Study finds that heel-down posture in great apes and humans confers a fighting advantage

February 15, 2017
A diagram of an experimental setup to test how a plantigrade (heel-down) stance affects the amount of force a person is able to apply to a pendulum. Credit: David Carrier

Walking on our heels, a feature that separates great apes, including humans, from other primates, confers advantages in fighting, according to a new University of Utah study published today in Biology Open. Although moving from the balls of the feet is important for quickness, standing with heels planted allows more swinging force, according to study lead author and biologist David Carrier, suggesting that aggression may have played a part in shaping our stance.

"This story is one more piece in a broader picture, a suite of distinguishing characters that are consistent with idea that we're specialized at some level for ," Carrier says.

Nature of man debate

Carrier studies biomechanics of how animals move and what the mechanics of movement suggests about the course of an animal's evolution. Such studies in primates and humans addresses a centuries-old controversy about human nature. Is humanity naturally aggressive and confrontational, made less violent through the recent controlling influences of governments, or inherently peaceful and benevolent, turning belligerent only when states and economies led to centralized power and ownership of resources?

Carrier says that when members of the same species compete for resources or mates the stakes are high and physical competition is costly, demanding peak performance from the musculoskeletal and cardiovascular system. The physiological traits that confer advantages in fighting are different from those required for other tasks. "The folks who line up for the Olympic marathon are not built the way the fighters are," he says. "They're not built the way sprinters are. If aggression was important in our past, we should see evidence of it in our anatomy." If the characteristics that distinguish humans and great apes from other primates are not beneficial for fighting, he says, then the hypothesis that aggression was important in our evolutionary past would be falsified. If, however, apes' distinguishing anatomical traits are beneficial to fighting success, then the hypothesis that physical competition helped shaped our evolution would be supported.

Apes' planted heels

Most species of mammals, including most primates, stand, walk and run with their heel elevated above the ground. These stances, called digitigrade and unguligrade, increases the economy of running by lengthening the limb and improving the storage and recovery of elastic strain energy in the tendons and ligaments of the lower limb. The heel-down posture of great apes, called plantigrade, is shared with other species that are less specialized for running, such as bears, wolverines and some rodents.

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A volunteer applies force to a pendulum. Credit: David Carrier

One hypothesis for the evolution of the great apes' stance has to do with how apes climb and forage in trees. Instead of walking on four limbs along the tops of branches like other primates, apes tend to hang using their arms and walk on their with balancing support from their arms on other branches. To facilitate this, apes may have shifted their center of mass toward the hind legs, which would yield a plantigrade stance.

Another hypothesis, which Carrier and colleague Christopher Cunningham of the University of Georgia explored, is that a plantigrade stance allows the arms more striking force by increasing the torque, or rotational force that can be applied to the ground.

Putting theory to the test

Carrier and Cunningham set up a force plate for volunteers to stand on that recorded the force applied to the ground while the volunteers struck and pushed a large weighted pendulum. By measuring the velocity that the volunteers imparted to the pendulum, along with the pendulum's known resistance to acceleration, the researchers calculated the work performed. Twelve volunteers completed the task with heels planted and heels up, either with one foot or two. The striking and grappling behaviors studied included lateral strikes and pushes, downward strikes, forward pushes and rearward pulls.

To further illustrate the significance of the rotational force applied by the feet, Carrier says, they also asked volunteers to push the pendulum while standing on a sheet of Teflon and wearing a fuzzy sock. With no ability to exert a rotational force on the ground, the volunteers simply spun in place.

In all cases, the force or energy applied was greater in plantigrade posture than digitigrade, confirming the team's hypothesis that a plantigrade stance allows a person or ape to exert more force and energy, an advantage in fighting. Physical aggression is clearly not the only behavior that influenced the evolution of our feet, Carrier says, but the results of this experiment are consistent with the hypothesis that selection on fighting performance played an important role.

"We're all familiar with the 'fight or flight' response of animals in danger," says Emily Carrington, a program director in the National Science Foundation (NSF)'s Division of Integrative Organismal Systems, which funded the research. "Certain species tend to be good at fighting or fleeing, but not both. This study provides insight into the basis for this trade-off. Animals that are able to use their heels to plant their feet firmly to the ground, like bears, badgers and great apes, are able to deliver stronger blows to their opponents."

