Autonomous exoskeleton reduces metabolic cost of human walking during load carriage

Abstract

BackgroundMany soldiers are expected to carry heavy loads over extended distances, often resulting in physical and mental fatigue. In this study, the design and testing of an autonomous leg exoskeleton is presented. The aim of the device is to reduce the energetic cost of loaded walking. In addition, we present the Augmentation Factor, a general framework of exoskeletal performance that unifies our results with the varying abilities of previously developed exoskeletons.MethodsWe developed an autonomous battery powered exoskeleton that is capable of providing substantial levels of positive mechanical power to the ankle during the push-off region of stance phase. We measured the metabolic energy consumption of seven subjects walking on a level treadmill at 1.5 m/s, while wearing a 23 kg vest.ResultsDuring the push-off portion of the stance phase, the exoskeleton applied positive mechanical power with an average across the gait cycle equal to 23 ± 2 W (11.5 W per ankle). Use of the autonomous leg exoskeleton significantly reduced the metabolic cost of walking by 36 ± 12 W, which was an improvement of 8 ± 3% (p = 0.025) relative to the control condition of not wearing the exoskeleton.ConclusionsIn the design of leg exoskeletons, the results of this study highlight the importance of minimizing exoskeletal power dissipation and added limb mass, while providing substantial positive power during the walking gait cycle.

Enhancing performance during inclined loaded walking with a powered ankle–foot exoskeleton

“…During the exercise test, assistance of the exoskeleton resulted in a reduction in net metabolic power of 8 % at lower intensities (walking without weights) and 10% at higher intensities (walking with 100% of maximal unpowered weight). This percentage reduction in net metabolic power is similar with previous research during assisted loaded walking (Mooney et al, 2014) or uphill walking without weights (Sawicki and Ferris 2009a) and emphasizes that subjects can benefit from plantarflexion assistance during uphill loaded walking. Also the lactate values that did not differ between both conditions in the beginning of the exercise test and that differed when intensities increased indicate that the lactate threshold is exceeded later in the exercise test in the powered condition and shows that plantarflexion assistance reduces the effort for a specific workload.…”

“…While most research is focused on technical enhancements, a quantitative evaluation of the effectiveness is often missing (Dollar and Herr 2008). The metabolic energy expenditure, often calculated as metabolic power (W•kg -1 ) based on oxygen consumption and carbon dioxide using a standard equation (Brockway 1987) and body weight normalization, is a key value in the evaluation of several exoskeleton devices (Galle et al 2013a;Malcolm et al 2013;Mooney et al 2014;Norris et al 2007a;Sawicki and Ferris 2008, 2009a, 2009bWehner et al 2013). Regardless of the functional goal of the device, reducing the metabolic power will improve the usability of the exoskeleton (Ferris et al 2007) and can therefore be considered a prime outcome when evaluating exoskeleton effectiveness, that can even be used to drive kinematic behavior with exoskeletons (Collins and Jackson 2013).…”

“…While they showed that it was possible to reduce metabolic power below the level of normal walking, their device was not autonomous, meaning that it needed power and air supply and extensive hardware which was not carried by the user. However, Mooney et al (2014) showed that it is possible to make a fully autonomous exoskeleton that assists plantarflexion and that can reduce the metabolic power of loaded walking with 8% versus normal walking if the design of the device is altered in order to reduce distal mass. Furthermore, there is increasing progress in energy recycling approaches (Collins and Kuo 2010;Donelan et al 2008;Li et al 2009;Malcolm et al 2013, Unal et al 2012) and soft exosuits (Wehner 2013), which makes it likely that autonomous ankle-foot exoskeletons can become a permanent fixture in daily life.…”

“…However, we are not aware of any succesfull attempts in increasing walking performance during maximal exercise intensities as current research is mainly focused on submaximal intensities (Galle et al 2013a;Malcolm et al 2013;Mooney et al 2014;Norris et al 2007a;Sawicki and Ferris 2008, 2009a, 2009b. Studying higher intensities is also useful for applications in specific populations (e.g.…”

“…Because they cannot achieve the high inflation and deflation frequency needed for running, higher intensities without drastically changing the walking pattern (Franz and Kram 2012;Harman et al 2000;Lay et al 2006;Lay et al 2007) can only be achieved during uphill walking and by adding external weights (Kramer 2010). It seems reasonable that subjects can benefit from push-off assistance during loaded uphill walking as previous research showed that subjects can benefit from an exoskeleton during uphill walking (Sawicki and Ferris 2009a) and during load carrying (Mooney et al 2014). Therefore, exoskeleton locomotion during maximal exercise intensities could be tested with the weighted walking test (Klimek and Klimek 2007) or a similar alternative.…”