Task goal and grip force dynamics

This study examined the effect of task goal on the structure of isometric force variability during precision grasping. In general, variability of isometric force production decreases when participants are asked to maintain a particular force output and are provided with visual feedback, although the irregularity of force output tends to increase under these conditions. In the current study we compared the tasks of holding an object using a precision grip and holding an object using a precision grip while matching a force target. Adults held an object between the index finger and thumb and force output was measured using load cells. The mass (92, 276, 460 g) and the grip aperture (5.5 and 8.5 cm) of the object were varied producing six different object conditions. The goal of the task was to either: (a) hold the object comfortably in a stable position (holding task) or (b) hold the object comfortably in a stable position while maintaining a constant target force level that matched the grip force of the holding condition (target task). The results showed that the amount of force variability in the target condition was lower than during the holding condition, while the force output was more regular in the holding condition. Increments in object mass increased force regularity in the holding condition whereas increments of force level decreased regularity in the target condition. The level of coherence between the two digits was very high (approximately 0.98) and maximum coherence occurred at a higher frequency during the target (0.94 Hz) as opposed to the holding (0.70 Hz) condition. The findings reveal that the goal of the task can qualitatively change the dynamical organization of the force output in prehension, even when the average force level produced is the same. This effect on the control strategy was mediated by visual information processes that interact with level of force output in determining the structure of variability. Theorizing about the organization of isometric force output should include the effects of task goals as well as the level of force per se.