The Advantage of Standing Up to Fight and the Evolution of Habitual Bipedalism in Hominins

Background: Many quadrupedal species stand bipedally on their hindlimbs to fight. This posture may provide a performance advantage by allowing the forelimbs to strike an opponent with the range of motion that is intrinsic to high-speed running, jumping, rapid braking and turning; the range of motion over which peak force and power can be produced. Methodology/Principal Findings: To test the hypothesis that bipedal (i.e., orthograde) posture provides a performance advantage when striking with the forelimbs, I measured the force and energy produced when human subjects struck from "quadrupedal' (i.e., pronograde) and bipedal postures. Downward and upward directed striking energy was measured with a custom designed pendulum transducer. Side and forward strikes were measured with a punching bag instrumented with an accelerometer. When subjects struck downward from a bipedal posture the work was 43.70 +/- 12.59% (mean +/- S. E.) greater than when they struck from a quadrupedal posture. Similarly, 47.49 +/- 17.95% more work was produced when subjects struck upward from a bipedal stance compared to a quadrupedal stance. Importantly, subjects did 229.69 +/- 44.19% more work in downward than upward directed strikes. During side and forward strikes the force impulses were 30.12 +/- 3.68 and 43.04 +/- 9.00% greater from a bipedal posture than a quadrupedal posture, respectively. Conclusions/Significance: These results indicate that bipedal posture does provide a performance advantage for striking with the forelimbs. The mating systems of great apes are characterized by intense male-male competition in which conflict is resolved through force or the threat of force. Great apes often fight from bipedal posture, striking with both the fore-and hindlimbs. These observations, plus the findings of this study, suggest that sexual selection contributed to the evolution of habitual bipedalism in hominins.