Design and clinical implementation of an open-source bionic leg.

In individuals with lower-limb amputations, robotic prostheses can increase walking speed, and reduce energy use, the incidence of falls and the development of secondary complications. However, safe and reliable prosthetic-limb control strategies for robust ambulation in real-world settings remain out of reach, partly because control strategies have been tested with different robotic hardware in constrained laboratory settings. Here, we report the design and clinical implementation of an integrated robotic knee-ankle prosthesis that facilitates the real-world testing of its biomechanics and control strategies. The bionic leg is open source, it includes software for low-level control and for communication with control systems, and its hardware design is customizable, enabling reduction in its mass and cost, improvement in its ease of use and independent operation of the knee and ankle joints. We characterized the electromechanical and thermal performance of the bionic leg in benchtop testing, as well as its kinematics and kinetics in three individuals during walking on level ground, ramps and stairs. The open-source integrated-hardware solution and benchmark data that we provide should help with research and clinical testing of knee-ankle prostheses in real-world environments.

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.

Amputee perception of prosthetic ankle stiffness during locomotion.

BACKGROUND: Prosthetic feet are spring-like, and their stiffness critically affects the wearer's stability, comfort, and energetic cost of walking. Despite the importance of stiffness in ambulation, the prescription process often entails testing a limited number of prostheses, which may result in patients receiving a foot with suboptimal mechanics. To understand the resolution with which prostheses should be individually optimized, we sought to characterize below-knee prosthesis users' psychophysical sensitivity to prosthesis stiffness. METHODS: We used a novel variable-stiffness ankle prosthesis to measure the repeatability of user-selected preferred stiffness, and implemented a psychophysical experiment to characterize the just noticeable difference of stiffness during locomotion. RESULTS: All eight subjects with below-knee amputation exhibited high repeatability in selecting their Preferred Stiffness (mean coefficient of variation: 14.2 ± 1.7%) and were able to correctly identify a 7.7 ± 1.3% change in ankle stiffness (with 75% accuracy). CONCLUSIONS: This high sensitivity suggests prosthetic foot stiffness should be tuned with a high degree of precision on an individual basis. These results also highlight the need for a pairing of new robotic prescription tools and mechanical characterizations of prosthetic feet.

Stiffness Perception During Active Ankle and Knee Movement.

OBJECTIVE: Recently, human joint impedance-the instantaneous mechanical response to a perturbation-has been quantified during gait, providing new insight beyond the traditional biomechanical descriptions of kinetics and kinematics. However, the role of joint impedance in neuromotor control and the development of exoskeletons and other wearable robotic systems remains unknown. One approach to studying the role of impedance in neuromotor control involves characterizing the human ability to discriminate changes in external impedance properties. Thus, the purpose of this work is to quantify the minimum detectable change in the stiffness component of impedance when interacting with an external mechanical impedance at the human ankle or knee. METHODS: A dynamometer coupled to subjects' right ankle or knee rendered the dynamics of a virtual rotational spring-mass-damper system. The minimum detectable change, or just noticeable difference, was determined via a weighted up-down staircase method in which subjects compared the stiffness values of two different controller configurations. RESULTS: We found that subjects could reliably detect stiffness changes of at least 12% at the ankle and 13% at the knee. CONCLUSION: Stiffness errors or variations produced by an external mechanical device will be undetected if they remain below the 12-13% threshold. SIGNIFICANCE: Our results provide novel insight into how the sensorimotor system senses joint impedance, information that may improve the design and control of impedance-based wearable robotic technologies.