Normal forces of the middle finger: group averages and standard errors (bars). In the legends in this and the following figures, the first letter indicates surface friction of the finger of interest (in this figure, the middle finger), and the second letter indicates surface friction of the thumb. The solid line corresponds to the finger at high-friction contact (HFC), and the dotted line corresponds to the finger at low-friction contact (LFC). The squares correspond to the thumb at HFC, and the circles to the thumb at LFC.