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.

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.

These observations lead to two predictions. First, mammals will strike with greater force and power from bipedal posture than from quadrupedal posture. Second, mammals are expected to exhibit greater force and power when they strike down than when they strike up with their forelimbs. These predictions would be best tested in habitual quadrupeds. Practical limitations, however, make such an experiment relatively difficult. Thus, to test these predictions, I quantified striking performance of human subjects (1) in bipedal (orthograde) posture and in simulated quadrupedal (pronograde) posture, and (2) when striking downward versus upward. Although humans are highly derived striding bipeds, our forelimbs do play a critical locomotor role in climbing. During climbing, the range of motion at the shoulder is largely similar to that used during terrestrial quadrupedalism and the muscles at the shoulder responsible for positive work during climbing are the retractors. Additionally, although quadrupedal posture is not the preferred fighting posture of humans, it does occur during grappling and ground fighting. Thus, human subjects do provide a valid test of the predictions.

The force-velocity relationship of striated muscle may also influence body posture during aggressive encounters. Bipedal posture allows quadrupeds to strike downward rather than upward on an opponent. Striking downward may increase the power of the limb because limb retractor muscles have a greater capacity for positive work than limb protractor muscles. In quadrupeds, retractor muscles are primarily responsible for the positive work associated with accelerating the body whereas protractor muscles apply force during braking and are therefore responsible primarily for negative work [4] , [5] . Muscle fibers produce more force during active lengthening (i.e., eccentric activity) that is required during braking than during active shortening (i.e., concentric activity) that is required for acceleration [6] . Thus, if quadrupeds need to slow down as rapidly as they need to speed up, one would expect the protractor muscles to have smaller physiological cross-sectional area than the retractor muscles. This appears to be true; protractor muscles are substantially smaller than retractor muscles in a variety of species [7] – [9] . Because striking downward requires concentric activity from the retractor muscles of the forelimbs whereas striking upward requires concentric activity from the smaller protractor muscles, quadrupeds may be able to do more work on an opponent when they strike downward.

Terrestrial vertebrates have evolved to do work against gravity during locomotion. This requires that the mobility and strength of limbs be oriented towards the substrate. Bipedal posture reorients the trunk from pronograde to orthograde, allowing quadrupeds to defend themselves and strike and manipulate an opponent with their forelimbs over the locomotor range of motion; the range of motion that can presumably produce the most force and power. Consider a galloping thoroughbred horse. At full speed, each forelimb is in contact with the ground for much less than a tenth of a second and, during that brief period, it applies a peak ground force of more than 2.5 times body weight [3] . Thus, bipedal posture repositions the axis of the body so that the locomotor range of motion of the forelimbs can be directed at an opponent, allowing quadrupeds to strike, grapple and defend themselves with their forelimbs' greatest capacity to do work.

Although bipedal locomotion is rare among mammals, many species stand bipedally on their hindlimbs when they fight. Fighting from a bipedal posture is commonly observed in anteaters, felids including domestic cats, lions and tigers; canids including foxes, wolves and domestic dogs; bears; wolverines; horses; and many species of rodents, lagomorphs and primates, including great apes. Why is this behavior so common among species that normally stand, walk and run on four legs? The simplest answer is that bipedal posture allows a quadruped to fight with its forelimbs. Among extant tetrapods, mammals are remarkable in the mobility of their forelimbs and their ability to grab, hold and manipulate objects with their forelimbs [1] , [2] . Given this mobility and dexterity, it is not surprising that many mammals fight with their forelimbs. Nonetheless, bipedal posture may also bestow specific advantages for fighting with the forelimbs that emerge from the mechanics of quadrupedal locomotion and the contractile physiology of striated muscle.

For all four types of strikes (downward, upward, side and forward), performance by the subjects was greater from bipedal than from quadrupedal posture. When striking downward and upward, subjects did 44 and 47% more work respectively when they performed from bipedal than from “quadrupedal” posture ( Table 1 ). When the subjects punched the transducer with a forward strike, peak forces averaged 49% greater and force impulses averaged 30% greater from bipedal than from quadrupedal posture ( Fig. 1A , Tables 2 and 3 ). When striking sideways, peak forces were on average 64% greater and force impulses averaged 43% greater from bipedal than quadrupedal posture ( Fig. 1B , Tables 2 and 3 ).

