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Session: Joints
Session starts: Friday 25 January, 13:00
Presentation starts: 13:00
Room: Lecture room 535

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Ronald C. van 't Veld (University of Twente)
Alejandro Moya Esteban (University of Twente)
Alfred C. Schouten (University of Twente)

Abstract:
Introduction Human limb mechanical properties can be quantified using joint impedance, the dynamic relation between joint angle deviations and corresponding joint torque, with joint stiffness as its position-dependent component. Joint stiffness is key in postural control and movement and well understood within (quasi-)static conditions [1]. However, muscle and joint mechanics change during functional movements, consequently also the joint stiffness changes [2,3]. Monitoring and understanding the continuous modulation of joint stiffness during functional movements in able-bodied subjects has several technical and clinical applications. For example, it can be applied to the design of next-generation biomimetic prostheses and exoskeletons or to advance clinical assessment and decision making. Therefore, the goal of this study is to develop an experimental protocol to assess how humans modulate ankle joint stiffness in time-varying conditions. Methods Six healthy adults (5 male, 24.2±1.0y) participated in this study. Participants were seated with their right foot connected to a single axis actuator using a footplate. Muscle activity of the soleus, gastrocnemius and tibialis anterior muscles was recorded using electromyography. The experiment consisted of three conditions: a 5Nm amplitude sinusoidal torque tracking task against a rigid actuator (static posture), and a 0.15rad amplitude slow (0.3Hz) and fast (0.6Hz) sinusoidal position tracking task against a virtual spring resulting in a torque amplitude of 9Nm (dynamic posture). Additionally, pseudo-random binary sequence (PRBS) perturbations were applied during the tasks (±0.015 rad, 0.15s switching time). System identification was performed using an ensemble-based algorithm, which produced ankle joint stiffness estimates by means of multiple short segments of the torque and angle recordings [2]. Results and Discussion Across subjects and conditions two maximum stiffness peaks, corresponding to plantarflexion and dorsiflexion phases, were observed followed by periods of decreased stiffness. The periods of high and low stiffness matched periods of high and low muscle activation. The average estimated joint stiffness magnitude across conditions ordered from low to high was: fast dynamic, static and slow dynamic posture. Our results highlight the variability in stiffness modulation strategies across conditions, especially across movement frequency. References [1] Kearney RE, Hunter IW. Crit Rev Biomed Eng. 1990; 18(I):55-87. [2] Ludvig D et al. Exp Brain Res. 2017; 235(10):2959-70. [3] Dietz V, Sinkjaer T. Lancet Neurol. 2007; 6(8):725-33.