Investigations of roll-over shape: implications for design, alignment, and evaluation of ankle-foot prostheses and orthoses.

PURPOSE: The purpose of this article is to provide an overview of our previous work on roll-over shapes, which are the effective rocker shapes that the lower limb systems conform to during walking. METHOD: This article is a summary of several recently published articles from the Northwestern University Prosthetics Research Laboratory and Rehabilitation Engineering Research Program on the topic of roll-over shapes. The roll-over shape is a measurement of centre of pressure of the ground reaction force in body-based coordinates. This measurement is interpreted as the effective rocker shape created by lower limb systems during walking. RESULTS: Our studies have shown that roll-over shapes in able-bodied subjects do not change appreciably for conditions of level ground walking, including walking at different speeds, while carrying different amounts of weight, while wearing shoes of different heel heights, or when wearing shoes with different rocker radii. In fact, results suggest that able-bodied humans will actively change their ankle movements to maintain the same roll-over shapes. CONCLUSIONS: The consistency of the roll-over shapes to level surface walking conditions has provided insight for design, alignment and evaluation of lower limb prostheses and orthoses. Changes to ankle-foot and knee-ankle-foot roll-over shapes for ramp walking conditions have suggested biomimetic (i.e. mimicking biology) strategies for adaptable ankle-foot prostheses and orthoses.

Net external energy of the biologic and prosthetic ankle during gait initiation.

The net external energy of the biologic human ankle joint and of some lower limb prosthetic ankle-foot systems was examined during gait initiation. The purpose of the study was to better understand the ankle's behavior during the acceleration phase of walking for use in the design of improved lower limb prostheses and orthoses. Quantitative gait data were collected from 10 able-bodied subjects and 10 persons with unilateral transtibial amputations during gait initiation. The behaviors of the biologic and prosthetic 'ankle' joints were examined by analyzing the relationship between sagittal plane ankle angles and moments. Net external energy at the ankle was estimated by calculating the area under the moment versus angle curves (hysteresis) created during the loading and unloading phases. Results indicate that able-bodied persons utilize energy input from the trailing ankle after the first step is made in gait initiation, most likely to help transition the body into steady-state walking. The passive prosthetic ankle-foot systems tested were unable to put energy into the system during gait initiation.

CIR Casting System for making transtibial sockets.

This paper describes a new casting system for transtibial socket fabrication. Like the earlier CIR Sand Casting System, the CIR Casting System is based on the 'dilatancy' principle that is similar to the packaging process for coffee beans by which loose beans become a solid mass when a vacuum is applied. The main difference from the CIR Sand Casting System is that the CIR Casting System uses light-weight, polystyrene beads in place of silica sand as the primary material for casting the negative mold. The formed negative mold can be converted into a positive sand model for modification and socket formation. With the new plaster-less casting system, the prosthetist can fabricate a transtibial prosthesis in about one hour. It reduces the set-up cost, overall weight and size of the casting system, and increases portability for service in remote areas. The System also creates minimal waste and is energy-conserving and environmentally-friendly.

Roll-over shapes of the able-bodied knee-ankle-foot system during gait initiation, steady-state walking, and gait termination.

A few investigators have described the movement of the center of pressure (COP) of the ground reaction force and the activation patterns of the lower limb muscles during gait initiation and termination. This study examines the effective rocker (roll-over shape) behavior of the knee-ankle-foot (KAF) system during gait initiation, steady-state walking (i.e. constant speed gait), and gait termination. The KAF roll-over shapes were characterized by transforming COP data of 10 able-bodied subjects from a laboratory-based coordinate system into a leg-based coordinate system. The resulting roll-over shapes (effective rockers) were characterized using a circular arc model. The KAF roll-over shapes exhibit an overall "flexed" orientation during the first step of gait initiation and an "extended" orientation during the last step of gait termination. Understanding the behavior of the anatomical KAF system during gait initiation and termination may aid in the design of prosthetic components, i.e. mechanical devices that replace complete anatomical structures. Prostheses that intend to mimic the overall behavior of physiological KAF systems (biomimetic designs) could be manufactured using approaches that are much simpler than attempting to reconstruct the complexity of the lower limb.

