Figure 4.

Endpoint stiffness was adapted differently in each force field. The stiffness ellipses for each of the eight subjects (S1–S8) participating in the study are shown for each of the four force fields (NF, filled gray; 0DF, dotted red; −45DF, blue; 80DF, thick green). Relative to the NF stiffness, the stiffness measured in the divergent force fields is larger and more anisotropic. The direction of the increased stiffness for each unstable force field is similar across all of the subjects. These directions are close to the directions of instability in the environment.