Schematic representation of the setup and the posture of the participants.
The joint angles were the same for all participants. (a) Side view of the setup. In green, the back and top frame are drawn. The back frame was attached to the back of the chair, while the top frame was positioned above the participant. The back frame prevented movement of the chair, while the shoulder straps kept the participant in the same position throughout the experiment. A sling was attached to the top frame, in which the lower arm was inserted, to make sure that only small movements of the elbow in the horizontal plane were possible. Above the top frame, a webcam was mounted to check the posture of the participants. In grey, the chair is shown, which could be adjusted in height and position before it was attached to the back frame. The HapticMaster and the infrared Optotrak markers on top are drawn in light and dark blue, while the participant is shown in black. In purple, the four planes in which the 26 force directions were defined are shown. In experiment 1, participants were blindfolded. In experiment 2, participants received visual feedback via the screen (green open rectangle), which they could see through a gap in the frame (dashed green line). A cloth (green, underneath frame) prevented them from seeing their own hand. (b) Top view of the setup, showing the 8 force directions that were defined in each of the planes. The orange directions are the ones that are present in every plane, resulting in 4 × 6 + 2 = 26 unique directions. The red dotted lines and arcs show the postural angles: α = 38°, β = 98°, γ = 125°.