Figure 1

Force vector map. An example of the force vector map (direction (left) and magnitude (right)). The colored lines in the left pane show the measurement x- and y-forces for the abductor pollicis longus (AbPL), opponens pollicis (OpP) and the flexor pollicis brevis (FPB) muscles. The determined muscle force directions are indicated by the grey lines. The small variations indicate that the angles are relatively constant throughout the operating range. The force vector map in the left pane is shown on top of an overview of a custom built setup for restraining wrist movements and measurement of thumb forces with two pre loaded single axis force sensors. The fitted sigmoidal recruitment curves for the three thumb muscles and the individual measurement points (steady state of step responses at different amplitudes) are shown on the right.