The effects of ground-irregularity-cancelling prosthesis control on balance over uneven surfaces

Motion Analysis of a Frontal Plane Adaptable Prosthetic Foot

Introduction

An objective of designing a prosthetic foot is to achieve the natural adaptability of the foot and ankle on various surfaces and different forms of gait. Frontal plane position of the foot relative to the shank changes with many functional aspects of gait, such as turning, stairs, and walking on uneven ground. Prosthetic foot designs have variable frontal plane adaptability. An investigation foot with a linkage with ±10° of frontal plane motion was developed to improve frontal plane response under various conditions. The purpose of this study was to compare the kinematics of locked and unlocked conditions of a frontal plane adaptable prosthetic foot and the person’s usual foot while walking forward on a level surface, on an unstable rock surface, and sidestep, using a crossover design. These different conditions result in changes in frontal plane motion in the anatomical foot and ankle, and the current study evaluates whether there are similar trends in prosthetic feet.

Materials and Methods

People were included if they had a unilateral below-knee amputation, intact residual limb skin, were over 16 years old, and were able to walk more than 400 m on level ground without using a walking aid and without an increase in pain. The control group was people without amputations who completed the procedures once. Participants with amputations completed forward walking on level ground, on an unstable rock surface, and sidestep with their usual foot. Then after 2 weeks of accommodation, participants repeated these tests with the investigational foot unlocked and locked. Motion analysis data were collected with a 12-camera optically based system. Primary outcomes were sagittal and frontal plane motions of the foot relative to the shank. In addition, step length, step width, and stride velocity were obtained from the kinematic measures. Paired t-tests were used for statistical inference for individual participant comparisons. Unpaired t-tests were used for comparisons between the controls and people with amputations.

Results

Twenty-one people with amputations and 10 controls completed the tests. Participants with amputation had 16 different usual feet. There was a wide variation in usual foot motion during forward walking, whereas investigational foot conditions showed less variability. During level walking, control subjects had more frontal plane motion than any of the foot conditions, and the unlocked had more frontal plane motion than the usual foot and locked condition. Walking across an unstable rock surface showed similar trends, with control participants having more sagittal and frontal plane ankle motion compared with any prosthetic foot condition. Also, the unlocked had statistically greater frontal plane motion than the usual foot or locked condition. Sidestep results were also consistent with other gait tests. The control participants’ sagittal plane ankle range of motion was significantly more than the prosthetic sagittal plane motion for all foot conditions, whether the prosthetic side was leading or trailing. There was significantly more frontal plane motion with the unlocked than the usual foot and locked condition when the prosthetic foot was trailing or leading.

Discussion and Conclusions

Wide variation in usual foot range of motions in the frontal and sagittal planes confirmed the need for additional controls when considering the effect of the linkage alone. The unlocked had increased frontal plane ranges of motion compared with the locked and the majority of usual foot for all gait conditions, including level walking. This finding demonstrated that people with amputations were functionally using the additional range of motion provided by the linkage. However, control subjects used more range of motion in both the sagittal and frontal planes for the unstable rock surface and sidestepping. Increased frontal plane range of motion did not translate into improved stride length and velocity, step width, or center of mass deviations.

Clinical Relevance

The person-specific functional activities should be considered when choosing a prosthetic foot. A prosthesis with frontal plane motion may be applicable for a person who moves in a sidestep pattern or on uneven ground.

Investigating the performance of soft robotic adaptive feet with longitudinal and transverse arches

Biped robots usually adopt feet with a rigid structure that simplifies walking on flat grounds and yet hinders ground adaptation in unstructured environments, thus jeopardizing stability. We recently explored in the SoftFoot the idea of adapting a robotic foot to ground irregularities along the sagittal plane. Building on the previous results, we propose in this paper a novel robotic foot able to adapt both in the sagittal and frontal planes, similarly to the human foot. It features five parallel modules with intrinsic longitudinal adaptability that can be combined in many possible designs through optional rigid or elastic connections. By following a methodological design approach, we narrow down the design space to five candidate foot designs and implement them on a modular system. Prototypes are tested experimentally via controlled application of force, through a robotic arm, onto a sensorized plate endowed with different obstacles. Their performance is compared, using also a rigid foot and the previous SoftFoot as a baseline. Analysis of footprint stability shows that the introduction of the transverse arch, by elastically connecting the five parallel modules, is advantageous for obstacle negotiation, especially when obstacles are located under the forefoot. In addition to biped robots' locomotion, this finding might also benefit lower-limb prostheses design.

