A lumbar-degenerative flat back is a condition caused by a decline in back muscle strength with aging. The symptoms of this condition include severe lower back pain and spinal disorders, which restrict people from performing activities of daily living (ADL). Recently, various wearable robots have been developed to assist the weakened muscles. However, despite their effectiveness, the wearables require either bulky and rigid structures (active wearables) or tend to constrain users’ motion (passive wearables). In addition, the complex biomechanics of the human spine have not been sufficiently investigated during the design process. To address these issues, we propose a human-inspired spine assistive robot with a routed twisted string actuator (RTSA, active element) and a flat-back alleviation mechanism (FAM, passive element). This innovative RTSA conforms to the complex human spine curvature while minimizing friction and assists the ADL with its unique prioritizing characteristics. The human spine-inspired FAM, on the other hand, was designed to prevent spinal disorder injury while performing ADL. The experimental results showed that the proposed system successfully reduced the electromyography signals while performing ADL. Furthermore, the spinal disorder injury during trunk motion could be prevented by reducing the lumbopelvic ratio of the users.

Redundant motion, which is possible when robotic manipulators are over-actuated, can be used to control robot arms for a wide range of tasks. One of the best known methods for controlling redundancy is the null space projection, which assigns a priority while executing desired tasks. However, when the manipulator is projected into null space, its motion would be limited, since the motion is only permitted in the direction that does not interfere with the primary task. In this study, we have analyzed the null space projector matrix to derive the appropriate direction of the redundant motion by quantifying the allowed motion in each direction. As a result, we have found an ellipsoidal boundary, in which the redundant motion is permitted to move. We have named this ellipsoidal boundary as ’null space quality’ in directions. The proposed null space quality shows similar aspects with that of the robot manipulability, but it reveals a decisively different value when the manipulator operates within the null space. The experimental results showed that the robotic manipulator tracked the sinusoidal input trajectory with reduced root mean square (RMS) error by 33.84%. Furthermore, we have demonstrated the obstacle avoidance of a robotic arm utilizing the null space projector while considering the null space quality.