Normal forces of the thumb: group averages and standard errors (bars). HFC = high-friction contact. LFC = low-friction contact. When one finger was at LFC, Combinations 1, 2, 3, and 4 corresponded to LHHH, HLHH, HHLH, and HHHL, respectively (the four-letter sequences here indicate high [H] and low [L] friction at the index, middle, ring, and little fingers, respectively). When two fingers were at LFC, Combinations 1, 2, 3, 4, 5, and 6 corresponded to LLHH, HLLH, HHLL, LHLH, HLHL, and LHHL, respectively. When three fingers were at LFC, combinations 1, 2, 3, and 4 corresponded to LLLH, HLLL, LLHL, and LHLL, respectively.