The effects of prosthetic foot roll-over shape arc length on the gait of trans-tibial prosthesis users.

The Shape&Roll prosthetic foot was used to examine the effect of roll-over shape arc length on the gait of 14 unilateral trans-tibial prosthesis users. Simple modifications to the prosthetic foot were used to alter the effective forefoot rocker length, leaving factors such as alignment, limb length, and heel and mid-foot characteristics unchanged. Shortening the roll-over shape arc length caused a significant reduction in the maximum external dorsiflexion moment on the prosthetic side at all walking speeds (p < 0.001 for main effect of arc length), due to a reduction in forefoot leverage (moment arm) about the ankle. Roll-over shape arc length significantly affected the initial loading on the sound limb at normal and fast speeds (p = 0.001 for the main effect of arc length), with participants experiencing larger first peaks of vertical ground reaction forces on their sound limbs when using the foot with the shortest effective forefoot rocker arc length. Additionally, the difference between step lengths on the sound and prosthetic limbs was larger with the shortest arc length condition, although this difference was not statistically significant (p = 0.06 for main effect). It appears that prosthesis users may experience a drop-off effect at the end of single limb stance on prosthetic feet with short roll-over shape arc lengths, leading to increased loading and/or a shortened step on the contralateral limb.

The effects of static friction and backlash on extended physiological proprioception control of a powered prosthesis.

In general, externally powered prostheses do not provide proprioceptive feedback and thus require the user to rely on cognitively expensive visual feedback to effectively control the prosthesis. Applying the concept of extended physiological proprioception (EPP) to externally powered prostheses provides direct feedback to the user's proprioceptive system regarding the position, velocity, and forces applied to the prosthesis. However, electric elbows with EPP controllers developed at the Northwestern University Prosthetics Research Laboratory have exhibited unexplained "jerky" behavior in both clinical fittings and bench-top operation. In addition, the development of limit cycles, a specific type of constant-amplitude oscillation, had been observed in bench-top use of these elbows. Backlash and static friction within the EPP system were found to be primarily responsible for the development of limit cycles. Reducing static friction and backlash improved the system's performance. These results suggest that to most effectively implement EPP, prosthesis manufacturers should design prosthetic components that minimize static friction and backlash.

Effects of adding weight to the torso on roll-over characteristics of walking.

Ten participants without physical impairment walked with 0 kg, 11.5 kg, and 23.0 kg of added weight equally distributed about the torso in a harness. At each weight level, the participants walked at slow, normal, and fast self-selected walking speeds. We examined the roll-over characteristics by determining the ankle-foot and knee-ankle-foot roll-over shapes. These shapes, which are the effective rockers created by the respective lower-limb systems between heel contact and opposite heel contact of walking, are found if one transforms the center of pressure of ground reaction force into body coordinate systems. The roll-over shapes of the ankle-foot and knee-ankle-foot systems did not change appreciably with added weight at any of the three walking speeds. The invariance of these biologic systems to added weight should be considered when prostheses and orthoses are designed that intend to replace and augment their function in walking.

Temporal symmetries during gait initiation and termination in nondisabled ambulators and in people with unilateral transtibial limb loss.

This study investigated the temporal characteristics of gait initiation and gait termination. Ten nondisabled adult volunteers and ten people with unilateral transtibial limb loss performed starting and stopping for slow, normal, and fast walking speeds. We used kinematic and anthropomorphic data to determine the body center of mass (BCOM) position of each subject. The BCOM acceleration was derived by double-differentiating the position data. An averaged BCOM acceleration was calculated by a filtering of the instantaneous acceleration data at a cutoff frequency set by the cadence for elimination of the step-to-step variation. We used this averaged acceleration to calculate the time the volunteers needed to initiate and terminate gait. The results support the hypothesis that both nondisabled ambulators and the subjects with unilateral transtibial limb loss initiate and terminate gait in approximately two steps, regardless of the steady-state walking speed.

