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This study investigated the effects on metabolic cost and gait biomechanics of using a prototype lower-body exoskeleton (EXO) to carry loads. Nine US Army participants walked at 1.34 m/s on a 0% grade for 8 min carrying military loads of 20 kg, 40 kg and 55 kg with and without the EXO. Mean oxygen consumption (VO(2)) scaled to body mass and scaled to total mass were significantly higher, by 60% and 41% respectively, when the EXO was worn, compared with the control condition. Mean VO(2) and mean VO(2) scaled to body mass significantly increased with load. The kinematic and kinetic data revealed significant differences between EXO and control conditions, such as walking with a more flexed posture and braking with higher ground reaction force at heel strike when wearing the EXO. Study findings demonstrate that the EXO increased users' metabolic cost while carrying various loads and altered their gait biomechanics compared with conventional load carriage. STATEMENT OF RELEVANCE: An EXO designed to assist in load bearing was found to raise energy expenditure substantially when tested by soldiers carrying military loads. EXO weight, weight distribution and design elements that altered users' walking biomechanics contributed to the high energy cost. To realise the potential of EXOs, focus on the user must accompany engineering advances.

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In this study, we describe the mechanical design and control scheme of a quasi-passive knee exoskeleton intended to investigate the biomechanical behavior of the knee joint during interaction with externally applied impedances. As the human knee behaves much like a linear spring during the stance phase of normal walking gait, the exoskeleton implements a spring across the knee in the weight acceptance (WA) phase of the gait while allowing free motion throughout the rest of the gait cycle, accomplished via an electromechanical clutch. The stiffness of the device is able to be varied by swapping springs, and the timing of engagement/disengagement changed to accommodate different loading profiles. After describing the design and control, we validate the mechanical performance and reliability of the exoskeleton through cyclic testing on a mechanical knee simulator. We then describe a preliminary experiment on three healthy adults to evaluate the functionality of the device on both left and right legs. The kinetic and kinematic analyses of these subjects show that the exoskeleton assistance can partially/fully replace the function of the knee joint and obtain nearly invariant moment and angle profiles for the hip and ankle joints, and the overall knee joint and exoskeleton complex under the applied moments of the exoskeleton versus the control condition, implying that the subjects undergo a considerable amount of motor adaptation in their lower extremities to the exoskeletal impedances, and encouraging more in-depth future experiments with the device.

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