Lei Ren

The three-dimensional locomotor dynamics of African (Loxodonta africana) and Asian (Elephas maximus) elephants reveal a smooth gait transition at moderate speed

We examined whether elephants shift to using bouncing (i.e. running) mechanics at any speed. To do this, we measured the three-dimensional centre of mass (CM) motions and torso rotations of African and Asian elephants using a novel multisensor method. Hundreds of continuous stride cycles were recorded in the field. African and Asian elephants moved very similarly. Near the mechanically and metabolically optimal speed (a Froude number (Fr) of 0.09), an inverted pendulum mechanism predominated. With increasing speed, the locomotor dynamics quickly but continuously became less like vaulting and more like bouncing. Our mechanical energy analysis of the CM suggests that at a surprisingly slow speed (approx. 2.2 m s−1, Fr 0.25), the hindlimbs exhibited bouncing, not vaulting, mechanics during weight support. We infer that a gait transition happens at this relatively slow speed: elephants begin using their compliant hindlimbs like pogo sticks to some extent to drive the body, bouncing over their relatively stiff, vaulting forelimbs. Hence, they are not as rigid limbed as typically characterized for graviportal animals, and use regular walking as well as at least one form of running gait.

Regression techniques for the prediction of lower limb kinematics.

This work presents a novel and extensive investigation of mathematical regression techniques, for the prediction of laboratory-type kinematic measurements during human gait, from wearable measurement devices, such as gyroscopes and accelerometers. Specifically, we examine the hypothesis of predicting the segmental angles of the legs (left and right foot, shank and thighs), from rotational foot velocities and translational foot accelerations. This first investigation is based on kinematic data emulated from motion-capture laboratory equipment. We employ eight established regression algorithms with different properties, ranging from linear methods and neural networks with polynomial support and expanded nonlinearities, to radial basis functions, nearest neighbors and kernel density methods. Data from five gait cycles of eight subjects are used to perform both inter-subject and intra-subject assessments of the prediction capabilities of each algorithm, using cross-validation resampling methods. Regarding the algorithmic suitability to gait prediction, results strongly indicate that nonparametric methods, such as nearest neighbors and kernel density based, are particularly advantageous. Numerical results show high average prediction accuracy (rho = 0.98/0.99, RMS = 5.63 degrees/2.30 degrees, MAD = 4.43 degrees/1.52 degrees for inter/intra-subject testing). The presented work provides a promising and motivating investigation on the feasibility of cost-effective wearable devices used to acquire large volumes of data that are currently collected only from complex laboratory environments.

A generic analytical foot rollover model for predicting translational ankle kinematics in gait simulation studies

The objective of this paper is to develop an analytical framework to representing the ankle-foot kinematics by modelling the foot as a rollover rocker, which cannot only be used as a generic tool for general gait simulation but also allows for case-specific modelling if required. Previously, the rollover models used in gait simulation have often been based on specific functions that have usually been of a simple form. In contrast, the analytical model described here is in a general form that the effective foot rollover shape can be represented by any polar function rho=rho(phi). Furthermore, a normalized generic foot rollover model has been established based on a normative foot rollover shape dataset of 12 normal healthy subjects. To evaluate model accuracy, the predicted ankle motions and the centre of pressure (CoP) were compared with measurement data for both subject-specific and general cases. The results demonstrated that the ankle joint motions in both vertical and horizontal directions (relative RMSE approximately 10%) and CoP (relative RMSE approximately 15% for most of the subjects) are accurately predicted over most of the stance phase (from 10% to 90% of stance). However, we found that the foot cannot be very accurately represented by a rollover model just after heel strike (HS) and just before toe off (TO), probably due to shear deformation of foot plantar tissues (ankle motion can occur without any foot rotation). The proposed foot rollover model can be used in both inverse and forward dynamics gait simulation studies and may also find applications in rehabilitation engineering.

A Phase-Dependent Hypothesis for Locomotor Functions of Human Foot Complex

The human foot is a very complex structure comprising numerous bones, muscles, ligaments and synovial joints. As the only component in contact with the ground, the foot complex delivers a variety of biomechanical functions during human locomotion, e.g. body support and propulsion, stability maintenance and impact absorption. These need the human foot to be rigid and damped to transmit ground reaction forces to the upper body and maintain body stability, and also to be compliant and resilient to moderate risky impacts and save energy. How does the human foot achieve these apparent conflicting functions? In this study, we propose a phase-dependent hypothesis for the overall locomotor functions of the human foot complex based on in-vivo measurements of human natural gait and simulation results of a mathematical foot model. We propse that foot functions are highly dependent on gait phase, which is a major characteristics of human locomotion. In early stance just after heel strike, the foot mainly works as a shock absorber by moderating high impacts using the viscouselastic heel pad in both vertical and horizontal directions. In mid-stance phase (~80% of stance phase), the foot complex can be considered as a springy rocker, reserving external mechanical work using the foot arch whilst moving ground contact point forward along a curved path to maintain body stability. In late stance after heel off, the foot complex mainly serves as a force modulator like a gear box, modulating effective mechanical advantages of ankle plantiflexor muscles using metatarsal-phalangeal joints. A sound understanding of how diverse functions are implemented in a simple foot segment during human locomotion might be useful to gain insight into the overall foot locomotor functions and hence to facilitate clinical diagnosis, rehabilitation product design and humanoid robot development.