Effects of shoe heel height on biologic rollover characteristics during walking.

This study investigated the effects of shoe heel height on the rollover characteristics of the biologic ankle-foot system. Ten nondisabled adult female volunteers walked using three pairs of shoes with varying heel heights and at three walking speeds with each pair of shoes. Kinematic and kinetic data needed to calculate the rollover shapes of the ankle-foot systems of the participants were collected. Rollover shapes are the effective rocker geometries that ankle-foot systems conform to between heel contact and opposite heel contact. Parameters of the best-fit circular arcs to the rollover shapes were used in an examination of the effects of shoe heel height on the ankle-foot system. The results support the notion that nondisabled humans automatically adapt their ankle-foot systems to accommodate a range of shoe heel heights, resulting in rollover shapes that do not change appreciably. Given physiologic constraints, this adaptation may not be possible for very high heels.

The human ankle during walking: implications for design of biomimetic ankle prostheses.

The non-disabled human ankle joint was examined during walking in an attempt to determine overall system characteristics for use in the design of ankle prostheses. The hypothesis of the study was that the quasi-stiffness of the ankle changes when walking at different walking speeds. The hypothesis was examined using sagittal plane ankle moment versus ankle angle curves from 24 able-bodied subjects walking over a range of speeds. The slopes of the moment versus ankle angle curves (quasi-stiffness) during loading appeared to change as speed was increased and the relationship between the moment and angle during loading became increasingly non-linear. The loading and unloading portions of the moment versus angle curves showed clockwise loops (hysteresis) at self-selected slow speeds that reduced essentially to zero as the speed increased to self-selected normal speeds. Above self-selected normal speeds, the loops started to traverse a counter-clockwise path that increased in area as the speed was increased. These characteristics imply that the human ankle joint could be effectively replaced with a rotational spring and damper for slow to normal walking speeds. However, to mimic the characteristics of the human ankle during walking at fast speeds, an augmented system would be necessary. This notion is supported by the sign of the ankle power at the time of opposite heel contact, which was negative for slow speeds, was near zero at normal speeds, and was positive for fast walking speeds.

Roll-over shapes of human locomotor systems: effects of walking speed.

OBJECTIVE: To examine the hypothesis that roll-over shapes of non-disabled lower limb systems do not change appreciably with walking speed. DESIGN: Repeated measures (n = 24). BACKGROUND: Roll-over shapes of three lower limb systems are presented. They are: roll-over shapes of the (1) foot, (2) ankle-foot, and (3) knee-ankle-foot systems. Roll-over shapes show the effective rocker (or cam) shapes that the lower limb systems conform to during the period in the stance phase of walking between heel contact and opposite heel contact. METHODS: Roll-over shapes were measured by transforming center of pressure data from a laboratory-based coordinate system into each of three body-based coordinate systems. Knee-ankle-foot roll-over shapes were further characterized using a circular arc model. RESULTS: From a statistical standpoint, the radii of the best-fit circular arcs did not change significantly with walking speed, while the forward shifts of the circular models did change significantly. However, the change in forward shift was not considered to be clinically significant. CONCLUSIONS: The biologic systems involved in developing the roll-over shapes adapt to changing conditions of walking speed, including increased loading amplitudes as speed is increased, to maintain similar effective roll-over geometries. RELEVANCE: Roll-over shapes provide insight into the workings of various lower limb systems by taking a new look at existing gait data. This insight could provide broad utility, helping to develop a better understanding of able-bodied and disabled human walking, and leading to the design of improved rehabilitation devices, surgeries, and therapies.

Roll-over characteristics of human walking on inclined surfaces.

