###### Abstract  
Difficulty walking in older adults affects their independence and ability to execute daily tasks in an autonomous way, which can result in a negative effect to their health status and risk of morbidity. Very often, reduced walking speed in older adults is caused by an elevated metabolic energy cost. Passive exoskeletons have been shown to offer a promising solution for lowering the energy cost of walking, and their simplicity could favor their use in real world settings. The goal of this study was to assess if a constant and consistent low torque applied by means of a passive exoskeleton to the hip flexors during walking could provide higher and more consistent metabolic cost reduction than previously achieved. Eight older adults walked on a treadmill at a constant speed of 1.1 m/s with and without the hip assistive device. Metabolic power and spatiotemporal parameters were measured during walking in these two conditions of testing. The hip assistive device was able to apply a low torque which initiates its assistive effect at mid-stance. This reduced the metabolic cost of walking across all the participants with respect to free walking (\(-4.2\pm 1.9\%\); \(p=0.002\)). There were no differences in the spatiotemporal parameters reported. This study strengthened the evidence that passive assistive devices can be a valuable tool to reduce metabolic cost of walking in older adults. These findings highlighted the importance of investigating torque profiles to improve the performance provided by a hip assistive device. The simplicity and usability of a system of this kind can make it a suitable candidate for improving older adults' independence. exoskeleton; hip assistance; elderly population; metabolic cost; human-robot interface; gait analysis +
Footnote †: journal:  
In the last years exoskeletons have been developed with the purpose of reducing metabolic cost during walking [8; 9; 10; 11]. To achieve this goal, engineers developed different devices which applied an assistive torque to the lower limbs: some of them are active, powered by batteries or external actuators [9; 10; 12], while others are passive [11; 13; 14; 15; 16], storing and releasing part of the mechanical energy generated by the human body at specific phases of the gait cycle by means of elastic mechanisms. Despite research conducted in this field, only 13 studies to date have reported successful reduction of metabolic cost of walking while using an exoskeleton, as listed in a recent detailed review on the exoskeletons' expansion [17]. Some of these studies have also shown that individuals exhibit significant physiological and neurological differences which can lead to a large variety, and sometime contrasting metabolic responses when using the same assisted device [9; 11; 18]. To this extent, [PERSON] and colleagues [11] reported a large variability of metabolic cost reductions between the participants involved in their study. These results were obtained from people walking with a passive ankle device varying the level of stiffnesses applied to the joint by means of a spring acting in parallel with the Achilles tendon [11]. Another study involving a powered hip exoskeleton [18] highlighted how time spent walking with the exoskeleton affects metabolic cost reduction, and how this familiarization time changes on an individual basis. Lastly, [PERSON] et al. [9] reported a reduction of 8 \(\pm\) 3% while walking with an active ankle exoskeleton relative to not wearing the exoskeleton. Previous literature [17] has underlined the importance of control architectures that are able to provide subject-specific devices' level of assistance to optimize metabolic savings for each individual [19; 20]. To this extent, [PERSON] and colleagues [19] conducted real time optimization of the torque patterns using an ankle exoskeleton emulator system which tuned four parameters: peak torque, timing of peak torque, and rise and fall timing.