Abstract
The end-state comfort effect (ESC) has been reliably observed in numerous studies conducted under vastly different conditions (e.g. Rosenbaum & Jorgenson, 1992; Hughes & Franz, 2008; Cohen & Rosenbaum, 2011). Explanations for the effect typically involve easing precision demands (e.g., Short & Cauraugh, 1996) or maximizing movement stability, control and speed (e.g., Rosenbaum, Heughten & Caldwell, 1996). However, all of these studies have confounded the “end-state” of the movement with the “goal-state” in that the locus of the task goal typically coincides with the terminal action requirement of the task. If the proposed explanations for ESC are correct, it would be expected that adoption of the most comfortable and/or stable posture would occur at any point throughout the movement wherever the most precision, control and stability is required. To test this hypothesis, we created a task in which the movement’s goal-state and end-state were distinct. Participants were required to pick up and then squeeze a bottle in order to activate a laser to be aimed at either circular targets (discrete task) or through two sine waves (continuous task). Three different normalized force requirements and three indices of difficulty presented in the middle of the movement were tested. We hypothesized that participants would demonstrate typical ESC grip selection at easier indices of difficulty and lower force conditions, but adopt GSC grip selection at higher indices and force conditions. Results suggest strong support for GSC, as opposed to ESC, as participants (n = 14) selected a comfortable grip posture during the aiming of the bottle 96.0% of the time even though this required participants to select uncomfortable grips when both initiating and ending the movement. Surprisingly, GSC grip selection was observed to occur independently of accuracy and force requirements. Results are discussed in the context of hierarchical models of motor control.Acknowledgments: National Sciences and Engineering Research Council (U.S.R.A.)