiffness increases with axial load independent of muscle activity

“…Our measures of ankle stiffness at axial loading equivalent to 50% body weight (0.055 ± 0.004 Nm/rad/N) are within the range of other studies that quantified frontal plane ankle stiffness at 50% BW [.044 -0.112 Nm/rad/N] (A. Ribeiro et al, 2018;Matos et al, 2021;Nalam et al, 2021). Our results also agree with previous studies showing that frontal-plane ankle stiffness increased with increasing weight on the ankle during standing (Matos et al, 2021;Nalam et al, 2021).…”

Sex Differences in Human Ankle Stiffness During Standing Balance

“…In order to overcome this limitation, robotic platforms, capable of applying perturbations to the ankle, have been used to characterize 2D ankle stiffness during standing balance (Ficanha et al, 2016;Nalam and Lee, 2019). These studies have demonstrated that ankle stiffness in the sagittal plane is significantly higher than in the frontal plane even during standing balance, but the difference is more pronounced than that in the seated studies Lee, 2017, 2018;Ribeiro et al, 2018).…”