When light insects like grasshoppers jump, the muscles in their hind legs power the jump and then return to their original position. A network of signals transmitted through their neurons can achieve this, but scientists have a found a way that evolution has made simpler.
University of Leicester neurobiologists have found that light insects have special joints that facilitate movement in one direction and then automatically reverse that movement. Thus, the way the insect has evolved has reduced the number of activators — like muscles and neuronal signals — required to achieve movement.
The joint the researchers looked at is a type of hinge joint in which one part of the limb (tibia) pivots on two tiny peg-like structures of another part (femur), like a pen swung between the tips of your index and middle fingers. The development could inspire simpler, cheaper prosthetic limbs in the future, with the same functional scope, with the use of these insect-inspired joints.
The study published in Current Biology on July 18, led by Dr. Tom Matheson, found that, apart from this clever biomechanical trick, joint structures varied from limb to limb also. Thus, they could have developed in response to muscular forces.
Many movements of insect limbs are caused by muscles, which pull on tendons and move one part of the limb relative to another. In the hind leg of the locust, an insect similar to grasshoppers which the researchers studied, the extensor muscle that powers jumps is much larger than the flexor muscle.
The joint forces help the weak flexor muscle counteract the strong extensor muscle.
“Without the joint forces, the flexor would have to be a lot larger. This would increase the animal’s weight and prevent it from jumping farther,” Dr. Matheson explained via email. The larger muscle would also be ‘metabolically costly’ for the animal to maintain. Since the passive joint flexion is ‘built into’ the joint, passive flexion automatically succeeds extension.
This means that the insect does not have to contract its flexor muscle, simplifying the patterns of motor nerve cell activity that it needs to generate to produce coordinated movements.
In prosthetic limbs that a subject can move, sophisticated control systems that simulate neuronal activity are necessary to operate the limb. Building in passive joint properties, such as in the insects, may permit prosthetic limbs to operate with fewer ‘muscles.’
“One actuator might extend the prosthetic, but a return movement could be generated by passive joint movement rather than an opposing actuator. This may also make the control easier, because a patient would only have to control movement in one direction, while movement in the other direction would occur automatically,” Dr. Matheson said.