Human-in-the-loop optimization of rocker shoes via different cost functions during walking

Personalised footwear could be used to enhance the function of the foot-ankle complex to a person's maximum. Human-in-the-loop optimization could be used as an effective and efficient way to find a personalised optimal rocker profile (i.e., apex position and angle). The outcome of this process likely depends on the selected optimization objective and its responsiveness to the rocker parameters being tuned. This study aims to explore whether and how human-in-the-loop optimization via different cost functions (i.e., metabolic cost, collision work as measure for external mechanical work, and step distance variability as measure for gait stability) affects the optimal apex position and angle of a rocker profile differently for individuals during walking. Ten healthy individuals walked on a treadmill with experimental rocker shoes in which apex position and angle were optimized using human-in-the-loop optimization using different cost functions. We compared the obtained optimal apex parameters for the different cost functions and how these affected the selected gait related objectives. Optimal apex parameters differed substantially between participants and optimal apex positions differed between cost functions. The responsiveness to changes in apex parameters differed between cost functions. Collision work was the only cost function that resulted in a significant improvement of its performance criteria. Improvements in metabolic cost or step distance variability were not found after optimization. This study showed that cost function selection is important when human-in-the-loop optimization is used to design personalised footwear to allow conversion to an optimum that suits the individual.

An Experimental Setup to Test Obstacle-Dealing Capabilities of Prosthetic Feet

Small obstacles on the ground often lead to a fall when caught with commercial prosthetic feet. Despite some recently developed feet can actively control the ankle angle, for instance over slopes, their flat and rigid sole remains a cause of instability on uneven grounds. Soft robotic feet were recently proposed to tackle that issue; however, they lack consistent experimental validation. Therefore, this paper describes the experimental setup realized to test soft and rigid prosthetic feet with lower-limb prosthetic users. It includes a wooden walkway and differently shaped obstacles. It was preliminary validated with an able-bodied subject, the same subject walking on commercial prostheses through modified walking boots, and with a prosthetic user. They performed walking firstly on even ground, and secondly on even ground stepping on one of the obstacles. Results in terms of vertical ground reaction force and knee moments in both the sagittal and frontal planes show how the poor performance of commonly used prostheses is exacerbated in case of obstacles. The prosthetic user, indeed, noticeably relies on the sound leg to compensate for the stiff and unstable interaction of the prosthetic limb with the obstacle. Therefore, since the limitations of non-adaptive prosthetic feet in obstacle-dealing emerge from the experiments, as expected, this study justifies the use of the setup for investigating the performance of soft feet on uneven grounds and obstacle negotiation.

Legged locomotion over irregular terrains: state of the art of human and robot performance

Legged robotic technologies have moved out of the lab to operate in real environments, characterized by a wide variety of unpredictable irregularities and disturbances, all this in close proximity with humans. Demonstrating the ability of current robots to move robustly and reliably in these conditions is becoming essential to prove their safe operation. Here, we report an in-depth literature review aimed at verifying the existence of common or agreed protocols and metrics to test the performance of legged system in realistic environments. We primarily focused on three types of robotic technologies, i.e., hexapods, quadrupeds and bipeds. We also included a comprehensive overview on human locomotion studies, being it often considered the gold standard for performance, and one of the most important sources of bioinspiration for legged machines. We discovered that very few papers have rigorously studied robotic locomotion under irregular terrain conditions. On the contrary, numerous studies have addressed this problem on human gait, being nonetheless of highly heterogeneous nature in terms of experimental design. This lack of agreed methodology makes it challenging for the community to properly assess, compare and predict the performance of existing legged systems in real environments. On the one hand, this work provides a library of methods, metrics and experimental protocols, with a critical analysis on the limitations of the current approaches and future promising directions. On the other hand, it demonstrates the existence of an important lack of benchmarks in the literature, and the possibility of bridging different disciplines, e.g., the human and robotic, towards the definition of standardized procedures that will boost not only the scientific development of better bioinspired solutions, but also their market uptake.