Reaching high speed with tiny steps and the powerful "fifth" leg

Current studies from all over the world show that locomotion analysis is not only a good tool for examining human beings: Also in animals - small or big - this type of analysis helps to gain new important insight.

Locomotion scientists from the University of Jena analyze the locomotion of ants

They are small, fast and agile: Thanks to an extremely dynamic rear-wheel-drive and an efficient light-weight build, but mainly because of a an ingenious stabilizing system, the little runabouts manage to develop absolute top speed. We do not talk about a new generation of top-speed compact cars. We talk about wood ants (formica polyctena). The animals cover up to 26 body lengths per second and reach a frequency of 16 steps per second in doing so.

"And all of that without lifting off" says locomotion scientist Lars Reinhardt from the university Friedrich-Schiller-Universität in Jena. "Because in difference to most of the other fast running mammals or birds, there is no flight phase within the locomotion of ants." Also when running fast, according to Reinhardt, the animals never use soil contact, but they reach the high speed using tiny steps. This is the result the junior researcher from the team of Prof. Dr. Reinhard Blickhan reached in a recent study. According to his comment in the journal "The Journal of Experimental Biology", ants use a locomotion pattern called "grounded running" (DOI 10.1242/jeb.098426).

In the present work the researchers from Jena did the very first comprehensive, biomechanical locomotion analysis of ants, measuring and analyzing not only the motion processes but also the forces within the animals' locomotion. This only was possible with a specially developed, highly sensitive sensor, that Lars Reinhardt designed during his PhD thesis (DOI: 10.1242/jeb.094177). The sensor consists of elastic polymer strips that are able to capture even the smallest forces within the micro-Newton-area in all three directions in space.

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The step pattern of ants, as Lars Reinhardt explains, remains the same at any speed: Three legs each of the altogether three leg pairs touch the soil. "The animals use the so-called alternating tripod-gait." They synchronically move the front and the hind leg of one side and the middle leg of the opposite side forward. Only when all three have soil contact again, the corresponding other three legs lift off. He claims that this is very energy-consuming. "But this way the ants get a very stable gait, also in rough areas," Reinhardt says. This also enables very fast changes of direction. "And this is more important for outdoor survival than saving energy."

The main drive for the forward movement of the ants is done by the hind leg pair, according to a further result of the latest study, while the front legs have more of a breaking effect and the legs in the body center contribute to the stabilization of the gait.

The motion researchers also found out that the ants briefly touch the soil between the steps with the hindquarters in regular intervals, also slightly slowing down the locomotion. "This way they put down odor trails indicating the way for their conspecifics," Reinhardt explains. This hampers on the one hand locomotion of the individual animal, on the other hand the ants also benefit from an established "road".

Credit: M. Großmann_pixelio.de

 

Original publications:
Reinhardt L, Blickhan R. Level locomotion in wood ants: evidence for grounded running, The Journal of Experimental Biology (2014) 217, 2358-2370, DOI: 10.1242/jeb.098426
Reinhardt L, Blickhan R. Ultra-miniature force plate for measuring triaxial forces in the micronewton range, The Journal of Experimental Biology (2014) 217, 704-710, DOI: 10.1242/jeb.094177

The kangaroo's tail propels and powers pentapedal locomotion

A Simon Fraser University study on how kangaroos use their tails as a ‘fifth’ leg is providing new insight into the diversity of biological movement, and specific insight into why we walk the way we do.

Published today in the Royal Society journal Biology Letters, the study, led by professor Max Donelan of SFU’s Locomotion Laboratory, found kangaroos, commonly viewed as hoppers, move with a “pentapedal” gait, planting their tails on the ground in combination with their front and hind legs.

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“We measured the forces the tail exerts on the ground and calculated the mechanical power it generates, and found that the tail is responsible for more propulsive force than the front and hind legs combined,” explains Donelan, an associate professor in the Department of Biomedical Physiology and Kinesiology (BPK).

“It also generates almost exclusively positive mechanical power, performing as much mechanical work as a human leg when walking at the same speed. Their muscular tail is used to propel and power their motion—just like a leg.” Measurements were carried out at the University of New South Wales in Sydney Australia back in 2001, but the researchers were only just recently able to analyze them properly, thanks to innovations by Donelan’s collaborator, Dr. Shawn O’Connor.

“One of the central findings of our human walking research is that it is very important to time the push-off of your back leg to make walking less effortful,” says Donelan, founder and scientific advisor to Bionic Power. The university spin-off company develops energy harvesting technology for those who depend on portable power. “People recovering from strokes or spinal cord injury can’t do this as well because their legs are partially immobilized, making walking more effortful.”

Kangaroos, meanwhile, have very short front legs that can’t be used to push off. “The timing and position of the tail, on the other hand, is perfect,” he says. “So, we wondered if they are able to use their tail just like a leg to push off and power their walking.”

What’s remarkable about this, suggests Donelan, is that the tail is anatomically quite different, being made up of more than 20 vertebrae taking on the roles of our feet, calves and thigh bones. “Animals have discovered many uses for their tails,” he says, “but as far as we know, this is the first use of one as a leg.”

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Beyond better understanding kangaroos’ movements, the research shows how important it is to push-off and help to redirect the body’s velocity when transitioning from one stance limb to the next, Donelan adds. “We know healthy humans do this nearly perfectly. We know that people with gait disorders and disabilities don’t do it as well, which increases the effort required for them to walk.

“Based on our original human research, fellow scientists and engineers have have built prosthetics and exoskeletons that help improve ability and make walking easier. And now we know that it is important enough that kangaroos have harnessed a limb originally evolved for swinging from trees to serve this role as functional fifth leg.”

Unusual gaits by unusual animals, such as pentapedal walking by kangaroos, provide insight into the breadth of solutions available to the same biomechanical problem, notes Donelan, who has also studied the movement of shrews, cats, crocodiles, giraffes and elephants.

And what’s not to find intriguing about kangaroos? “Their hopping is incredibly fast, powerful and efficient. Their walking, on the other hand, is as awkward as their hopping is graceful, but underlying the walking is this entirely new use for a tail. Biomechanically, it is all fascinating.”

Videos
Video explaining the pentapedal walking gait Credit: SFU Locomotion Lab
Video of a red kangaroo walking at the Fowlers Gap Arid Zone Research Station Credit: Catharina Vendl, Fowlers Gap