Meeting Abstract

P3.152  Monday, Jan. 6 15:30  Testing the protective buttressing hypothesis of hominin hand proportions. HORNS, J.J.*; JUNG, R.; CARRIER, D.R.; Univ. of Utah, Salt Lake City; Univ. of Utah, Salt Lake City; Univ. of Utah, Salt Lake City jjhorns@gmail.com

In comparison to other apes, hominins have short palms and fingers (i.e., digits 2-5), but long, strong and mobile thumbs. These characters are thought to have evolved in response to selection for enhanced manual manipulation. Recently, we have suggested that another factor that may have influenced the evolution of hominin hand proportions is sexual selection for improved striking performance during hand-to-hand combat by males. The bones of hominin hands are proportioned in a way that may provide supportive buttressing that protects the hand from injury when striking with a fist. To test the hypothesis of protective buttressing we measured the strain experienced by the second metacarpal of 3 human cadaver arms during striking with buttressed (normal) and unbuttressed fist postures. Cadaver arms were mounted on a frame that allowed adjustment of the tension of the various muscles of the hand and wrist. This allowed us to modify the posture of the hand. The frame on which the arm was mounted swung as a pendulum and the hand struck a mass instrumented with an accelerometer; providing a measure of the impact force. These recordings show that the dorsal surface of the second metacarpal undergoes tensile strain during the impact of a punch. This is consistent with the bone being loaded primarily in bending, rather than compression. Most importantly, striking with a buttressed fist decreased the strain experienced by the metacarpal. At the highest striking forces, peak principle strains were 50 to 80% greater in the unbuttressed posture than in the buttressed posture. These results are consistent with the hypothesis that the human fist provides protective buttressing of the musculoskeletal elements of the hand.