EMG OF THE HUMAN FLEXOR POLLICIS LONGUS MUSCLE - IMPLICATIONS FOR THEEVOLUTION OF HOMINID TOOL USE

Modern humans possess a distinct and well-developed flexor pollicis lo ngus muscle, an extrinsic thumb flexor which is ''either rudimentary o r absent'' in great apes (Straus, 1942, p. 228). Previous workers (e.g ., Napier, 1962; Susman, 1988) have related the origin of a well-devel oped flexor pollicis longus muscle to the acquisition of precision gra sping and stone tool making capabilities in early hominids. The propos ed functional association between flexor pollicis longus activity, pre cision grasping, and stone tool manufacture has, however, never been t ested experimentally. This study uses electromyographic techniques (EM G) to investigate the role of flexor pollicis longus during a variety of tool making, tool using, and manipulatory behaviors in order to det ermine the functional and evolutionary significance of the human flexo r pollicis longus muscle. Our results indicate that flexor pollicis lo ngus is recruited during forceful tool using and stone tool making beh aviors, regardless of the power or precision grip used to hold the too l. In particular, both stone tool use and stone tool making employing three-and four-jaw chuck precision grips elicit consistently high leve ls of FPL activity. Flexor pollicis longus activity increases most whe n resistance is increased to the thumb's volar pad during these hammer ing, cutting, and knapping behaviors. In contrast, we observed relativ ely low levels of flexor pollicis longus activity during the fine mani pulation of food items, the making of slender wooden probes, and the u se of these probes as tools. The paleontological, archaeological, and experimental data suggest that a well-developed flexor pollicis longus muscle functioned initially in the hominid lineage to stabilize the t erminal pollical phalanx against loads applied to the thumb's apical p ad during the frequent and forceful use of unmodified stones as tools. (C) 1998 Academic Press Limited.