‘Double-play’ motion keeps critters stable and agile

"What is immediately obvious in the slow-motion videos is that the fish constantly move their fins to produce opposing forces," says Eric Fortune. "This arrangement is rather counter-intuitive, like two propellers fighting against each other." (Credit: Will Kirk/JHU)

Animals that push forward and back at the same time aren’t wasting effort. They are maximizing both stability and maneuverability simultaneously, a new study shows.

The finding—while it could lead to more agile robots—serves primarily to shed light on a question that has baffled biologists: why do animals exert force in ways that don’t move them toward their destination? A robot designer would likely avoid the side-to-side sashaying of a running lizard or cockroach, movements that seem inefficient. So why do the animals behave this way?

(credit: Will Kirk/JHU)
(Credit: Will Kirk/JHU)


A research team led by Johns Hopkins University engineers says that the extra exertion isn’t necessarily wasteful after all. It allows at least some animals to accomplish a double-play often described as impossible in engineering textbooks.

“One of the things they teach you in engineering is that you can’t have both stability and maneuverability at the same time,” says Noah Cowan, the associate professor of mechanical engineering who supervised the research. “The Wright Brothers figured this out when they built their early airplanes. They made their planes a little unstable to get the maneuverability they needed.”

Animal mechanics

When an animal or vehicle is stable, it resists unwanted changes in direction. On the other hand, if it is maneuverable, it has the ability to quickly change course when desired. Generally, engineers have assumed that a system can rely on one property or the other—but not both. Not so.

“Animals are a lot more clever with their mechanics than we often realize,” Cowan says. “By using just a little extra energy to control the opposing forces they create during those small shifts in direction, animals seem to increase both stability and maneuverability when they swim, run, or fly.”

Cowan says this discovery, reported in the Proceedings of the National Academy of Sciences, could help engineers simplify and enhance small robots that fly, swim, or move on mechanical legs.

Knifefish in slow motion

The scientists used slow-motion video to study the fin movements of the tiny glass knifefish. These shy fish, each about 3 inches, prefer to hide in tubes and other shelters, where they avoid being eaten by predators in their Amazon basin habitat. In a lab, the team filmed the fish at 100 frames per second to study how they used their fins to stay in one place in these tubes, even when facing a steady flow of water.

“What is immediately obvious in the slow-motion videos is that the fish constantly move their fins to produce opposing forces. One region of their fin pushes water forward, while the other region pushes the water backward,” says Eric Fortune, a professor of biological sciences at the New Jersey Institute of Technology who was a co-author of the paper. “This arrangement is rather counter-intuitive, like two propellers fighting against each other.”

If the fish wants to move forward or backward instead of hovering, it can adjust the proportion of fin pushing in either direction. The research team developed a mathematical model that suggested that this odd arrangement enables the animal to improve both stability and maneuverability.

The team then tested that model with a robot that mimicked the fish’s fin movements. This biomimetic robot was developed in the lab of Malcolm MacIver, associate professor of mechanical and biomedical engineering at Northwestern University and a co-author.

Hummingbirds and bees do it

“We are far from duplicating the agility of animals with our most advanced robots,” MacIver says. “One exciting implication of this work is that we might be held back in making more agile machines by our assumption that it’s wasteful or useless to have forces in directions other than the one we are trying to move in. It turns out to be key to improved agility and stability.”

The mutually opposing forces that help the knifefish become both stable and maneuverable can also be found in the hovering behavior of hummingbirds and bees, says senior author Cowan, who directs the Locomotion in Mechanical and Biological Systems Lab at Johns Hopkins’ Whiting School of Engineering.

“As an engineer, I think about animals as incredible, living robots,” says lead author Shahin Sefati, a doctoral student advised by Cowan. “It has taken several years of exciting multidisciplinary research during my PhD studies to understand these ‘robots’ better.”

The National Science Foundation and Office of Naval Research funded the study.

Source: Johns Hopkins University