Figure 2.

Illustration of the three Modes of normal forces associated with the principal components computed from the data and the simulations, across all subjects, and conditions [adapted Rácz et al., (2012)]. Please note that the loadings are the unit vectors describing the multidimensional correlation defining each PC. Therefore the loadings for this PC show that the thumb, index and middle finger forces all co-vary in this Mode. We refer to these three PCs as: (i) the task-irrelevant Grasp Mode, along [0.81 0.41 0.41]^T, as it reflects synchronous increases and decreases in the three normal forces, which are also known as grasp forces, (ii) the Compensation Mode, along [0.0 −0.71 0.71]^T, reflecting the out-of-phase opposition, or compensation, of thumb normal force by either the index or middle finger normal force, and (iii) the task-relevant Hinge Mode, along [0.6 −0.5 −0.5]^T, reflecting an increase (decrease) in thumb normal force accompanied by a simultaneous decrease (increase) in the index and middle finger normal forces, which would typically occur if the object was accelerated by the thumb, thus violating the mechanical task requirements of static grasp (without loss of generality, the violation of static grasp by purely rotating the object using tangential forces is not considered here, see Discussion).