System Design of a Soft Underwater Exosuit to Reduce Metabolic Cost Across Multiple Aquatic Movements During Diving

Assisting underwater movements improves divers' efficiency and reduces the risk of decompression sickness from physical activity. Although exoskeletons have been developed for numerous land-based scenarios, their application in underwater diving remains unexplored. This article proposes a soft underwater lower-limb exosuit designed to assist three aquatic movements: flutter kick, breaststroke kick, and underwater walk. We presented the mechanical design of the exosuit that is capable of assisting bidirectional leg movements in full kicking/gait cycle, while ensuring natural leg mobility without impeding normal leg function. A cascade force integral controller is also designed to resolve issues related to uncontrollable states and stiffness variations within the system. To verify the assistive performance of the system, experiments were conducted with nine participants to assess how the proposed exosuit aids in reducing metabolic cost across various motion patterns and frequencies. The findings indicate that the underwater exosuit effectively reduces the air consumption rate by $29.77\pm 7.68$% during flutter kick, $25.70\pm 5.99$% during breaststroke kick, and $18.35\pm 4.53$% during underwater walk.