Movement Effort does not alter the Planning Horizon of Sequential Reaching Movements

Everyday tasks often involve multiple reaching movements in a sequence, where the choices made for one movement also affect the conditions for subsequent movements. Previous research indicates that humans are capable of planning ahead to optimize such sequences. Here, we ask whether increasing the resistance to movement prompts participants to plan even further ahead to reduce overall movement effort. Participants (n=28) were shown 14 targets that varied in size, value and location, and were instructed to accumulate as many points as possible. To collect each target, they reached while holding the handle of a robotic manipandulum that applied a resistive force. We analyzed their target choices using a planning model that takes into account the size, value and distance of several future targets. We expected that, as resistance – and therefore the effort required to move between two targets – increased, participants would plan further ahead to minimize total movement distance. Results show that participants typically planned 2-4 targets ahead. However, increasing resistance did not significantly affect the planning horizon. We conclude that movement effort is not a primary constraint on the planning horizon in sequential reaching movements.