Fig. 6.

Dependency of the digit normal force on the load for different friction sets. Group averages. At all loads, the group averages are based on 32 individual observations (8 subjects × 4 trials). Numbers in the figure are the $k_i^{\left(3\right)}$ coefficients and the coefficients of correlation squared (all r² ≥ 0.98). A: thumb forces. Friction sets with the thumb at a low-friction contact (LLL, LHH, LHL, and LLH) are printed with dotted lines. Solid lines represent the tasks with the high-friction contact at the thumb. LOCAL FRICTION EFFECT is the scattering induced by the high- or low-friction contact at the thumb. Two other smaller figure brackets show the synergic effects (i.e., the effect of friction at other digits on the thumb force). Two distinct groups were mainly determined by the local friction effect. B: middle finger forces. Dotted lines designate the low-friction contact at the middle finger. Two groups of the regression lines were mainly distinguished by the friction at the thumb (the synergic effect). Two small figure brackets show the local friction effect (i.e., the spreading induced by the high-or low-friction contact at the middle finger). At a given thumb friction, the forces were larger at the low-friction contact at the middle finger. C: index finger force. Dotted lines designate the low-friction contact at the index finger. In contrast to the thumb and the middle finger, the regression lines do not cluster into 2 distinct groups. Largest forces are exerted in the HLL and LLL tasks when both the index and middle finger were at the low-friction contact. Smallest forces were exerted in the HHH task when all the digits contacted high-friction surfaces.