Discussion

The advantage of fighting from bipedal posture The results of this study indicate that humans are capable of striking with 40–50% higher force and energy from bipedal than quadrupedal posture and can impart more than 200% greater energy when striking downward than upward. The increase in work done in downward and upward strikes when subjects switched from quadrupedal to bipedal posture likely reflects the difference in the range of motion of the arm in these two postures. When subjects struck vertically from a bipedal posture, they used approximately the full range of motion of the arm. In contrast, when they struck vertically from a quadrupedal posture, the motion was restricted to relatively protracted angles, limiting the range of motion by roughly half of the full range. Additionally, although kinematics were not quantified, subjects tended to raise their arm into extreme protraction in preparation for the quadrupedal strikes. Power production is likely to be limited by the length-tension relationship of the extrinsic shoulder muscles at these joint angles. The greater performance in side and forward directed strikes from bipedal posture is partially a function of a transfer of energy from the legs and trunk that bipedalism makes possible. Although the contribution of legs and trunk to the work of side and forward strikes was not addressed in this study, energy transfer from the legs and trunk is generally recognized to be important in fighting. However, because one forelimb remains in contact with the ground, quadrupedal posture largely eliminates significant contribution from the trunk and legs. The greater performance in side and forward strikes from bipedal posture is also likely a consequence of the relative strength of the different shoulder muscles producing these two movements. From a bipedal posture, both side and forward strikes require (1) adduction of the humerus, produced by the pectoralis major and anterior deltoid muscles, and (2) anteversion of the arm on the trunk produced by the serratus anterior muscle. From quadrupedal posture, however, side and forward strikes require lateral “elevation” of the arm in which the humerus is brought closer to the head. Elevation of the arm is produced primarily by the middle deltoid muscle. In humans, the combined physiological cross-sectional area of the pectoralis major and anterior deltoid muscles is approximately 130% larger than that of the middle deltoid muscle [10], [11]. Additionally, side and forward striking from a quadrupedal posture eliminates the contribution from the powerful serratus anterior muscle that likely occurs during bipedal horizontal strikes. The more than two-fold greater work done in downward than in upward directed strikes is consistent with the greater strength of the retractor than the protractor muscles of the forelimb. As mentioned above, retractors of the forelimb tend to have greater physiological cross-sectional area than do the protractors in mammals [8], [9]. This is also true of humans, in which the latissimus dorsi, the sternocostal part of the pectoralis major, the teres major and the long head of the triceps all act as retractors of arm. In comparison, only the anterior and middle portions of the deltoid and the clavicular part of the pectoralis major muscle have a capacity to protract the arm. Nevertheless, for the biomechanical reasons described above, a greater capacity to strike downward than upward is likely to be true for most species of mammals, including the quadrupedal ancestors of hominins. The imbalance of strength in retractor versus protractor muscles of the limbs of quadrupeds, such as hares and dogs, is almost certainly a consequence of the mechanics of running on four legs and the force velocity relationship of skeletal muscle, as explained above. Given that habitual bipedalism evolved over 4 million years ago in hominins, why have humans retained this imbalance in muscular strength in our forelimbs? One possibility is the role that forelimb retractors play in the production of positive and negative work during climbing. Another factor that may have been important is that our ancestors evolved overhand motor behaviors associated with aggression, long before the evolution of habitual bipedalism. Chimpanzees, bonoboos and gorillas all strike opponents with overhand motions of the forelimb. Chimpanzees also throw with an overhand motion of the forelimb. Overhand striking and throwing are more common in apes than underhand versions of these same behaviors presumably because of the greater capacity of their forelimbs to do positive work during retraction than during protraction. We inherited overhand motor control of these behaviors from our quadrupedal ancestors. The necessity of high power production during striking and throwing may be why humans retained a greater capacity for power production during retraction rather than protraction of the forelimb. The fact that humans are habitual bipeds reduces the relevance of humans as a model organism for this study. Obviously, the biomechanical predictions of this study would be better tested in a species that walks and runs quadrupedally. Collecting similar data from chimpanzees or bonoboos may be possible, but will be difficult for a variety of reasons and confounded by questions of motivation and training. Nevertheless, a study similar to this one in another species of great ape would be worthwhile. In summary, humans are capable of striking with greater force and energy from bipedal than quadrupedal posture and can impart much more energy when striking downward than upward. The magnitude of the greater energy imparted in downward directed strikes suggests that the most important reason quadrupeds stand bipedally to fight is that it allows them to strike downward on an opponent.

A functional basis for the attractiveness of tall males All else being equal, the much greater energy that can be delivered in downward than in upward directed strikes provides a tall individual with a performance advantage over a shorter opponent. This height dependent advantage may be the basis of the observed female preference for tall men. Several studies have found that women are more attracted to tall than short men [12]–[14]. Tall men receive more responses to dating advertisements [15] and women report dating tall men more often than short men [16]. In the latter study, men one standard deviation above the mean height had twice as many dates as did men one standard deviation below the mean. Tall men also have more attractive partners [17], report greater relationship satisfaction and have lower levels of cognitive or behavioral jealousy than short men [18], [19]. These differences in male attractiveness and relationship confidence appear to give taller men a fitness advantage. Tall men have a greater number of children than shorter men [20]–[22]. Furthermore, given that stature is highly hertiable [23], females who mate with tall men are more likely to have tall sons, who in turn would be preferred by females. The greater attractiveness and reproductive success of taller males is generally assumed to be due to stature serving as an indicator of good genes. Height is correlated with cognitive abilities and is positively associated with a number of metrics of social and financial success [24], [25]. Height has also been found to be correlated with physical health and with morphological symmetry [26], [27]. Yet, if the greater attractiveness and reproductive success of taller men were solely a function of the correlation with somewhat greater intelligence, health and social success, we could expect taller women to be more attractive to men and to have greater reproductive success for the same reasons. In western societies, this is not the case. Women who are short or of average height are perceived as more attractive by men [16], [28], have lower levels of jealousy [18], and have greater reproductive success [29] than tall women. Thus, the presence of “good genes” is unlikely to account fully for the greater attractiveness and reproductive success of taller males. A performance advantage in male-male competition could also be part of the explanation for the greater attractiveness of taller males. Although larger size (i.e., body mass) provides an advantage during physical competition, the results of this study suggest that greater height, by itself, is associated with an enhanced capacity to strike downward on an opponent. Short individuals have to strike upward to hit a tall person in the head, but tall fighters swing downward to hit the most vulnerable targets of a shorter opponent. Consistent with this, is the observation that tall men are perceived to be more dominant and assertive than shorter individuals [30]. Thus, early in hominin evolution, an enhanced capacity to strike downward on an opponent may have given tall males a greater capacity to compete for mates and to defend their resources and offspring. If this were true, females who chose to mate with tall males would have had greater fitness.