Viewing hand grip enhances observer's grip force in a body-part-specific manner

We examined whether visual information on the dynamic aspect of the actions performed by an individual can influence an observer's action. Sixteen participants cyclically generated an isometric precision grip force with their right thumb and index finger in synchronization with the contraction (in-phase) or relaxation phase of an experimenter's hand, foot, and mouth movements presented in videos. Visual information of the hand action significantly enhanced the observer's grip force, and this enhancement exclusively occurred during the in-phase condition. These results suggest that the effector matching between the observed and the performed actions and the temporal coincidence in the movement phase are the key factors when visual information on the dynamics of the action of an individual influences the observer's action.