The mechanics of elevation control in locust jumping

Abstract

How do animals control the trajectory of ballistic motions like jumping? Targeted jumps by a locust, which are powered by a rapid extension of the tibiae of both hind legs, require control of the take-off angle and speed. To determine how the locust controls these parameters, we used high speed images of jumps and mechanical analysis to reach three conclusions: (1) the extensor tibiae muscle applies equal and opposite torques to the femur and tibia, which ensures that tibial extension accelerates the centre of mass of the body along a straight line; (2) this line is parallel to a line drawn from the distal end of the tibia through the proximal end of the femur; (3) the slope of this line (the angle of elevation) is not affected if the two hind legs extend asynchronously. The mechanics thus uncouple the control of elevation and speed, allowing simplified and independent control mechanisms. Jump elevation is controlled mechanically by the initial positions of the hind legs and jump speed is determined by the energy stored within their elastic processes, which allows us to then propose which proprioceptors are involved in controlling these quantities.