The Influence of Time Spent in Beginning and End-State Postures on Grasp Choice

The choice to end comfortably often requires adoption of uncomfortable beginning states, demonstrating that a sequence of movement is planned in advance of movement onset. Many factors may influence the choice of comfortable end-state postures including the greater precision and speed afforded by postures at joint angle mid-ranges (Short & Cauraugh, 1999). There has been no evaluation of the hypothesis that postures are chosen based on minimizing time spent in postures. Typically, the time spent in beginning and end-states are not explicitly constrained, but the end-state posture is likely held for the longer amount of time due to greater precision or task demands (Fitts, 1954). The aim of this experiment was to examine how the relative time required to hold a beginning and end-state posture influenced the choice of posture. We predicted that we would see more thumb-up postures for positions held longer regardless of the end or beginning state of grasp. Participants completed four conditions: unconstrained beginning and end state, constrained beginning state, constrained end state, and constrained beginning and end state. Within each condition, participants moved a wooden dowel rod from one location to another with the requirement to grasp the object with either a thumb up or thumb down posture and to place a specified color of the object down. In addition, by using a conductive object and surface, we were able to measure reaction time movement times, and grasp times. Two strategies for planning emerged. The majority of participants chose to end comfortably regardless of the time required for the beginning state grasp. A smaller group of participants appeared to minimize the effort of motor planning by choosing beginning state comfort on trials that required the beginning state to be held longer