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Prosthetic devices are capable of restoring motor functions that were lost due to amputations caused by diseases, traumas or injuries. Advanced prosthetic hands are greatly desired since manipulating objects are a great part of daily life. Research have focused on developing enhanced robotic hands with many degrees of freedom and different control mechanisms that makes use of Electromiography (EMG) or Electroencephalography (EEG) signals, for example. Restoring proprioceptive and tactile feedback of prosthetic devices, however, is still a major challenge to the field. Object manipulation consists of complex interactions between the hand and the object and sensory feedback is crucial for proper scaling grip force to avoid slip and excessive force. Therefore, studying how grip forces are adjusted under different conditions can provide insights for developing control schemes that can be added to prosthetic devices in order to make them easier to use and more reliable. In this paper, we investigated how grip force is modulated during pronation and supination movements. We found that changes in force occur in parallel to changes in object orientation, suggesting that it is not a feedback based control scheme where force is adjusted gradually after a change in the orientation is detected. Furthermore, there is a synergism between the forces generated by the thumb and the index finger to maintain grasp stability. |
