Anmin Liu

Functional units of the human foot

Functional units in the human foot provide a meaningful basis for subdivisions of the entire foot during gait analysis as well as justified simplifications of foot models. The present study aimed to identify such functional units during walking and slow running. An invasive method based upon reflective marker arrays mounted on intracortical pins was used to register motion of seven foot bones. Six healthy subjects were assessed during walking and four of them during slow running. Angle-angle diagrams of corresponding planar bone rotations were plotted against each other and used to establish functional units. Individual functional units were accepted when the joints rotated temporally in phase and either (i) in the same direction, (ii) in the opposite direction, or (iii) when one of the two joints showed no rotation. A functional unit was generalized if all available angle-angle diagrams showed a consistent pattern. A medial array from the navicular to the first metatarsal was found to perform as a functional unit with parts rotating in the same direction and larger rotations occurring proximally. A rigid functional unit comprised the navicular and cuboid. No other functional units were identified. It was concluded that the talus, navicular, and medial cuneiform should neither be regarded as one rigid unit nor as one segment during gait analysis. The first and fifth metatarsals should also be considered separately. It was further concluded that a marker setup for gait analysis should consist of the following four segments: calcaneus, navicular-cuboid, medial cuneiform-first metatarsal, fifth metatarsal.

An In Vivo Experimental Validation of a Computational Model of Human Foot

Reliable computational foot models offer an alternative means to enhance knowledge on the biomechanics of human foot. Model validation is one of the most critical aspects of the entire foot modeling and analysis process. This paper presents an in vivo experiment combining motion capture system and plantar pressure measure platform to validate a three-dimensional finite element model of human foot. The Magnetic Resonance Imaging (MRI) slices for the foot modeling and the experimental data for validation were both collected from the same volunteer subject. The validated components included the comparison of static model predictions of plantar force, plantar pressure and foot surface deformation during six loading conditions, to equivalent measured data. During the whole experiment, foot surface deformation, plantar force and plantar pressure were recorded simultaneously during six different loaded standing conditions. The predictions of the current FE model were in good agreement with these experimental results.

Error in the description of foot kinematics due to violation of rigid body assumptions

Kinematic data from rigid segment foot models inevitably includes errors because the bones within each segment move relative to each other. This study sought to define error in foot kinematic data due to violation of the rigid segment assumption. The research compared kinematic data from 17 different mid and forefoot rigid segment models to kinematic data of the individual bones comprising these segments. Kinematic data from a previous dynamic cadaver model study was used to derive individual bone as well as foot segment kinematics. Mean and maximum errors due to violation of the rigid body assumption varied greatly between models. The model with least error was the combination of navicular and cuboid (mean errors < = 1.3 degrees, average maximum error < = 2.4 degrees). Greatest error was seen for the model combining all the ten bones (mean errors < = 4.4 degrees, average maximum errors < = 6.9 degrees). Based on the errors reported a three segment mid and forefoot model is proposed: (1) Navicular and cuboid, (2) cuneiforms and metatarsals 1, 2 and 3, and (3) metatarsals 4 and 5. However the utility of this model will depend on the precise purpose of the in vivo foot kinematics research study being undertaken.

Optimization of legged robot locomotion by control of foot-force distribution

The contribution of this paper is the introduction of three new pseudo-inverse formulations for the real-time control of foot-force distribution in multi-legged walking machines. Three alternative locomotion performance objectives are proposed for the purpose of optimizing the foot-force distribution. An exhaustive search method has been used to obtain truly optimal results, which are then used for comparison with the results obtained using suboptimal pseudo-inverse formulations that are suitable for real-time control. Simulation results show that, by using the appropriate pseudo-inverse formulation, a good approximation to the corresponding optimal foot-force distribution can be obtained. Furthermore, it is clear that the friction duty factor formulation provides an excellent real-time solution for minimizing the risk of foot-slip.

