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Stepping forward in understanding the Metabolic Cost of Human Walking

Periodic Reporting for period 1 - Metabolic cost of walking (Stepping forward in understanding the Metabolic Cost of Human Walking)

Reporting period: 2015-04-01 to 2017-03-31

Walking accounts for up to 30% of an adult's daily energy cost. The cost of walking is greatly increased in elderly and impaired people (up to 100% compared to young adults), which negatively impacts mobility, independence and quality of life. It is currently impossible to explain the metabolic cost of walking. Unravelling the mechanisms underlying this metabolic cost is of both fundamental and applied interest. For example, it is essential to improve locomotion and thereby quality of life in elderly.
To understand the mechanisms underlying the metabolic cost of walking, we need to know how the energetic cost at the level of an organism as a whole –measured through O2 uptake– is related to that at the level of the motors: the muscles. The energetics of muscles contracting under circumstances resembling locomotion are unknown. We propose to study its key determinants by using an integrative approach, from physiology at the muscle-fibre level to biophysical modelling at the whole-body level.
A central part in our approach is to perform experiments using a unique set-up for directly measuring of O2 consumption of muscle-fibres contracting under circumstances akin to walking; this has never been done before. The experimental results will be used to improve and validate the mechanical and energetic behaviour of our muscle-tendon complex models. Detailed models of the human musculoskeletal system will be used in an attempt to predict in vivo muscle contractile states and from that O2 uptake during isolated human knee extension/flexion movements against different loads, and during human walking. Our integrative approach to study energetics at levels ranging from muscle-fibres to the whole body is expected to have a broad impact on various fields of theoretical and applied life sciences.
We have performed the state-of-the-art oxygen consumption experiments. We have tried to first focus on the relative contribution of cross-bridge cycling and calcium pumping to the efficiency of muscle contractions that are relevant for locomotion. The results were that the overall minimal muscle efficiency was 14% during concentric contractions, of which 37% was due to pumping of Calcium. A paper dealing with this study will be sent out for review within one month.
We have performed extensive set of human experiments measuring the oxygen consumption during isolated knee movements. These experiments were done to answer if Huxley/Hill type muscles can adequately predict metabolic cost during concentric work-loops and if concentric work-loops are energetically more expensive than isometric contractions. These experiments were technically more demanding than anticipated and we will have to rerun some conditions with an adjusted protocol. Nevertheless, these results are promising and it is anticipated that we will be able to get at least one article from these experiments.
We have run very extensive experiments on the energetic cost, kinematics and mechanics of walking on a treadmill versus over ground, both in young adults and healthy elderly. These data are of very high quality and will lead to an article dealing with the finding that elderly walking on an instrumented treadmill initially use more oxygen than their healthy young peers. Second, these data will be used to estimate metabolic cost of walking of a detailed Optimal Control model of a human. This is still work in progress.
Walking is a hard problem to solve with computer simulation. First, we need to incorporate a validated ground contact model for the foot, which is not trivial from both a computational perspective and energetic perspective (foot-ground collisions are a major source of energy dissipation). Second, optimizing the model using computers is technically demanding. Two technical problems were solved separately while addressing issues related to the main topic of this EU funded research. Firstly, we published a paper using an optimal control musculoskeletal model of a jumping human, which is very similar to a walking model. Secondly, we have published another paper in which we used our optimal control model to the sit-to-stand task, which involved a special technique (multi-phase optimal control) that we will need for walking as well.
We are finalizing a study with the validation of the mechanical and energetic behavior of our muscle-tendon model during work-loops. The outcome will hopefully allow us to use a model to adequately predict oxygen consumption or metabolic cost of muscle contractions. Such a model has two major major advantages. First, having a validated model of a muscle-tendon unit reduces the necessity of performing invasive animal studies. Second, a model allows us to estimate the mechanic and energetic variables that are not accessible during human experiments.
To be able to adequately predict behavior of human muscles, we need to extrapolate the mice experimental data and the mouse muscle-tendon model and is done in a second line of research. This study is a crucial step in linking mouse oxygen consumption experiments to a full body optimal control model of human walking capable of predicting the metabolic cost of walking.
One of our goals that should be able to make a substantial societal impact is to investigate the energetics of walking in elderly. Even though many studies report that elderly walk with an increased energetic cost, it is very difficult to compare results across different studies due to differences in methodologies and due to differences in analyses. We have ran an extensive meta-analysis on the topic of the relationship between age and metabolic cost of walking. The main conclusion of the paper was that there is indeed a substantial and significant increase in the energetic cost of walking the elderly. However, we found a possible confounding factor and that was the use of a treadmill. In particular, we found evidence that suggests that the energetic cost of walking on a treadmill is increased in elderly, but not in young adults with respect to normal over-ground walking.
This led us to perform a large experimental study that, in line with our expectations, showed that the elevation in metabolic cost of walking in elderly is due to walking on a treadmill, which is different for older adults than for younger adults. We first showed that there is no statistical elevation in elderly compared to young adults when walking over-ground. Second, during the first encounter with walking on a treadmill, elderly walk with substantial higher metabolic cost that their healthy peers. However, this elevations is completely gone after ample training on the treadmill.
Interestingly, the writing of this article was delayed because we encountered a flaw in a standard method that is very often used for movement analysis (“inverse dynamics analysis”). We have found a solution to that problem and have written an article on how to adjust this.
We have published a long commentary on a new theory of walking that was published in an esteemed journal. The original paper suggested that people could use gravity to propel ourselves forward and could even walk uphill for free when adopting a particular walking technique; this goes against the law of thermodynamics. Because it attracted so much attention in the media, we felt compelled to write a detailed commentary on their paper in which we clearly explained why the mechanics and energetics of this paper is flawed.