Visual effects on grip force control for two-digit manipulation of objects with different center of mass

Abstract

Dexterous object manipulation requires suitable control of grip force, load force and digit positions. To keep an object stable in the air, force magnitude, force direction and digit positions should be coordinated, producing compensatory torque that could balance the external torque and maintain a minimal object roll. Six males and 6 females enrolled in the study. In the experiment, subjects were required to grasp an object with their thumb and index finger and hold the object stably in the air. The object had three different external torques. Effects of visual feedback on digit force and moment control were analyzed. The compensatory torque was generated by the load force, the grip force and the digits placement. The fitting compensatory torque made the minimal object roll to come true. The results showed that the visual feedback plays an important role in object manipulation especially during stable grip. For both the left and right hands, the average roll angle was significantly increased without visual feedback, leading to an internal rotation of the object. And there was a significant negative correlation between the distance of two digits placements and the difference of two digits load force. This founding helped us to get more knowledge about vision feedback in stable grip and how to coordinate force magnitude and digit position keeping the asymmetrical mass object balance with the thumb and index finger.