A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing

Frozen clamps can hold tendons and ligaments tightly and transmit high loads, from 4 kN to 13 kN, without slippage, yet they are complex and expensive. The existing non-frozen serrated jaw clamp is simple to fabricate and use, but the maximal tensile force it can sustain is only about 2.5 kN, which is not enough in many biomechanical tests. In this study, a new type of non-frozen clamp, which has lateral block boards and asymmetrical teeth jaws, was designed. The lateral block boards made of titanium alloy were used to prevent the soft tissues from being squeezed out during compressing, while the asymmetrical teeth jaws made of nylon were used to grip and keep holding soft tissues. The capability of this new type of clamp was tested by stretching five cattle tendons to failure on the tensile and compression testing machine, none of them displayed any slippage before rupture, the maximum tension force was 6.87 kN. This non-frozen asymmetrical teeth jaw clamp was designed for gripping tendons in foot and ankle dynamic simulation test, but it can also be applied to other in vitro tests, such as hip and knee dynamic tests.

Development and evaluation of prefabricated antipronation foot orthosis

Our aim was to develop and evaluate a new antipronation foot orthosis that addressed problems perceived by clinicians and users with existing foot orthoses. Clinicians and users were engaged to develop a user specification for the orthosis, and orthotic geometry and materials were developed using clinical reasoning. The orthotic material properties were tested and the ability of the orthosis to reduce foot pronation evaluated on 27 individuals. Clinicians expressed concern that current prefabricated orthoses often did not offer sufficient support to the foot because of a combination of the shape and materials used, and users concurred but also highlighted issues of durability and hygiene. The geometry of the new orthosis was, therefore, adjusted to enable individual foot size orthoses to be produced. A material was selected that was harder and more durable than materials used in many prefabricated orthoses. When the new orthosis was being worn, maximum rear foot eversion was reduced in both walking (mean reduction -3.8 degrees, p < 0.001) and running (mean reduction -2.5 degrees, p < 0.001). Through a structured process, orthotic design decisions were made that addressed the specific concerns of clinicians and users and the new orthosis was proven to reduce rearfoot pronation.

Kinematics of lower limbs of healthy Chinese people sitting cross-legged.

Background: Sitting cross-legged as an activity of daily living and its kinematics have significantly different demands on the arthroplasty of hip and knee, which can be referred in the design of joint arthroplasty. Objectives: The purpose of this study was to obtain the kinematics of the lower limb in Chinese people during cross-legged sitting. Study design: This study identified the necessary requirements for joint arthroplasty to carry out sitting cross-legged activity. Methods: A total of 40 healthy Chinese participants (average age = 23.8 years) performed six cross-legged sitting trials per person. Three-dimensional kinematic data of ankle, knee, and hip joints were collected; the means of the three-dimensional angles of these three joints were calculated. Results: At the hip, the range of motion of the flexion was 101.7°, the abduction reached 43.2°, and the range of motion of the external rotation was 36.4°. At the knee, the range of motion of the flexion was 131.9° and the range of motion of the internal rotation reached 32.4°. At the ankle, the range of motion in three planes was not great. Conclusions: Such motion ranges were likely to result in prosthetic dislocation. The results would provide valuable references for prosthesis design in the Chinese population. Clinical relevance The kinematic data of sitting cross-legged activity provided the baseline information for physicians and therapists concerned with the surgical and functional rehabilitation, and offered reference for lower limbs prosthesis designing.

Does a specific MR imaging protocol with a supine-lying subject replicate tarsal kinematics seen during upright standing? / Bildet ein spezifisches MR-Verfahren mit rücklings liegendem Probanden die tarsale Kinematik unter stehenden Bedingungen nach?