Regarding the broader question of whether or not selection on aggressive behavior influenced the evolution of our species, Carrier points out that "the shape of our feet is one of a series of distinguishing , from our faces to our heels, that increase fighting performance."

Explore further: The cost of being on your toes

More information: Biology Open, DOI: 10.1242/bio.022640

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Dug
5 / 5 (1) Feb 15, 2017
Wow, I thought boxers and marshal artists were always instructed stay on their toes? Perhaps these researchers might want to research a little more of both marshal sports and their basic physics. Up on the toes position allows the body to be not only more mobile to avoid blows, but when hit reduces the impact by allowing the body to absorb the energy as it rocks back. Most fighters learn first think never to be caught flat footed - heels planted. Standing flat footed, weight on heels is generally considered to be an extreme disadvantage to a fighter. Planting the heels additionally does not increase the force of a punch thrown, only one received. A punch thrown depends on the weight and velocity of the fist and those connected to it, or the foot - at impact. Additionally, to through a punch or sling a kick, the implementer necessarily has to come of his heels as rotates. Authors should spend some time watching humans fight. The totally missed this in their poor theo
rrwillsj
2.7 / 5 (7) Feb 16, 2017
Dug, my apologies in advance for blunt criticism but you missed the point of the study of the plantigrade fighting stance.

Among us Great Apes and Bears, Martial Arts training is a very recent invention. If you watch animals fighting (NOT hunting) they bear an uncanny resemblance too a schoolyard fight between howling mobs of twelve year olds. And, I suspect for exactly the same reasons.

Martial Arts, that I am familiar with, are centered around the concepts of self-discipline, honor and respect for others.

In the wild, that does not describe combative males of any species. It makes me cringe to think anyone would be so egotistical as to attack an adult Silverback or any of the Brown Bears with fancy Martial Arts kicks and blows. Frankly, I am of the opinion all you will do is piss them off. Right before they rip your head off!
Kron
5 / 5 (2) Feb 16, 2017
@Dug
Staying nimble on the toes is advantageous for the stick and move type of fighter. A power puncher fights with a flat footed stance.

If you're ever fighting a bigger and stronger individual I'd advise you to stick and move. If you're the bigger more powerful male I'd tell you to knock your opponents head off.

Sticking and moving will make for a long drawn out fight (and really, makes you look scared to engage (and in nature, when it comes to competing for mates, this is not the appearance you want to convey)).

Standing your ground and brawling is the way to assert your dominance in conflict. Is this the best strategy to ensure victory? Not always. But it shows you do not fear your opponent, and that you are the dominant alpha willing to stand your ground (flat :))
Kron
not rated yet Feb 16, 2017
"Why waltz 10 rounds with an opponent if you can KO him in one?"
OceanDeep
2 / 5 (4) Feb 20, 2017
Intriguing research. I recently thought of another idea while watching combat sports. Given how easily the unprotected human fist can break when punching with the knuckles, and given how padded the edges of our hands are, I wonder if the first, best fighting by our primate ancestors was by using "hammer fists" more than boxing punches. Hammer fists can do a lot of damage to the opponent while avoiding injuries to the aggressor's knuckles and fingers.
Da Schneib
5 / 5 (3) Feb 20, 2017
While doubling the pinky when using a hammer-fist blow does help protect it some, it's still vulnerable, particularly in case of a partly missed target. The most powerful blow you can throw with your hand is an open-palm blow; this is because it engages the large muscles in your upper arm, not just your shoulder, and because it relies on your radius and ulna in compression, which is how they are strongest. Professional football players on the lines use this blow all the time with great effectiveness.

Getting back to the article, open-palm is a blow that relies upon this same heel-down posture for its force. And for that matter, so is the hammer-fist.

If you watch martial arts practitioners who break, for example, stacks of boards or concrete blocks, you will see that they either use your hammer-fist technique, @Ocean, or an open palm. Most peoples' finger bones just aren't strong enough for that sort of stuff.
Da Schneib
5 / 5 (2) Feb 20, 2017
As far as engaging an animal capable of crushing rocks in its hands, such as a gorilla or orangutan, or one with claws, such as a bear, using human-style martial arts, that's fantasy. Don't get put in a position where combat is an option. You're smarter than either a gorilla or a bear.
OceanDeep
2 / 5 (4) Feb 20, 2017
Thanks @Da Schneib.

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