Figure 20.

Tangential forces of the thumb versus tangential forces of the virtual finger across 25 trials. Data from a representative subject are shown. The subjects performed 25 trials at each of the external torques −1.0 Nm, −0.5 Nm, 0 Nm, 0.5 Nm and 1.0 Nm while the total load was always 14.9 N. In individual trials the forces were different. However, they were along the same line such that their sum was always constant. All the coefficients of correlation were −1.00. Because the moment of the tangential forces is proportional to the difference between the tangential forces, different location of the forces along the line is indicative of the different moment of tangential forces. The trial-to-trial variations of the moment of tangential forces were compensated by the matching changes of the moments of normal forces (not shown in the figure). Adapted by permission from J.K. Shim, M.L. Latash, and V.M. Zatsiorsky (2003). Prehension synergies: tria-to-trial variability and hierarchical organization of stable performance. Experimental Brain Research. 2003, 152: 173–184.