Abstract Magnetic resonance (MR) imaging is becoming increasingly important in the study of foot biomechanics. Specific devices have been constructed to load and position the foot while the subject is lying supine in the scanner. The present study examines the efficacy of such a newly developed device in replicating tarsal kinematics seen during the more commonly studied standing loading conditions. The results showed that although knee flexion and the externally applied load were carefully controlled, subtalar and talo-navicular joint rotations while lying during MR imaging and when standing (measured opto-electrically with markers attached to intracortical pins) did not match, nor were they systematically shifted. Thus, the proposed MR protocol cannot replicate tarsal kinematics seen during upright standing. It is concluded that specific foot loading conditions have to be considered when tarsal kinematics are evaluated. Improved replication of tarsal kinematics in different postures should comprehensively consider muscle activity, a fixed hip position, and a well-defined point of load application. Zusammenfassung Die Magnetresonanz- (MR) Tomographie gewinnt in der Fußbiomechanik immer mehr an Bedeutung. Um den Fuß positionieren und belasten zu können, während der Proband rücklings im Tomographen liegt, wurden spezifische Aufbauten konstruiert. Die vorliegende Studie prüft die Effektivität eines derartigen, neu entwickelten Aufbaus hinsichtlich der Imitation der tarsalen Kinematik, die sich unter den üblicherweise untersuchten stehenden Bedingungen ergibt. Die Ergebnisse zeigten, dass trotz sorgfältiger Kontrolle der Knieflexion und der äußeren Last die Rotationen des unteren Sprunggelenks sowie talo-navikularen Gelenks während dem Liegen im MR-Tomo-graphen nicht mit denen während des Stehens (opto-elektrisch gemessen anhand von im Knochen fixierten Drähten) übereinstimmen, wobei die Ergebnisse auch nicht systematisch verschoben sind. Das vorgeschlagene MR-Verfahren ist daher nicht in der Lage, die tarsale Kinematik während des Stehens abzubilden. Folglich sind bei Betrachtungen der tarsalen Kinematik die spezifischen Belastungen des Fußes zu bedenken. Eine verbesserte Imitation der tarsalen Kinematik in verschiedenen Körperhaltungen sollte sorgfältig die Aktivität der Muskulatur, eine fixierte Hüfte sowie einen exakt definierten Kraftangriffspunkt berücksichtigen.

A novel method of using accelerometry for upper limb FES control

This paper reports on a novel approach to using a 3-axis accelerometer to capture body segment angle for upper limb functional electrical stimulation (FES) control. The approach calculates the angle between the accelerometer x-axis and the gravity vector, while avoiding poor sensitivity at certain angles and minimizing errors when true acceleration is relatively large in comparison to gravity. This approach was incorporated into a state-machine controller which is used for the real-time control of FES during upper limb functional task performance. An experimental approach was used to validate the new method. Two participants with different upper limb impairments resulting from a stroke carried out four different FES-assisted tasks. Comparisons were made between angle calculated from arm-mounted accelerometer data using our algorithm and angle calculated from limb-mounted reflective marker data. After removal of coordinate misalignment error, mean error across tasks and subjects ranged between 1.4 and 2.9°. The approach shows promise for use in the control of upper limb FES and other human movement applications where true acceleration is relatively small in comparison with gravity.

A review of the design and clinical evaluation of the ShefStim array-based functional electrical stimulation system.

Functional electrical stimulation has been shown to be a safe and effective means of correcting foot drop of central neurological origin. Current surface-based devices typically consist of a single channel stimulator, a sensor for determining gait phase and a cuff, within which is housed the anode and cathode. The cuff-mounted electrode design reduces the likelihood of large errors in electrode placement, but the user is still fully responsible for selecting the correct stimulation level each time the system is donned. Researchers have investigated different approaches to automating aspects of setup and/or use, including recent promising work based on iterative learning techniques. This paper reports on the design and clinical evaluation of an electrode array-based FES system for the correction of drop foot, ShefStim. The paper reviews the design process from proof of concept lab-based study, through modelling of the array geometry and interface layer to array search algorithm development. Finally, the paper summarises two clinical studies involving patients with drop foot. The results suggest that the ShefStim system with automated setup produces results which are comparable with clinician setup of conventional systems. Further, the final study demonstrated that patients can use the system without clinical supervision. When used unsupervised, setup time was 14min (9min for automated search plus 5min for donning the equipment), although this figure could be reduced significantly with relatively minor changes to the design.