People optimize reaching to make straight and smooth movements. We performed experiments characterizing human sensitivity to hand path deviations from a straight reach. Vision of the arm was blocked. Subjects either moved the hand along paths of constrained curvature, or a robot moved the relaxed limb along similar trajectories (active and passive conditions, respectively). Subjects responded after each trial whether or not they thought the movement curved convex right. In a series of three experiments, we tested the effects of modifying visual feedback of hand position to suppress curvature, isotonic muscle activation, and a distracter task on subjects ability to detect curvature during reaching. We found that both active reaching and artificial minimization of visual hand path deviations significantly decreased proprioceptive curvature sensitivity. Specifically, isotonic contraction of muscles antagonistic to the movement decreased sensitivity to curvature while agonistic contraction had no effect. The distracter task did not significantly affect proprioceptive sensitivity, though it did interfere with the detrimental effect of minimizing visual error feedback. These findings demonstrate that: 1) "antagonist" muscle activation decreases efficacy of proprioceptive feedback during hand path curvature estimation, and 2) vision's dominance over proprioception can be manipulated by altering the attentional demands of the task.