Meeting Abstract
P2.107 Monday, Jan. 5 The evolution of the human hand: making a fist. MORGAN, M*; CARRIER, D; University of Utah, Salt Lake City; University of Utah, Salt Lake City mhmorgan2@mac.com
Anatomical features that distinguish human hands from the hands of other apes undoubtedly facilitate our ability to make and use tools. However, the high proclivity of our species for aggression led us to suspect that the anatomical configuration of our hands may also protect the hand from injury when striking an opponent. Relative to other apes, humans have shorter fingers (digits 2-5) and a thumb (digit 1) that is much larger, more mobile and muscular. These changes allow humans to form a fist in which digits 2-4 are firmly buttressed by contact of the fingertips against the palm and by the wrapping of the thumb around the distal phalanges of digits 2, 3 and 4. We suggest that this buttressing provides greater stiffness at the metacarpal-phalangeal and phalangeal-phalangeal joints, and thereby protects the fist from injury when delivering a blow. The importance of buttressing may be found in a common injury called the Brawlers Fracture, which is characterized by fractures of the necks of the relatively un-buttressed 4th and/or 5th metacarpals. To test the hypothesis, that the proportions of the human hand provides protection against injury during fighting, we measured the stiffness of the 2nd metacarpal-phalangeal joint in a closed fist using a force transducer and a linear transducer. Subjects used three different fist postures: full buttressing, no thumb buttressing, and no buttressing. Results confirm that the buttressing inherent in a formed fist increases the stiffness of the metacarpal-phalangeal joints. On average, the no thumb posture exhibited approximately 1/2 the stiffness of a fully buttressed fist and the no buttressing posture exhibited 1/3 the stiffness of a fully buttressed fist. These results are consistent with the idea that the selection for aggressive behavior played a role the evolution of the human hand.
