Perceptions and biomechanical effects of varying prosthetic ankle stiffness during uphill walking: A case series.

BACKGROUND: Prosthetic foot stiffness, which is typically invariable for commercially available prosthetic feet, needs to be considered when prescribing a prosthetic foot. While a biological foot adapts its function according to the movement task, an individual with lower limb amputation may be limited during more functionally demanding gait tasks by their conventional energy storing and return prosthetic foot. RESEARCH QUESTION: How do changes in prosthetic foot stiffness during incline walking affect biomechanical measures as well as perception of participants. METHODS: Kinetic and kinematic data were collected during incline walking, for five participants with trans-tibial amputation. A mixed model analysis of variance was used to analyse the effects of changing the stiffness during incline walking, using a novel variable-stiffness unit built on a commercially available prosthetic foot. Biomechanical results were also analysed on an individual level alongside the participant feedback, for a better understanding of the various strategies and perceptions exhibited during incline walking. RESULTS: Statistically significant effects were only observed on the biomechanical parameters directly related to prosthetic ankle kinematics and kinetics (i.e., peak prosthetic ankle dorsiflexion, peak prosthetic ankle power, dynamic joint stiffness during controlled dorsiflexion). Participant perception during walking was affected by changes in stiffness. Individual analyses revealed varied perceptions and varied biomechanical responses among participants. SIGNIFICANCE: While changes in prosthesis mechanical properties influenced the amputee's experience, minimal immediate effects were found with the overall gait pattern. The reported inter-participant variability may be due to the person's physical characteristics or habitual gait pattern, which may influence prosthesis function. The ability to vary prosthetic foot stiffness during the assessment phase of setting up a prosthesis could provide useful information to guide selection of the appropriate prosthetic device for acceptable performance across a range of activities.

Task dependent changes in mechanical and biomechanical measures result from manipulating stiffness settings in a prosthetic foot.

BACKGROUND: Adaptation of lower limb function to different gait tasks is inherently not as effective among individuals with lower limb amputation as compared to able-bodied individuals. Varying stiffness of a prosthetic foot may be a way of facilitating gait tasks that require larger ankle joint range of motion. METHODS: Three stiffness settings of a novel prosthetic foot design were tested for level walking at three speeds as well as for 7,5° incline and decline walking. Outcome measures, describing ankle range of motion and ankle dynamic joint stiffness were contrasted across the three stiffness settings. Standardized mechanical tests were done for the hindfoot and forefoot. FINDINGS: Dorsiflexion angle was incrementally increased with a softer foot and a faster walking speed / higher degree of slope. The concurrent dynamic joint stiffness exhibited a less systematic change, especially during INCLINE and DECLINE walking. The small difference seen between the stiffness settings for hindfoot loading limits analysis for the effects of stiffness during weight acceptance, however, a stiffer foot significantly restricted plantarflexion during DECLINE. INTERPRETATIONS: Varying stiffness settings within a prosthetic foot does have an effect on prosthetic foot dynamics, and differences are task dependent, specifically in parameters involving kinetic attributes. When considering the need for increased ankle range of motion while performing more demanding gait tasks, a foot that allows the users themselves to adjust stiffness according to the task at hand may be of benefit for active individuals, possibly enhancing the user's satisfaction and comfort during various daily activities.

Variable stiffness foot design and validation.

Energy storing and returning prosthetic feet are commonly prescribed. Research has demonstrated advantages to use these types of prosthetic feet. However, their stiffness in the sagittal plane is fixed and cannot adapt to different walking tasks and user preference. In this paper, we propose a novel prosthetic foot design capable of modulating its stiffness in the sagittal plane. The Variable Stiffness Ankle unit (VSA) is mounted on a commercially available prosthetic foot. The stiffness of the foot is adjusted with a lightweight servo motor controlled wirelessly. The stiffness change is accomplished by moving the supports points on the glass fiber leaf spring of the VSA ankle unit. We described the design and characterized changes in ankle stiffness using a mechanical test bench. A novel method was used to capture mechanical test data using a six degree of freedom load cell, allowing us to contrast mechanical and biomechanical data. A transtibial unilateral amputee performed level ground walking on an instrumented treadmill. The VSA prosthetic foot exhibited ankle stiffness change in the mechanical test bench. Ankle stiffness changes were also confirmed during the biomechanical analysis. Future work will involve additional subjects. The VSA prosthetic foot could improve user satisfaction and help prosthetist to fine tune prosthetic feet during fittings.

