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脚趾短是为奔跑进化而成
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http://www.sciencenet.cn/htmlnews/2009/2/216584.html
First published online February 13, 2009
Journal of Experimental Biology 212, 713-721 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019885
Walking, running and the evolution of short toes in humans
1 Department of Anthropology, Harvard University, Cambridge, MA 02138, USA
2 Department of Kinesiology, University of Massachusetts, Amherst, MA 01003, USA
3 School of Medicine, Vanderbilt University, Nashville, TN 37232, USA
Author for correspondence (e-mail: cprolian@ucalgary.ca )
Accepted 25 November 2008
The phalangeal portion of the forefoot is extremely short relative to body mass in humans. This derived pedal proportion is thought to have evolved in the context of committed bipedalism, but the benefits of shorter toes for walking and/or running have not been tested previously. Here, we propose a biomechanical model of toe function in bipedal locomotion that suggests that shorter pedal phalanges improve locomotor performance by decreasing digital flexor force production and mechanical work, which might ultimately reduce the metabolic cost of flexor force production during bipedal locomotion. We tested this model using kinematic, force and plantar pressure data collected from a human sample representing normal variation in toe length (N=25). The effect of toe length on peak digital flexor forces, impulses and work outputs was evaluated during barefoot walking and running using partial correlations and multiple regression analysis, controlling for the effects of body mass, whole-foot and phalangeal contact times and toe-out angle. Our results suggest that there is no significant increase in digital flexor output associated with longer toes in walking. In running, however, multiple regression analyses based on the sample suggest that increasing average relative toe length by as little as 20% doubles peak digital flexor impulses and mechanical work, probably also increasing the metabolic cost of generating these forces. The increased mechanical cost associated with long toes in running suggests that modern human forefoot proportions might have been selected for in the context of the evolution of endurance running.
Key words: phalanges, gait, foot biomechanics, bipedalism, Australopithecus
http://blog.wired.com/wiredscience/2009/02/runningtoes.html
If you've ever wondered why humans don't have long, prehensile toes that would turn our feet into extra hands, here's an answer: stubby toes may be custom-made for running.
Biomechanical analysis shows that long toes require more energy and generate more shock than short toes, making them one of many adaptations that may have helped our savannah-dwelling ancestors chase their prey.
"Longer toes require muscles to do more work, and exert stronger forces to maintain stability, compared to shorter toes," said University of Calgary anthropologist Campbell Rolian. "So long as we were engaged in substantial amounts of running, natural selection would favor individuals with shorter toes."
Most primates — including our closest relative, the chimpanzee — have proportionately longer toes than humans. Our own are comparatively dwarfish and two-dimensional, capable only of extending and flexing. Most animals that run, however, also have extremely short toes. Some species, such as cats and dogs, have paws composed almost entirely by palms. This led Rolian's team to wonder if our foot's physiology could be explained by running.
The importance of running to early Homo is, of course, conjectural. But it does make sense: few other animals are capable of long-distance running, and none can do so under a blazing sun. (Wolves and hyenas, for example, require cold weather or nightfall for long-distance hunting; otherwise they overheat.) Endurance running might have set early humans apart from the pack.
According to study co-author and Harvard University anthropologist Daniel Lieberman, many modern anatomical features make sense in the context of savannah marathons. Achilles tendons act as springs to store energy. Our hind limbs have extra-large joints. Our buttocks muscles are perfect for stabilization, as are regions of the brain uniquely sensitive to the physical pitching generated by the motion of running.
Toes may belong to this class of adaptations.
"Humans are well-adapted for endurance running. That's much of what makes the human body what it is," said Lieberman. "We're actually terrible sprinters, but the world's best long-distance runners."
The long-runner hypothesis is not universally accepted. "Walking and running use the same body parts," said University of Wisconsin paleoanthropologist John Hawks, who was not involved in the study. "It's hard to argue that these are specifically crafted for running" — and, to be even more specific, for long-distance running.
However, Rolian's study, published recently in the Journal of Experimental Biology, makes at least a plausible case for the importance of toes for running. During the moment of propulsion, when one foot is in the air and the other is on the ground, between one-half and three-quarters of a body's weight falls squarely on the forefoot.
"When you're walking, before you push off to start the next step, your other foot has already hit the ground. You've transferred some of your body weight," said Rolian. "Your toes have to do much more work in running, to push you."
When his team analyzed force exerted by fifteen test subjects running and walking on a pressure-sensitive surface, they found that increasing toe length by just 20 percent produced a doubling of motor force. This can be explained in terms known conversationally known from the action of a see-saw: levering force is magnified by the distance between pressure and a fulcrum.
Rolian also observed that longer toes require an additional energy investment when "braking," or using them to guide the forward-falling motion that underlies both running and walking.
The additional work required by long toes, and a resulting increase in muscle stress and damage, likely made them a victim of natural selection. The fossil record, though spotty, provides a fitting narrative: the toes of great apes are longer than those of Australopithecus — the first bipedal hominid — which in turn are longer than the toes of Homo, the genus to which modern humans belong.
Hawks notes that long-distance running is now extremely rare, and "where it exists, it is supported by very sophisticated cultural adaptations, including tracking, water storage and staged transport of meat back to home bases. There is presently little or no evidence for these cultural adaptations in early Homo."
But Lieberman points out that early Homo and its descendants clearly ate large game, though the projectile technologies ostensibly necessary to slay them were invented just several thousand years ago.
"How did our ancestors, those weak little primates, kill big animals? The answer is that we chased them. We made them gallop. They can't pant and gallop at the same time," said Lieberman. "We can run down a gazelle not through speed, but through endurance."
Of course, in the modern world of grocery stores and restaurants, long-distance running is a recreational activity, and hard-soled shoes absorb much of the shock felt by a bare foot. Freed from ancient evolutionary pressures, what will happen to our feet?
It's too soon to tell, and nothing at all may happen, but "that's generally a question you could ask about many features of the human anatomy," said Rolian. Because it isn't required to push off, he said, "There's talk about whether the pinkie toe is eventually going to disappear."
Citation: "Walking, running and the evolution of short toes in humans." By Campbell Rolian, Daniel E. Lieberman, Joseph Hamill, John W. Scott and William Werbel. Journal of Experimental Biology, Vol. 212, Issue 5. March 1, 2009.
（插图由牛文鑫硕士论文所建立三维计算机模型提供）
https://blog.sciencenet.cn/blog-2189-217130.html
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