Proprioceptivity and upper-extremity dynamics in robot-assisted reaching movement

Reaching-against-gravity movements feature some remarkable aspects of human motion, like a wide exploration of the upper extremity workspace and high dynamics. In clinical rehabilitation protocols the recovery of the reaching movement capability is considered as a "paradigm" because of its fundamental role as a precursor for the use of the hand in activities of daily living. Reaching-based protocol may take advantage of robot usage, which has become a standard procedure in rehabilitation of neurological patients although the efficacy of the robot-assisted treatment is still matter of discussion. Even fewer studies in literature investigate proprioception, upper-extremity dynamics and their mutual relationship. Robot-assistance introduces alterations in the dynamics of movements, e.g. limited maximum velocities and accelerations, partial upper-extremity weight support, interaction forces between the robot and a subject. As a consequence, the subjects' proprioception may be altered too. The purpose of this preliminary work is to investigate the relationship between upper-extremity dynamics and proprioception by comparing the estimation of shoulder torques and EMG activation pattern with the evaluation given by the subjects on the quality of the perceived movements during different reaching trials with and without robot assistance. Results show that slow free (nonassisted) reaching movements are felt as uncomfortable and figure large shoulder torques and EMG cocontraction levels. Comfortable movements are those displaying shoulder torques and cocontraction levels comparable to those in natural free reaching, suggesting the strong correlation of torques patterns and co-contractions in motion comfort.