Even though both arms are just being moved by the robot, people feel like they’re working harder mentally when both arms are moving together than when only one arm is moving.

When people’s arms are moved by a robot without seeing where they’re supposed to go, their muscles don’t work as hard as when they can see the target path—even if they’re not moving on their own.

When both arms are moved together by a robot while watching a screen, healthy people’s muscles work harder than when only one arm is moved, even if they’re not trying to move it themselves.

When people get both a visual target and a gentle push from the robot at the same time, they hit the target more accurately and with less muscle strain than when they get only one kind of cue.

When healthy people move their arm with a robot using just one kind of feedback (like seeing a target or feeling resistance), they make more mistakes and use more muscle effort than when they get both kinds of feedback at once.