Feasibility Study of a Thruster-based Wearable Robot for Aquatic Knee Exercise

Aquatic therapy exercises utilize buoyancy and hydrodynamic resistance forces, providing a safe and effective exercise environment for participants, particularly elderly individuals, those with obesity, and patients undergoing rehabilitation. However, conventional aquatic exercise tools, such as buoyant aids and rigid fins, offer limited capacity for providing active assistance or resistance. To address these challenges, this study proposes a wearable robotic device for aquatic lower-limb exercises, equipped with a bidirectional thruster. The robot measures knee joint angles in real time and controls the thrust moment to enable various exercise modes, including passive exercises such as stretching and continuous-passive-motion (CPM), as well as active exercises with assistance and resistance modes. To evaluate the performance of the proposed system, a dynamic model was derived, and its parameters and relationships were experimentally identified. A prototype was subsequently fabricated and attached to a motor-driven lower-leg mock-up. Control experiments were then conducted underwater across the proposed exercise modes to assess the feasibility of the proposed exercise mode. These results verify the feasibility of closed-loop, thruster-based assistance and resistance for lower-limb aquatic exercise, laying the groundwork for forthcoming human-subject trials aimed at elderly and post-operative populations.