Segmental Kinematic Coupling of the Human Spinal Column during Locomotion

As one of the most important daily motor activities, human locomotion has been investigated intensively in recent decades. The locomotor functions and mechanics of human lower limbs have become relatively well understood. However, so far our understanding of the motions and functional contributions of the human spine during locomotion is still very poor and simultaneous in-vivo limb and spinal column motion data are scarce. The objective of this study is to investigate the delicate in-vivo kinematic coupling between different functional regions of the human spinal column during locomotion as a stepping stone to explore the locomotor function of the human spine complex. A novel infrared reflective marker cluster system was constructed using stereophotogrammetry techniques to record the 3D in-vivo geometric shape of the spinal column and the segmental position and orientation of each functional spinal region simultaneously. Gait measurements of normal walking were conducted. The preliminary results show that the spinal column shape changes periodically in the frontal plane during locomotion. The segmental motions of different spinal functional regions appear to be strongly coupled, indicating some synergistic strategy may be employed by the human spinal column to facilitate locomotion. In contrast to traditional medical imaging-based methods, the proposed technique can be used to investigate the dynamic characteristics of the spinal column, hence providing more insight into the functional biomechanics of the human spine.

Co-deposition of titanium/polytetrafluoroethylene films by unbalanced magnetron sputtering

Abstract Graded nanocomposite coatings, consisting of a polytetrafluoroethylene (PTFE)-rich surface layer and functionally graded titanium–titanium carbide–PTFE mixed sublayer, were deposited onto stainless steel substrates using a radio frequence (RF) unbalanced magnetron sputter system with the purpose of improving the tribological performance of the substrate and endowing the surface with hydrophobicity. The results show that decomposition of PTFE during RF plasma sputtering results mainly in the evolution of fluoropolymer species. High incident power results in low F/C ratio in the resulting films, and low incident power results in high F/C ratio films. The tribological performance of the coatings depends on the fluoropolymer content in the film bulk, as well as on the film growth process. The films with high fluoropolymer content, with a gradient multilayer structure, possess good tribological performance especially at the initial stage of testing. During the co-deposition process, the segments were inlaid into the titanium matrix and became strongly mechanically bonded. It is speculated that some of the carbon atoms may react with titanium to form titanium carbide in the coatings. Multilayer structure attributed to the decrease in the stress developed between layers. All these factors attributed to the improvement of the tribological performance of the stainless steel substrate.

Surface modification of PTFE by plasma treatment

Abstract Plasma treatment of polymer surfaces is a well established method for improving surface properties. In this paper, the surface structure and adhesive bonding properties of PTFE treated by three types of plasma are reported. The results indicate that different plasma gases have different effects on the surface structure. Argon plasma treatment produced a highly cross-linked honeycomblike structure, while air and oxygen plasma treatment resulted in a surface displaying high aspect ratio protrusions. All experimental plasma treatments caused a marked improvement in overlap shear strength, with the highest shear strength achieved after oxygen plasma treatment. It was found that the overlap shear strength was also influenced by plasma power and plasma treatment time although excessive plasma treatment caused damage to the surface layer leading to decreased shear strength. The change in surface properties and roughened microstructure together contributed to the improvement in shear strength.

Computational Models to Synthesize Human Walking

The synthesis of human walking is of great interest in biomechanics and biomimetic engineering due to its predictive capabilities and potential applications in clinical biomechanics, rehabilitation engineering and biomimetic robotics. In this paper, the various methods that have been used to synthesize human walking are reviewed from an engineering viewpoint. This involves a wide spectrum of approaches, from simple passive walking theories to large-scale computational models integrating the nervous, muscular and skeletal systems. These methods are roughly categorized under four headings: models inspired by the concept of a CPG (Central Pattern Generator), methods based on the principles of control engineering, predictive gait simulation using optimisation, and models inspired by passive walking theory. The shortcomings and advantages of these methods are examined, and future directions are discussed in the context of providing insights into the neural control objectives driving gait and improving the stability of the predicted gaits. Future advancements are likely to be motivated by improved understanding of neural control strategies and the subtle complexities of the musculoskeletal system during human locomotion. It is only a matter of time before predictive gait models become a practical and valuable tool in clinical diagnosis, rehabilitation engineering and robotics.

A unified non-steady non-linear tyre model under complex wheel motion inputs including extreme operating conditions

This study is to describe the transient force and moment characteristics of tyres involving large lateral slip, longitudinal slip, turn-slip and camber, and to develop a dynamic tyre model applicable to vehicle dynamic simulation and control for extreme operating conditions. Based on the steady state USES tire model [4,6], the effective slip ratios and quasi-steady concept are introduced to represent the non-linear dynamic tire properties in large slip cases. A high-order non-steady tyre model is presented. Special attention has been paid on the relationship between turn-slip and camber. Various kinds of experiments are performed to verify the tire model.

Assessment of 3D dynamic interactions between backpack and bearer using accelerometers and gyroscopes

INTRODUCTION During load carriage, the dynamic pack interaction forces exerted on the bearer’s torso relate directly to perceived discomfort, fatigue, and the risk of injury, e.g. rucksack palsy and back problems [2,3]. A better understanding of these interaction forces would help to improve the design of future load carriage systems [1]. Unfortunately, in contrast to the contact pressure distribution [4], the interaction forces and moments between pack and torso cannot be measured directly.