Figure 5.

The characteristics of the endpoint stiffness were modified according to the instability in the environment. A, Changes in the shape of the endpoint stiffness ellipse in the four environments. Compared with the stiffness ellipse in the NF force fields, the stiffness ellipses in all three divergent fields were significantly more anisotropic, indicating that the endpoint stiffness had been selectively increased in a particular direction. Mean values across eight subjects with error bars indicating the SD are shown. Statistics indicate the results of Dunnett's t post hoc test of the difference relative to the NF (**p < 0.001). B, The orientation of the endpoint stiffness ellipse in the four environments. The variability in orientation of the NF stiffness is illustrated by the large SD. After subjects adapted to a divergent force field, the orientation shifted closer to the direction of the instability. Compared with the orientation of the 0DF stiffness ellipse, the −45DF stiffness ellipse rotated significantly in the counterclockwise direction, whereas the 80DF stiffness ellipse rotated significantly in the clockwise direction. Statistics indicate the results of Dunnett's t post hoc test of the difference relative to the DF (*p < 0.05; **p < 0.001). The orientation of the force field is indicated with the gray dotted line for each of the divergent fields.