A major goal of powered lower-limb exoskeletons is to act in parallel with the user’s leg muscles and reduce metabolic energy con-sumption during locomotion. Recent designs have focused on portable devices that can mimic the normal torque output of the lower-limb joints over the full gait cycle using large, powerful motors under high gain force control. Powerful motors are heavy, require bulky gears and mounting frames, and rely on even larger power sources. Furthermore, we are unaware of any study to date that de-monstrates a metabolic savings during walking with a portable lower-limb exoskeleton. On the other hand, a recent study indicates that when humans don tethered (i.e. nonportable), bilateral, lightweight, pneumatically powered ankle exoskeletons that replace only ~63% of the ankle muscletendon mechanical work during push-off, they reduce their metabolic energy consumption by 10-12% du-ring treadmill walking . Thus, supplying mechanical energy at a single joint (i.e. the ankle) during a key propulsive phase of wal-king (i.e. pushoff) can have appreciable metabolic benefits. Our goal in this study was to develop a portable device capable of providing ankle joint mechanical assistance during walking without using external power from onboard actuators (i.e. an ‘energy-neutral’ solution). Human walkers exploit a key passive dynamic principle of locomotion: elastic energy storage and return. Early in stance, strain energy is stored in the Achilles’ tendon and then it is recovered later, providing up to 60% of the ankle joint mechanical work during push-off . We hypothesize that a passive wearable device using parallel elastic elements during the walking cycle is capable of recycling a significant portion of the ankle joint mechanical work and could reduce the metabolic cost of walking by up to 18% .

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