In vivo thickness and dynamic behaviour of the heel fat pad during ageing: biomechanical and anthropological impact of footwear.

The heel pad is a complex structure between the heel bone and the skin which is important as a shock absorber for healthy gait, and therefore it affects quality of life. However, its ability to do so deteriorates during ageing because its mechanical properties change. It is unclear whether these changes result from ageing as such, and/or from overuse. The former cannot be changed, but the latter can be mitigated by several factors, including footwear.

An individual's footwear choice often depends on culture and design, more than on function - known as the shoe's biomechanics - and therefore these are important to consider. This interdisciplinary study will bring together elements of anthropology (what are cultural and design drivers in footwear choice?) with functional anatomy and biomechanics. We will collect laboratory (UK) and field (India, Namibia) data on the properties of the heel pad and assess how they change with ageing and footwear. We will study young and older populations with different footwear use to disentangle the effects of ageing and footwear habits on heel pad properties and gait.

Access to sophisticated laboratory equipment in the UK will allow us to use 3D X-Ray video recordings to show how the heel pad deforms during walking barefoot, and in footwear: minimal and cushioned. We will also use MRI to build a 3D foot model including the heel pad, record the motions of the leg and foot, and measure the forces between the substrate and the foot or shoe. We will study a younger (18-40) and an older (60+) population to answer the questions: (1) how do changes in foot function and estimated tissue loading (using Finite Element Modelling) relate to heel pad changes between these groups, and (2) what are the here-and-now effects of various amounts of footwear cushioning?

Our field study in India study will address the question: what is the effect of long-term use of footwear? We will answer this by comparing participants that wear shoes daily, to participants who never wear shoes - a considerable sub-population in rural parts of South India. If footwear affects heel pad degradation, we expect to see differences between the two groups, when matched for age and other factors.

Our Namibia study will work with participants from the San (Ju|'hoansi bushmen) community, one of the most genetically diverse populations on earth. They display a range of footwear habits from Western-type shoes to indigenous hunting sandals. They also habitually walk on natural substrates. We will investigate whether the relationships between the heel pad and gait are similar between populations. We will also aim to understand some of the reasons behind footwear choice in our populations.

In all studies, we will use portable ultrasound imaging to measure thickness and compressibility of the heel pad, an in-house designed strain measurement device to estimate its mechanical properties, video to measure gait, and a wearable data logger to measure foot strike impact and ankle movements. We will assess cultural versus functional drivers for footwear choice through structured interviews.

Combined, these studies will allow us to disentangle the effects of ageing and footwear habits on heel pad properties and gait, laying a foundation for understanding the impact of heel pad health, ageing and quality of life. These studies will also support the transition from primarily lab-based, Western biomechanics to a more inclusive, diverse, and interdisciplinary research field. We will develop new knowledge to determine if, and what type of footwear can prevent deterioration of the heel pad during ageing, and if footwear with individually tuned cushioning can help mitigate existing loss in heel pad quality. Doing so will result in better foot function, effective gait, and better quality of life in older people.

We will assess the effects of ageing and footwear on the degradation of the heel pad, and how this affects gait. This observational study uses techniques from biomechanics and anthropology to study European, African and Asian populations; the inclusion of three diverse populations will lead to more generalisable results.

UK lab-based experiments for in-depth characterisation will use biplanar X-Ray videography (100 fps) on younger (18-40) and older (60+) volunteers (N=14 per group) matched for basic biometrics (height, mass, BMI, appendicular body fat) footwear habits and activity levels. Subjects will walk barefoot, in minimally cushioned, and in cushioned shoes. Synchronised ground reaction force recordings (Kistler) and MRI-based hindfoot reconstructions (0.4 mm isotopic resolution) and published tissue properties will be used to build a Finite Element Model of heel pad and rearfoot loading.

We will work with habitual barefoot and shod walkers from India, to disentangle the effects of ageing and (over)use on heel pad degradation, and participants from the San community to understand the impact of culture on footwear choice. All participants will have ultrasound imaging (Clarius) to determine heel pad and skin thickness, shear strain (in-house) to determine fundamental mechanical properties, plus portable accelerometry and ankle goniometry (Biometrics) with video analyses (4k, 50 fps) to measure foot strike impact and gait kinematics (speed, stride length and frequency, duty factor). Overseas experiments will recruit a total of 90 subjects per site (30 younger, 30 age 40-60, 30 older).

All participants will be interviewed regarding cultural and design anthropological aspects of their habitual footwear. This is important because such aspects often dominate over functional properties when selecting footwear.

Data from the project will inform work to develop individualised footwear to stimulate optimal foot health and gait throughout the life course.