Are animals’ graceful movements bestowed on them by a nervous system that controls all details of motion? When most animals swing their legs during walking, their nervous system typically activates muscles on one side of the leg joint to bring the leg forward and, as the leg is about to touch the ground, activates muscles on the opposite side of the joint to slow down the leg. However, stick insects may have simplified this strategy. Their nervous system only provides the initial input to accelerate the leg and provides no input to slow the limb down. Arndt von Twickel and colleagues at the University of Cologne, Germany, wondered how stick insects could pull off this stunt. From their previous work, the team had learned that stick insect muscles are able to produce a burst of force when stretched, without any neural activation, so they hypothesized that a burst of force – produced when the muscles that slow down the leg are stretched – could provide the brake power without the necessity for additional input from the nervous system.
The team chose to study how the force production of muscles around the stick insects’ femur–tibia joint – analogous to a human knee joint – controls the leg movement during the swing phase of walking. In the first step in their analysis, the team measured the force burst generated by the flexor muscle, which could provide a brake on movement of the femur–tibia joint, when stretched by the extensor muscle on the other side of the joint. To do this, they manoeuvred the joint over angles ranging from 20 to 140 deg at speeds of up to 1200 deg s−1, which are typical of the movements performed by a stick insect during a stride.
By looking at the burst of force generated by the stretched flexor muscle during the swinging motion, the team found that faster speeds and larger movement ranges increased the force burst. The muscle also generated the maximal force when the leg was bent at about 110 deg, reaching values that were more than 10 times greater than the team expected. This suggests that the flexor muscle could supply the brake power to slow down the leg during swing.
To understand further how the force could contribute to movement control during a swing manoeuvre, the team combined their observations with computer simulations to calculate the forces from the extensor and flexor muscles and determine their effect on the swing movement. The simulation showed that the force burst elicited by the flexor muscle when passively stretched by the extensors was enough to slow down the femur–tibia joint. This suggests that the burst of force from stretched flexor muscles could allow the stick insects’ simple control strategy.
Passively induced stretch forces could also provide a straightforward control mechanism for other small animals, because their relatively light limbs are very sensitive to bursts of force, possibly allowing muscles to relieve the nervous system of its otherwise detailed control when the animals move.