Joshua Horns, University of Utah
There are numerous arresting differences between the hands of humans and those of chimps, principally among them the shortening of metacarpals 2-5, the relative increase in length of the thumb, and the development of separate musculature controlling flexion of the thumb. In addition to improving manual dexterity, these changes allow humans, in contrast to chimps and other apes, to make a buttressed fist with the phalanges pressed against the central palm and the thumb adducted onto the dorsal surface of the phalanges. We believe this hand posture greatly reduces the strain experienced by a fully formed fist when striking with force, thereby rendering the human hand as a more effective weapon. We tested this idea by manipulating the tendons in a cadaver arm to induce the hand to form into buttressed and non-buttressed conformations, and then tested each conformation by having the hand strike a hanging instrumented mass to measure the force of each strike. Additionally, the 2nd metacarpal of the hand was fitted with a strain gauge so that strain in the bone could be compared to the force of the strikes. The results of these tests showed that there was a significant drop in strain in the buttressed fist (in comparison to the non-buttressed) for a given force.