Metabolic energy consumption during walking increases considerably with disease, injury and age. We poorly understand why, because experimental estimates of energetic cost are limited to average whole-body measures. This lack of knowledge hinders the design of interventions that target walking energetics. Model-based computer simulations have the potential to predict energetics of individual muscles over time, and can be used to elucidate causal relationships between musculoskeletal properties and walking energetics. But current simulations vastly underestimate changes in energetic cost across walking conditions. I hypothesize that errors in predicting the energetic cost of walking are due to using Hill-type models and energetics models, which employ phenomenological relations derived from controlled experiments that do not capture muscle operating conditions during walking. In contrast, biophysical muscle models capture the mechanisms underlying muscle mechanics and energetics, but have not been used in simulations of walking. This project aims to gain fundamental insight into walking energetics through incorporating biophysical muscle models into simulations of walking, revealing contributions of individual muscles and gait phases to overall energetic cost, and predicting the energy savings enabled by assistive devices. To achieve the research aim, I designed 4 work packages. The first work package (WP0) is focused on Training that will help me obtain computational and transferrable skills that are key to realizing the project aims. These skills also contribute to my professional development, and facilitate reaching my career goals. In the next work package (WP1), I will develop a biophysical model that – when integrated in musculoskeletal simulations – captures whole-body energetics. I will use this model to identify mechanisms underlying mechanics and energetics of muscle shortening and lengthening at the single-joint level (Aim 1). In the next work package (WP2), I will use the developed simulations to study muscle energetics across a range of walking conditions. I will use these simulations to infer how individual muscles and gait phases contribute to the energetic cost of walking across walking conditions (Aim 2). Lastly, in the final work package (WP3), I will test the ability of the framework to predict exoskeleton assistance that maximizes energy savings. I will use this framework to provide proof of concept for the computer-aided design of devices to reduce the energetic cost of walking (Aim 3). Key insights into walking energetics obtained during the project will help us understand why metabolic energy consumption during walking varies with movement conditions, impairments and assistance from devices.
In summary, our poor understanding of the mechanisms underlying the energetic cost of walking hinders the development of effective interventions and assistive devices to alleviate the increased energetic cost of impaired walking.
The overall aim of this project is therefore to advance insights into the mechanisms underlying the energetic cost of walking. To accomplish this overall aim, I will pursue the three sub-aims below.
Sub-aim 1 – Identify mechanisms underlying mechanics and energetics of muscle shortening and lengthening at the single-joint level.
Sub-aim 2 – Infer how individual muscles and gait phases contribute to the energetic cost of walking across walking conditions.
Sub-aim 3 – Provide proof of concept for the computer-aided design of devices to reduce the energetic cost of walking.
