Comparison of Muscular Activity and Movement Performance in Robot-Assisted and Freely Performed Exercises

End-effector-based robotic systems are, in particular, suitable for extending physical therapy in stroke rehabilitation. An adequate therapy and thus the recovery of movement can only be guaranteed if the physiological muscular activation and movement performance are influenced as little as possible by the robot itself. Yet, this relation has not been investigated in the literature. Therefore, 20 healthy subjects performed free and robot-assisted exercises under different control settings supported by an end-effector-based system. The control settings differed concerning changes in the end-effector velocity and the stiffness of the robot joints. During the exercises, data from inertial measurement unit sensors, robot kinematics, and surface electromyography were collected for the upper limbs. The results showed an increase in muscular activity during robot-assisted movements compared to freely performed movements and also differences in movement performance. The change of the control setting influenced the muscular activation, but not the movement performance. The results of the study revealed that the robot could not be regarded as only a passive element. This should be kept in mind in future robotic rehabilitation systems in order to reduce the influences of the robot itself and thus to optimize the therapy.