Metabolic Effects Induced by a Kinematically Compatible Hip Exoskeleton During STS.

OBJECTIVE: Show the benefit of kinematically compatible joint structures in exoskeletons for improving their performance in reducing metabolic consumption.
METHODS: Subjects were fitted with a hip exoskeleton, with misalignment compensation for all degrees of freedom and were instructed to perform recurring sit-to-stand motions for 5 min. This was executed three times: Unequipped (i.e., not wearing the exoskeleton), assisted, and unassisted. During each trial, oxygen consumption and muscle activity were monitored.
RESULTS: An increased oxygen consumption was observed between the unequipped and the unassisted trial. During the assisted trial, oxygen consumption was reduced to levels seen in the unequipped state. Muscle activity increased for rectus femoris and tibialis anterior and decreased for biceps femoris and gluteus maximus.
CONCLUSION: Oxygen consumption only increases in accordance with the added mass. No added penalty was seen related to increased inertia or hindrance of natural motion patterns. This indicates that the mechanism operates as intended. The increased muscle activity can be explained by the nature of the actuation system, which is not optimized for sit-to-stand tasks. A more targeted actuation system can easily reduce muscle activity, and therefore, induce a reduced oxygen consumption, below unequipped levels. SIGNIFICANCE: Because the benefits induced by using these systems are independent of user capabilities or deficiencies, it is applicable in a wide range of exoskeleton applications. The design presented here, allows for the realization of compact and light devices, that have a minimal impact on the metabolic cost of their user. This allows to maximally exploit the metabolically beneficial effects of a well-designed actuation system.