Speed Adaptable Prosthetic Foot: Concept Description, Prototyping and Initial User Testing.

This article presents a novel design of a prosthetic foot that features adaptable stiffness that changes according to the speed of ankle motion. The motivation is the natural graduation in stiffness of a biological ankle over a range of ambulation tasks. The device stiffness depends on rate of movement, ranging from a dissipating support at very slow walking speed, to efficient energy storage and return at normal walking speed. The objective here is to design a prosthetic foot that provides a compliant support for slow ambulation, without sacrificing the spring-like energy return beneficial in normal walking. The design is a modification of a commercially available foot and employs material properties to provide a change in stiffness. The velocity dependent properties of a non-Newtonian working fluid provide the rate adaptability. Material properties of components allow for a geometry shift that results in a coupling action, affecting the stiffness of the overall system. The function of an adaptive coupling was tested in linear motion. A prototype prosthetic foot was built, and the speed dependent stiffness measured mechanically. Furthermore, the prototype was tested by a user and body kinematics measured in gait analysis for varying walking speed, comparing the prototype to the original foot model (non-modified). Mechanical evaluation of stiffness shows increase in stiffness of about 60% over the test range and 10% increase between slow and normal walking speed in user testing.

Assessing the Involvement of Users During Development of Lower Limb Wearable Robotic Exoskeletons: A Survey Study.

OBJECTIVE: To explore user-centered design methods currently implemented during development of lower limb wearable robots and how they are utilized during different stages of product development. BACKGROUND: Currently, there appears to be a lack of standardized frameworks for evaluation methods and design requirements to implement effective user-centered design for safe and effective clinical or ergonomic system application. METHOD: Responses from a total of 191 experts working in the field of lower limb exoskeletons were analyzed in this exploratory survey. Descriptive statistics were used to present responses and measures of frequency, and chi-square tests were used to contrast the answers of respondents who identified as clinicians versus engineers. RESULTS: A vast majority of respondents involve users in their development, in particular at the initial and iterative stages, although some differences were found between disciplines. A variety of methods and metrics are used to capture feedback from users and test devices, and although valuable, some methods used may not be based on validated measures. Guidelines regarding tests on safety of exoskeletons also lack standardization. CONCLUSION: There seems to be a consensus among experts regarding the importance of a user-centered approach in exoskeleton development; however, standardized frameworks with regard to appropriate testing methods and design approaches are lacking. Such frameworks should consider an interdisciplinary focus on the needs and safety of the intended user during each iteration of the process. APPLICATION: This exploratory study provides an overview of current practice among engineers and clinicians regarding the user-centered design of exoskeletons. Limitations and recommendations for future directions are identified.

Frontal plane pelvis and hip kinematics of transfemoral amputee gait. Effect of a prosthetic foot with active ankle dorsiflexion and individualized training - a case study.

Following a transfemoral amputation (TFA), numerous changes in movement patterns during gait can occur. Frontal plane hip and pelvis compensatory strategies are recognized among individuals with a TFA, some thought to aid in safe foot clearance during the swing phase of gait. The aim of this case study was to evaluate the effect of an active ankle dorsiflexion provided by a microprocessor-controlled prosthetic foot, as well as the effect of individualized training on these parameters. In this case study, a 42-year-old male underwent 3 D gait analysis. Data were captured for two conditions; with a microprocessor-controlled prosthetic foot with active/inactive ankle dorsiflexion, during two sessions; before and after 6 weeks of individualized training. The main outcomes analyzed were frontal plane pelvis and hip kinematics. Prior to training, pelvic lift decreased slightly, coupled with an increase in hip abduction, during gait with the active ankle dorsiflexion of a prosthetic foot, compared to inactive dorsiflexion. After the training period, the pelvic lift was further decreased and an increase in hip adduction was concurrently seen. The results of this case study indicate a positive effect of the active dorsiflexion of the prosthetic foot but highlight the need for specific training after prescription of a microprocessor prosthetic foot. Implications for rehabilitation Decreased compensatory changes seen in this case study indicate a positive effect of the active dorsiflexion of the prosthetic foot, especially after a 6-week training period. Individualized training should be aimed at helping the user utilizing the benefits of the active dorsiflexion of the microprocessor prosthetic foot, implementing exercises that improve gait quality, technical training for this specific foot, strength training and balance exercises.