The impact of object weight, reach distance, discomfort and muscle activation on the location of preferred critical boundary during a seated reaching task.

Successful performance of a goal-directed action requires the prospective actor to perceive the environment relative to their action capabilities and tailor their movements accordingly. The current study examined the roles of reach distance, object (power drill) weight, gender, discomfort, and muscle activation (anterior deltoid, upper trapezius, biceps, ventral and dorsal forearm) in determining the location of the transition between an arm-only and an arm-and-torso reach (preferred critical boundary) during a seated reach task in which participants had to direct a power drill toward a target. Generalized Estimating Equations (GEE) used extrinsic (independent of the participant) and intrinsic measures (relative to the biodynamic properties of the participant) of reach distance and drill weight, discomfort judgments, and EMG integral recordings for the five muscles to identify factors that best predicted the type of reach used. GEE revealed that intrinsic measures of reach distance and drill weight were superior predictors compared to extrinsic measures. Discomfort judgment and upper trapezius activity were also significant predictors of the location of the preferred critical boundary.