Mechanics and energetics of stable bipedal locomotion in uneven terrain: Does a trade-off exist between economy and stability?

Studies of walking and running in humans and birds have revealed that all two legged animals (bipeds) move in similar ways over level ground, despite differences in leg anatomy. This finding has led to elegantly simple theoretical models for economic walking and running. These models have inspired technological advances such as the fastest running tracks, the most economic bipedal robots, and simple prosthetics devices, including the Cheetah Flex-Foot worn by track athlete Oscar Pistorious. Despite these advances, the design of prosthetics that can move with both stability and economy over uneven terrain remains a challenge. Walking and running in natural terrain requires frequent adjustment for bumps, steps, holes and obstacles. Yet, current prosthetic devices for human locomotion perform poorly over uneven terrain. Although the flex-foot prosthetic worn by Oscar Pistorious provides high economy, allowing him to run long distances, it does not adjust for variations in terrain such as kerbs or stairs. Advances such as motorised prosthetic devices can improve mobility in uneven terrain, but currently have limited use because they consume too much energy and run out of battery power quickly. New prosthetics and other mobility aids that perform well in varied terrain could vastly improve quality of life for those with limited mobility, including amputees and ageing individuals at risk for falls. In comparison to these technologies, humans and birds achieve remarkable stability and economy of movement in varied terrain. However, we understand very little about how they accomplish this. Current models of walking and running are based mostly on studies of movement over completely smooth, uniform terrain (such as a track or treadmill), which rarely exist in nature. It has long been supposed that a trade-off exists between economy and stability of locomotion. This idea has never been tested because so little research exists on stability of walking and running. This project will address this problem by by comparing stabilising strategies and energy cost of locomotion among six species of ground birds. We will measure mechanics, stability and energetic cost as birds walk and run over uneven terrain conditions. This will reveal how bipeds choose among different movement strategies depending on terrain condition, speed and gait. Ground birds are ideal study animals for this research because they are diverse bipedal athletes, live in broad range of habitats, and span a large size range: from quail under 100 grams to ostrich over 100 kilograms. Through study of ground birds, this project will develop simple models for stable bipedal locomotion that apply to a broad range of animals and terrain conditions. The models will reveal basic principles for stability and economy that could directly impact upon the design of prosthetics, orthotics and legged robots. One important observation is that small birds walk and run with a crouched, bent-leg posture, and large birds move with a relatively straight-leg posture. The difference in leg posture and anatomy likely reflects greater adaptation for stability in small birds and economy in large birds. For small and large animals moving in the same terrain, any change in terrain height will be a larger fraction of a small animal's leg length. Consequently, the world is a relatively 'rough' place for small animals, requiring robust stability. This project will compare stability strategies used by small and large birds and determine whether small birds are more stable. This research will reveal how bipeds walk and run over uneven terrain without falls or injury. Mobility without falls and injury is particularly important for the wellbeing of ageing and gait impaired individuals. The findings are likely to have high impact in these areas, inspiring innovation in human fall prevention, gait rehabilitation, and the design of prosthetics, orthotics and other mobility assistance devices.

It has been hypothesized that a trade-off exists between stability and economy of locomotion: leg anatomy of small animals reflects optimisation for stability in relatively 'rough' uneven terrain, and that of large animals reflects minimisation of energy cost in relatively uniform terrain. This idea has not been tested because almost nothing is known about locomotion in uneven terrain. By comparing stability in uneven terrain across species, this study will be the first to investigate this trade-off. This project will use an integrative systems approach at the whole-animal level to examine stability and energy cost of locomotion. I will combine experiments and hypothesis driven models to investigate behavioural strategies used by ground birds to negotiate uneven terrain. Six species will be studied, spanning a 500-fold range in body mass (quail, partridge, pheasant, guinea fowl, turkey and ostrich). This will allow analysis of how stability scales with body size, to reveal general principles for bipedal stability. The first half of this project will investigate mechanics of stable locomotion in uneven terrain, and the second half will investigate the link between stability mechanics and energy cost. First, I will experimentally measure mechanics of locomotion in uneven terrain as a function of obstacle height, speed and gait (Aim 1), and test whether current models for bipedal locomotion extend beyond uniform conditions to predict stability in uneven terrain (Aim 2). Second, I will measure energy cost of locomotion over uneven terrain (Aim 3) and compare measured values to predictions based on three fundamental mechanisms: external force, external work, and postural costs (Aim 4). This study will provide a framework for understanding trade-offs between economy and stability across all bipeds from birds and humans to legged robots. The results will have high potential for innovation in human fall prevention, gait rehabilitation and prosthetic design.

The PI is undertaking a number of activities in parallel with this project (described below) to ensure that the insights from this research have the opportunity to inspire innovation, advance methodology and inform educators and policy makers in identified priority areas. 1) Technology development: Engineering of bio-inspired legged robots, and rehabilitation robotics and mobility assistance devices. The PI collaborates with researchers in legged robotics and human rehabilitation sciences to develop models for stable locomotion that can be applied to control robotic exoskeletons and other assistive devices for rehabilitation of patients with mobility impairment. 2) Human lifelong health and wellbeing: Mobility assistance and fall prevention in ageing individuals and those with mobility limiting injury or disease. The PI collaborates with researchers in human rehabilitation and human fall prevention to investigate how insights from this project can be used to inspire novel training and rehabilitation strategies that improve mobility and fall prevention among ageing and gait impaired individuals. 3) Animal health & welfare: The PI is contributing to collaborative research at the RVC on broiler chicken locomotion to improve understanding how the altered body and leg conformation of domestic poultry contributes to gait pathologies, a common problem that substantially impacts the welfare of these animals. The PI will also interface with the BBSRC funded RVC Centre for Animal Welfare to translate her research into applications that raise welfare standards for farmed poultry. 4) Systems approach to biological research: Integrating function from genes, proteins and cells to tissue, functional systems and whole animal behaviour. Ground birds (e.g., quail, chickens and guinea fowl) have been key model animals in genetics, limb development and neural development, providing broad interdisciplinary knowledge that can advance systems biology. The PI has multidisciplinary collaborations at the organ system and whole animal level, and is developing new collaborations in integrative limb function, using proteomic profiles to investigate links between myokine function and whole animal behaviour. Communication and public engagement: Biomechanics is a charismatic field that can motivate and inspire a broad audience to think about the physics behind everyday life. Many principles can be explained in simple terms or shown through video and interactive demonstrations. The PI contributes to mentoring and science education of students from school age through undergraduate and postgraduate levels and her research has been featured numerous times in popular science media. The Structure and Motion Lab (SML) team host an image and video gallery through our web page, visit schools and communicate our science in a range of popular science journals and newspapers. Most recently, the SML team have contributed to 'Animal Autopsy'- a documentary that will provide scientific context for understanding anatomy of animals including giraffe, lion and crocodile. It will air this year on Channel 4 in the UK and National Geographic worldwide. The research in the current project will lend itself to media coverage, which will be pursued through the SML team's established media contacts. Intellectual property and commercial development: There is some likelihood that basic principles from this research could lead directly to improved design for stable legged robots or mobility aids for elderly or disabled people. In the event that insights from this project lead to intellectual property with commercial potential, the PI will work closely with RVC Enterprise, the College's technology transfer team, which has a strong track record in commercialisation of life-science research to pursue commercial development of the technology.