Characterization and clinical implications of ankle impedance during walking in chronic stroke.

Individuals post-stroke experience persisting gait deficits due to altered joint mechanics, known clinically as spasticity, hypertonia, and paresis. In engineering, these concepts are described as stiffness and damping, or collectively as joint mechanical impedance, when considered with limb inertia. Typical clinical assessments of these properties are obtained while the patient is at rest using qualitative measures, and the link between the assessments and functional outcomes and mobility is unclear. In this study we quantify ankle mechanical impedance dynamically during walking in individuals post-stroke and in age-speed matched control subjects, and examine the relationships between mechanical impedance and clinical measures of mobility and impairment. Perturbations were applied to the ankle joint during the stance phase of walking, and least-squares system identification techniques were used to estimate mechanical impedance. Stiffness of the paretic ankle was decreased during mid-stance when compared to the non-paretic side; a change independent of muscle activity. Inter-limb differences in ankle joint damping, but not joint stiffness or passive clinical assessments, strongly predicted walking speed and distance. This work provides the first insights into how stroke alters joint mechanical impedance during walking, as well as how these changes relate to existing outcome measures. Our results inform clinical care, suggesting a focus on correcting stance phase mechanics could potentially improve mobility of chronic stroke survivors.

Ankle Mechanical Impedance During the Stance Phase of Running.

OBJECTIVE: Differences in locomotor biomechanics between walking and running provide fundamental information about human ambulation. Joint mechanical impedance is a biomechanical property that governs the body's instantaneous response to disturbances, and is important for stability and energy transfer. Ankle impedance has been characterized during walking, but little is known about how humans alter joint impedance during running. The purpose of this study was to estimate ankle impedance during the stance phase of running, and compare to previously reported estimates during walking. METHODS: Perturbations were applied to the ankle using a one-degree-of-freedom (DOF) mechatronic platform. Least-squares system identification was performed using a parametric model consisting of stiffness, damping, and inertia. RESULTS: The model accounted for 89% ± 16% of variance. Ankle stiffness reached a maximum of 10 Nm/rad/kg at the end of mid-stance, decreasing in terminal stance phase to values previously reported during swing phase. Quasi-stiffness values differed significantly from stiffness across the stance phase of running. Comparing ankle impedance estimates between walking and running showed differences in both magnitude, and temporal variation. CONCLUSION: Ankle impedance differs significantly between walking and running. SIGNIFICANCE: This study provides novel information about the biomechanics of running and broadens our understanding of how the mechanical impedance of the ankle joint differs between locomotor tasks, motivating the need for future studies.

Ankle Mechanical Impedance During Waling in Chronic Stroke: Preliminary Results.

Dynamic joint mechanics, collectively known as mechanical impedance, are often altered following upper motoneuron disease, which can hinder mobility for these individuals. Typically, assessments of altered limb mechanics are obtained while the patient is at rest, which differs from the dynamic conditions of mobility. The purpose of this study was to quantify ankle impedance during walking in individuals post-stroke, determine differences from the healthy population, and assess the relationship between impedance impairment and clinical outcome measures. Preliminary data were collected in four individuals post-stroke. Displacement perturbations were applied to the ankle during stance phase, and least-squares system identification was performed to estimate ankle impedance. In comparison to the healthy population, the paretic ankle showed reduced variation of stiffness during mid-stance of walking, and damping estimates during early and mid-stance were increased. Clinical measures obtained during dynamic tasks showed strong correlation with changes to the stiffness component of impedance, while clinical measures obtained passively were not correlated to stiffness. Impairment in ankle damping was not correlated with any of the measures tested. This work provides novel, preliminary insight into paretic ankle impedance during walking, differences from healthy data, and elucidates how current clinical metrics correspond to the true values of ankle stiffness and damping during gait.

Damping Perception During Active Ankle and Knee Movement.

The mechanical impedance of the leg governs many important aspects of locomotion, including energy storage, transfer, and dissipation between joints. These mechanical properties, including stiffness and damping, have been recently quantified at the ankle joint during walking. However, little is known about the human ability to sense changes in impedance. Here, we investigate the ability to detect small changes in damping coefficients when interacting with a mechanical system coupled to the ankle or knee joint. Using a psychophysical experiment (adaptive, weighted staircase method) and an admittance-controlled dynamometer, we determined the 75% minimum detectable change by tasking subjects to compare the damping values of different virtual spring-mass-damper systems. The Weber fraction for damping coefficient ranged from 12% to 31%, with similar performance across the ankle and knee. Damping perception performance was similar to previous stiffness perception results, suggesting that both the stiffness and damping of the environment are important for the human sensorimotor system and motivating further investigation on the role of damping in biomechanics, motor control, and wearable robotic technologies.

Perception of Mechanical Impedance During Active Ankle and Knee Movement.

During locomotion, energy flow through the legs is governed by the mechanical impedance of each joint. These mechanical properties, including stiffness and damping, have recently been quantified at the ankle joint. However, the relevance of these properties in human sensorimotor control is unclear. An important aspect of sensorimotor control is the ability to sense small changes in stimuli. Thus, we investigated the human ability to detect small changes in the stiffness and damping components of leg joint impedance when interacting with a mechanical system coupled to the ankle or knee. The perception threshold was determined via a psychophysical paradigm that required subjects to compare the mechanical impedance of virtual spring-mass-damper systems. Subjects reliably detected impedance changes of 11% and 12% at the ankle and knee, respectively. Additionally, the perception of stiffness and damping were comparable, indicating that the biomechanical relevance of the stiffness and damping components of impedance may be similar. Finally, these results offer novel insight into the design and control of impedance-based technologies, such as prostheses and exoskeletons.

Mechanical Impedance of the Ankle During the Terminal Stance Phase of Walking.

Human joint impedance describes the dynamic relationship between perturbation induced change in position and the resulting response torque. Understanding the natural regulation of ankle impedance during locomotion is necessary to discern how humans interact with their environments, and provide a foundation for the design of biomimetic assistive devices and their control systems. This paper estimates ankle impedance during terminal stance phase of walking using a parametric model consisting of stiffness, damping, and inertia. The model accurately described ankle torque, accounting for 90% ± 7.7% of the variance. Stiffness was found to decrease from 3.7 to 2.1 Nm/rad/kg between 75% and 85% stance. Quasi-stiffness-the slope of the ankle's torque-angle curve-showed a similar decreasing trend but was significantly larger at the onset of terminal stance phase. The damping component of impedance was constant during terminal stance phase, and was increased relative to values previously reported during early and mid-stance phases, indicating an increase in damping in preparation for toe-off. Inertia estimates were consistent with previously reported inertia values for the human ankle. This paper bridges a gap in our understanding of ankle impedance during walking, and provides new insight into how ankle impedance is regulated during regions when substantial mechanical energy is added.