The Neuromuscular Origins of Kinematic Variability during Perturbed Walking

We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes using a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increased antagonist leg muscle coactivation during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment

Physics of food structure breakdown and bolus formation during oral processing of hard and soft solids

Recent studies on the oral processing of hard and soft foods are presented with consideration for the underlying physics involved during the transformation of food to a semifluid bolus for swallowing. Significant insights are being realised about the temporal aspects of the dominant processes of comminution, agglomeration, hydration and dilution, and connections to the dominant textural properties are emerging. The field is still challenged by inter-individual differences in oral physiology, but in vitro approaches to characterise the evolution of the food bolus have the potential to provide structure-property-oral processing relationships. Integrated approaches and development of techniques to measure the in-use physics of oral processing are critical for advanced food structure design

Community context and individual factors associated with arrests among young men in a South African township.

BackgroundIn high-income countries, individual- and community-level factors are associated with increased contact with the criminal justice system. However, little is known about how these factors contribute to the risk of arrest in South Africa, which has one of the highest rates of arrests globally. We examine both individual- and community-level factors associated with arrests among young men living in the townships of Cape Town.MethodsData were collected from a stratified community sample of 906 young men aged 18-29 years old living in 18 township neighborhoods. Communities with high and low rates of arrest were identified. Logistic regression models were used to assess which individual-level (such as substance use and mental health status) and community-level (such as infrastructure and presence of bars and gangs) factors predict arrests.ResultsSignificant predictors of arrests were substance use, gang activity, being older, more stressed, and less educated. Living in communities with better infrastructure and in more recently established communities populated by recent immigrants was associated with having a history of arrests.ConclusionsWhen considering both individual- and community-level factors, substance use and gang violence are the strongest predictors of arrests among young men in South Africa. Unexpectedly, communities with better infrastructure have higher arrest rates. Community programs are needed to combat substance use and gang activity as a pathway out of risk among South African young men.Trial registrationClinicalTrials.gov registration #NCT02358226, registered Nov 24, 2014

The role of saliva in oral processing: Reconsidering the breakdown path paradigm

We discuss food oral processing research over the last two decades and consider strategies for quantifying the food breakdown model, originally conceptualised by Hutchings and Lillford . The key innovation in their seminal 1988 paper was shifting the focus from intact food properties, measured in the lab, towards strategies to capture the dynamic nature of eating. This has stimulated great progress in the field, but a key aspect missing in oral processing research is the conversion of the Hutchings and Lillford breakdown path conceptual model into quantifiable parameters considered in the context of physiological factors such as saliva and oral movements. To address these short comings, we propose the following analysis: Hutchings’s and Lillford’s definitions of “Structure” and “Lubrication” are incomplete and they comprise many and varied physicochemical properties. We offer, here, a deeper analysis of each parameter, and propose strategies for researchers to consider in their quantification as an update of the Hutchings and Lillford Breakdown path

Enabling the Rational Design of Low-Fat Snack Foods: Insights from In Vitro Oral Processing

Texture perception can be conceptualized as an emergent cognitive response to several of the physical and chemical properties of a food. Contemporary oral processing research is focused on revealing the relationships between the sensory perceptions and the food properties, with the goal of enabling rational product design. One major challenge is the complexity of molecular and biocolloid interactions, underpinning even simple texture properties. Here, we will introduce the in vitro oral processing approach, which divides oral processing into discrete units of operation (first bite, comminution, granulation, bolus formation, and tribology) and then systematically investigates the material properties that govern each specific oral processing unit operation without the added complexity inherent to biological systems. We will describe how we used the approach to rationally design a low-fat potato chip by investigating the impact from adding back, to a low fat potato chip, a small amount of oil mixed with the surface active agent polyglycerol polyricinoleate (PGPR). The relevance of instrumental measures was validated by sensory assessment wherein panelists ranked the perceived oiliness of three different types of potato chips. The sensory results indicated that perceived oiliness was higher when the low- fat potato chip was supplemented with a 0.5% by weight topical coating (0.5% by weight 15% by weight PGPR in oil mixture) compared to the unaltered low-fat potato chip. The perceived difference in oiliness was found to correspond to in vitro transient friction of saliva in the presence and absence of PGPR. These results illustrate how dividing oral processing into distinct phases allows one to more readily align sensory and in vitro measures, allowing for integration of the two disciplines and more rational design when modifying macronutrients

Influence of fluid viscosity and wetting on multiscale viscoelastic lubrication in soft tribological contacts

