Insects May Not Need Muscle To Move Limbs
July 18, 2013

Insects May Not Need Muscle To Move Limbs

Brett Smith for - Your Universe Online

New research from the University of Leicester in the United Kingdom has found that insect limbs are capable of moving without the use of muscles - a finding that could have implications for the fields of medicine and robotics.

According to the UK scientists' report in Current Biology, "passive joint forces" allows some insects to return their limbs to a preferred resting position after being extended.

Lead researcher Tom Matheson, a neurobiologist at the university, said insects use neuronal control and "clever biomechanical tricks" to control some of their movements.

"It is well known that some animals store energy in elastic muscle tendons and other structures," Matheson said. "Such energy storage permits forces to be applied explosively to generate movements that are much more rapid than those which may be generated by muscle contractions alone. This is, for example, crucial when grasshoppers or fleas jump."

The UK team found that some insect structures have been specifically adapted to provide force that compliments muscle-driven actions.

"Our work set out to identify how the biomechanical properties of the limbs of a range of insects influence relatively slow movements such as those that occur during walking, scratching or climbing," Matheson said. "The surprising result was that although some movements are influenced by properties of the muscles and tendons, other movements are generated by forces that arise from within the joints themselves."

"Even when we removed all of the muscles and associated tissues from a particular joint at the 'knee' of a locust, the lower part of the limb (the tibia) still moved back towards a midpoint from extended angles," he explained.

The researchers said they expected to see the spring-like properties of limbs, but were surprised to find passive forces that contribute to almost every movement the limbs made.

"In the locust hind leg, which is specialized for jumping and kicking, the extensor muscle is much larger and stronger than the antagonist flexor muscle. This enables the animal to generate powerful kicks and jumps propelled by extensions of the tibia that are driven by contractions of the extensor muscle," Matheson said.

"When locusts prepare to jump, large amounts of energy generated by the extensor muscle are stored in the muscle's tendon and in the hard exoskeleton of the leg," he said. "Surprisingly, we noticed that when the muscles were removed, the tibia naturally flexed back towards a midpoint, and we hypothesized that these passive return movements might be counterbalancing the strong extensor muscle."

Besides their anatomical examinations of the insects, the team also referenced previous data on insect legs where the extensor and flexor muscles are similar in size or strength, or where the flexor is stronger than the extensor.

"We found that the passive joint forces really do counterbalance the stronger of the flexor or extensor muscle in the animals and legs we looked at," said co-author Jan M. Ache, a researcher from the University of Cologne.

"We hope that our work on locusts and grasshoppers will spur a new understanding of how limbs work and can be controlled, by not just insects, but by other animals, people, and even by robots," Matheson concluded.