Key pointsr The neuromotor system generates flexible motor patterns that can adapt to changes in our bodies or environment and also take advantage of assistance provided by the environment.r We ask how energy minimization influences adaptive learning during human locomotion to improve economy when walking on a split-belt treadmill. We use a model-based approach to predict how people should adjust their walking pattern to take advantage of the assistance provided by the treadmill, and we validate these predictions empirically.r We show that adaptation to a split-belt treadmill can be explained as a process by which people reduce step length asymmetry to take advantage of the work performed by the treadmill to reduce metabolic cost.r Our results also have implications for the evaluation of devices designed to reduce effort during walking, as locomotor adaptation may serve as a model approach to understand how people learn to take advantage of external assistance.Abstract In everyday tasks such as walking and running, we often exploit the work performed by external sources to reduce effort. Recent research has focused on designing assistive devices capable of performing mechanical work to reduce the work performed by muscles and improve walking function. The success of these devices relies on the user learning to take advantage of this Natalia Sánchez holds a BS in Biomedical Engineering from the Universidad EIA-CES, Colombia, and MS and PhD degrees in Biomedical Engineering from Northwestern University, researching the neuropathophysiology of stroke. She completed her postdoctoral training at the Division of Biokinesiology and Physical Therapy at the University of Southern California, where she is now an Assistant Professor of Research. Her research focuses on investigating gait adaptation in healthy and pathological populations using experimental and statistical methods. Her research is funded by the American Heart Association and the Mentored Career Development in Clinical and Translational Science programme (KL2). Surabhi Simha holds a Bachelor of Engineering in Biotechnology from Sri Jayachamarajendra College of Engineering, India. She is currently pursuing her PhD in Biomedical Physiology and Kinesiology at Simon Fraser University, Canada. In her research, she uses experiments and computer simulations to understand the algorithms employed by the human nervous system to control walking. N. Sánchez and S. N. Simha contributed equally to this work. This article was first published as a preprint: Sánchez N, Simha SN, Donelan JM, Finley JM (2018). Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split-belt treadmill walking. bioRxiv https://doi.N. Sánchez and others J Physiol 597.15external assistance. Although adaptation is central to this process, the study of adaptation is often done using approaches that seem to have little in common with the use of external assistance. We show in 16 young, healthy participants that a common approach for studying adaptation,...

Changes in the control of the lower extremities poststroke lead to persistent biomechanical asymmetries during walking. These asymmetries are associated with an increase in energetic cost, leading to the possibility that reducing asymmetry can improve walking economy. However, the influence of asymmetry on economy may depend on the direction and cause of asymmetry. For example, impairments with paretic limb advancement may result in shorter paretic steps, whereas deficits in paretic support or propulsion result in shorter nonparetic steps. Given differences in the underlying impairments responsible for step length asymmetry, the capacity to reduce asymmetry and the associated changes in energetic cost may not be consistent across this population. Here, we identified factors explaining individual differences in the capacity to voluntarily reduce step length asymmetry and modify energetic cost during walking. A total of 24 individuals poststroke walked on a treadmill, with visual feedback of their step lengths to aid explicit modification of asymmetry. We found that individuals who took longer paretic steps had a greater capacity to reduce asymmetry and were better able to transfer the effects of practice to overground walking than individuals who took shorter paretic steps. In addition, changes in metabolic cost depended on the direction of asymmetry, baseline cost of transport, and reductions in specific features of spatiotemporal asymmetry. These results demonstrate that many stroke survivors retain the residual capacity to voluntarily walk more symmetrically on a treadmill and overground. However, whether reductions in asymmetry reduce metabolic cost depends on individual differences in impairments affecting locomotor function.

A combination of new surface and subsurface structural data, new stratigraphic data on conventional provenance, facies and palaeocurrents, low-temperature thermochronology and detrital zircon U–Pb provenance data provides a comprehensive account of the timing of deformation in the intermountane Middle Magdalena basin of the Central Colombian Andes, and allows evaluation of the style of foreland basin deformation associated with tectonic inversion. This robust dataset enabled documentation of focused tectonic activity in two competing low-relief basement structures to the east and west of the present Middle Magdalena Valley during the Palaeogene, earlier than previously recognized. Cenozoic sediment accumulation of a sedimentary pile up to 7 km thick in the Middle Magdalena Basin created a large original taper angle in this part of the north Andes. At that time, when the detachment rocks were deeply buried, the original larger taper angle facilitated the forelandward advance of deformation instead of promoting its stagnation.Supplementary material:Raw data results from geochronometrial analyses are available at: http://www.geolsoc.org.uk/SUP18627

Split-belt treadmill adaptation can be seen as a process where people learn to acquire positive work from the treadmill to reduce energetic cost. Though we know what people should do to reduce energetic cost, this strategy is not observed during adaptation studies. We extended the duration of adaptation and show that people continuously adapt their gait to acquire positive work from the treadmill to reduce energetic cost. This process requires longer exposure than traditionally allotted.

35In everyday tasks such as walking and running, we exploit the work performed by external 36 sources such as gravity to reduce the work performed by muscles. There has been 37 considerable recent effort to design devices capable of performing mechanical work to 38 improve walking function or reduce effort. The success of these devices relies on the user 39 adapting their natural control strategies to take advantage of assistance provided by the 40 device. Although locomotor adaptation is central to this process, the study of adaptation is 41 often done using approaches that on the surface, seem to have little in common with the 42 use of external assistance. Here, we show that one of the most common approaches for 43 studying this process, which is adaptation to walking on a split-belt treadmill, can be 44 understood from a perspective in which people learn to take advantage of mechanical work 45 performed by the treadmill. During adaptation, people systematically adjust their step 46 lengths, defined as the distance between the feet at heel strike, from one step to the next. 47 alternative explanations for why the nervous system may adapt to reduce negative step