Friction (and lubrication) between soft contacts is prevalent in many natural and engineered systems and plays a crucial role in determining their functionality. The contribution of viscoelastic hysteresis losses to friction in these systems has been well-established and defined for dry contacts; however, the influence of fluid viscosity and wetting on these components of friction has largely been overlooked. We provide systematic experimental evidence of the influence of lubricant viscosity and wetting on lubrication across multiple regimes within a viscoelastic contact. These effects are investigated for comparatively smooth and rough elastomeric contacts (PTFE-PDMS and PDMS-PDMS) lubricated by a series of Newtonian fluids with systematically controlled viscosity and static wetting properties, using a ball-on-disc tribometer. The distinct tribological behaviour, characterised generally by a decrease in the friction coefficient with increasing fluid viscosity and wettability, is explained in terms of lubricant dewetting and squeeze-out dynamics and their impact on multi-scale viscoelastic dissipation mechanisms at the bulk-, asperity-, sub-asperity- and molecular-scale. It is proposed that lubrication within the (non-molecularly) smooth contact is governed by localised fluid entrapment and molecular-scale (interfacial) viscoelastic effects, while additional rubber hysteresis stimulated by fluid-asperity interactions, combined with rapid fluid drainage at low speeds within the rough contact, alter the general shape of the Stribeck curve. This fluid viscosity effect is in some agreement with theoretical predictions. Conventional methods for analysing and interpreting tribological data, which typically involve scaling sliding velocity with lubricant viscosity, need to be revised for viscoelastic contacts with consideration of these indirect viscosity effects

Responsive polysaccharide-grafted surfaces for biotribological applications

The elucidation of biolubrication mechanisms and the design of artificial biotribological contacts requires the development of model surfaces that can help to tease out the cues that govern friction in biological systems. Polysaccharides provide an interesting option as a biotribological mimic due to their similarity with the glycosylated molecules present at biointerfaces. Here, pectin was successfully covalently grafted at its reducing end to a polydimethylsiloxane (PDMS) surface via a reductive amination reaction. This method enabled the formation of a wear resistant pectin layer that provided enhanced boundary lubrication compared to adsorbed pectin. Pectins with different degrees of methylesterification and blockiness were exposed to salt solutions of varying ionic strength and displayed responsiveness to solvent conditions. Exposure of the grafted pectin layers to solutions of between 1 and 200 mM NaCl resulted in a decrease in boundary friction and an increase in the hydration and swelling of the pectin layer to varying degrees depending on the charge density of the pectin, showing the potential to tune the conformation and friction of the layer using the pectin architecture and environmental cues. The robust and responsive nature of these new pectin grafted surfaces makes them an effective mimic of biotribological interfaces and provides a powerful tool to study the intricate mechanisms involved in the biolubrication phenomenon

"Liquid, gel and soft glass" phase transitions and rheology of nanocrystalline cellulose suspensions as a function of concentration and salinity

The colloidal size and rod morphology of nanocrystalline cellulose (NCC) lead to suspensions with useful phase and gelation behaviours as well as complex rheologies. However, these have not been comprehensively evaluated previously. Here we report the detailed phase behaviour of sulphonated NCC aqueous suspensions as a function of concentration and salinity. Four phases - liquid, viscoelastic, repulsive glass and attractive glass/gel - are identified in terms of their distinct rheological behaviours. The liquid-solid transitions (LSTs) are determined rheologically, and these are supported by a simplified model based on the DLVO theory that indicates the importance of charge in determining the phase behaviour. Rheology is also used to investigate the solid-solid transition from a repulsive glass to an attractive gel with increasing salt at high NCC concentrations. A time-dependent aging phenomenon is observed in suspensions with a composition just below the LSTs, and the implications of this on the dynamics occurring during gelation processes are discussed. This work can be directly applied to the development of structure-function relationships and the expanding utilisation of NCC suspensions, whilst also providing a basis for the study of charged colloidal rods more generally and evaluation of theoretical models

Particle-wall tribology of slippery hydrogel particle suspensions

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows. We specifically discover that for two completely different sets of microgels, the sliding stress at which slip occurs scales with both the modulus of the particles and the bulk suspension modulus. This suggests that hysteresis losses within the viscoelastic particles contribute to friction forces and thus slip at the particle-surface tribo-contact. It is also found that slip during large amplitude oscillatory shear and steady shear flows share the same generic features

Tribology and its growing use towards the study of food oral processing and sensory perception

Here we provide a comprehensive review of the knowledge base of soft tribology, the study of friction, lubrication and wear on deformable surfaces, with consideration for its application towards oral tribology and food lubrication. Studies on 'soft-tribology' have emerged to provide knowledge and tools to predict oral behaviour and assess the performance of foods and beverages. We have shown that there is a now a comprehensive set of fundamental literature, mainly based on soft contacts in the Mini-traction machine with rolling ball on disk configuration, which provides a baseline for interpreting tribological data from complex food systems. Tribology-sensory relationships do currently exist. However, they are restricted to the specific formulations and tribological configuration utilised, and cannot usually be applied more broadly. With a careful and rigorous formulation/experimental design, we envisage tribological tools to provide insights into the sensory perception of foods in combination with other in vitro techniques such as rheology, particle sizing or characterisation of surface interactions. This can only occur with the use of well characterised tribopairs and equipment; a careful characterisation of simpler model foods before considering complex food products; the incorporation of saliva in tribological studies; the removal of confounding factors from the sensory study and a global approach that considers all regimes of lubrication