Roll-over characteristics of able-bodied human subjects walking on ramped surfaces were examined in this study. Ten subjects walked at their normal self-selected speed on a level surface, a 5-deg ramp, and a 10-deg ramped surface. Ramps were designed such that ground reaction forces and center of pressure of the ground reaction forces could be measured on their surfaces. This set-up facilitated calculation of the effective rockers that the ankle-foot (AF) and knee-ankle-foot (KAF) systems conformed to during single-limb stance (contralateral toe off to contralateral heel contact). Since our original "roll-over shapes" were characterized between heel contact and opposite heel contact, we label the shapes found during single-limb stance as "truncated roll-over shapes". We hypothesized that the ankle-foot system would adapt to the various surfaces, creating a roll-over shape that would change in orientation with different levels of inclination. The truncated AF roll-over shapes supported this hypothesis for uphill walking but did not support the hypothesis for downhill walking. However, truncated roll-over shapes of the KAF system did adjust their orientation to match both the positive and negative levels of surface inclination. In general, the ankle appears to be the main adapting joint when walking up inclined surfaces while the knee becomes important for the overall adaptation in downhill walking. Knowledge of physiological lower-limb roll-over characteristics on ramped surfaces may help in the development of biomimetic prostheses and orthoses that will automatically adapt to changes in walking surface inclination.

The 'shape&roll' prosthetic foot: I. Design and development of appropriate technology for low-income countries.

The Shape&Roll Prosthetic Foot (patent pending) is an artificial foot designed for use in low-income countries, and may also be useful in industrialised nations. Its design is based on the theory that the roll-over shape of a prosthetic foot should mimic that of the non-disabled physiological foot-ankle complex during walking. This article presents the S&R foot including the unique features incorporated into its design. The results of mechanical tests indicate that the roll-over shape of the foot closely mimics the roll-over shape of the non-disabled ankle-foot complex and an expensive 'high-performance' prosthetic foot. The fatigue testing process for the S&R foot is also described. The foot has been shown to be durable according to the International Organization for Standardization (ISO) standards, with more than five samples tested to date. The Shape&Roll Foot is low in cost, simple to fabricate, light in weight, low in profile, and is highly functional for walking in respect of roll-over characteristics.

The 'shape&roll' prosthetic foot: II. Field testing in El Salvador.

A field test was performed in El Salvador to evaluate the usefulness of the Shape&Roll prosthetic foot, a foot developed for low-income countries, involving 12 participants. Quantitative gait parameters were measured with a Direct Ultrasound Ranging System (DURS). Qualitative information was obtained from questionnaires administered before and after a three-week trial. The results indicate that the Shape&Roll foot widened the speed range of all participants. According to the questionnaires, the Shape&Roll foot eases walking, enabling participants to walk significantly longer distances. The participants rated its roll-over as very natural and smooth, resulting in a self-perceived reduced walking effort. Handling inclined and uneven surfaces was also rated superior to their current Solid Ankle Cushioned Heel (SACH)-like feet.

A simple method for determination of gait events.

Simple and effective methods for determining the timing of gait events are necessary for the proper normalization and statistical analysis of gait data when a variety of gait measurements are available. The approach presented was developed for cases in which overall center of pressure under the body and marker trajectories are being measured over multiple steps. The new method presented uses the relative positioning of the overall center of pressure and an ankle marker in the direction of forward progression for the determination of "heel-contact" and "toe-off" events. The difference between the locations of the overall center of pressure and the ankle in the direction of progression readily delineates the timing of these events. The new method was tested against force records from individual force platforms and was found to detect "heel-contact" events an average of 1 sample (at a sampling frequency of 120Hz, 0.00833s) before the event found using the individual force platforms. "Toe-off" events were found an average of 2 samples (0.0167s) prior to the events found using individual force plates. The method appears new and is attractive because of its simplicity in determining gait events when the appropriate gait measurements are available.