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The Jump Routine

If you watch a grasshopper closely as it jumps, you will see that it always goes through a set routine (a motor programme) before it actually takes off.

The Motor Programme

1. Initial Flexion. The first things that happens is that the hind legs flex fully. This is brought about by contraction of the flexor muscle. Full flexion is an essential pre-requisite for jumping (the reason will be explained later) - if it is prevented by, for instance, a lump of dirt getting between the tibia and the femur, the animal simply cannot jump.
   

2. Co-Activation. The flexor and extensor muscles contract together. You obviously can't see the actual muscles from the outside, but you can see their effects. The contraction of the flexor muscle keeps the tibia in the fully flexed position, so that the simultaneous contraction of the extensor muscle bends the springs in the joint, rather than extending the leg. The extensor muscle contraction is quite slow (it can take up to half a second), and this means that the muscle can contract with maximum force. The energy of the contraction is stored in the distortion of the cuticle springs (and also in stretching the extensor tendon), just like the energy from an archer's arm is stored in the bending of a bow.
   
3. Trigger Relaxation of Flexor. The leg extension is triggered by a sudden relaxation of the flexor muscle. This allows the leg to extend rapidly and forcefully, using the energy stored in the springs.

In the cartoons below, the whole animal is shown on the left, while what is happening inside the leg is shown on the right.

jump diagram

motor program

A point of detail is that the tibial extension is not caused directly by contraction of the extensor muscle, since this largely takes place before the jump. The extension is actually driven by the expansion of the spring. So the tibia acts as a second-order lever, rather than the normal first-order lever - the extensor muscle insertion acts as the pivot point, while what is normally the pivot becomes the point where the force is applied.

Science stuff: quantitative information on energy storage and power amplification

Had Enough?

Things get just a little bit more complicated after this. If you want to keep to the overview, you may want to skip to the comparative stuff at the end. However, if you want to find out some more details about the mechanics and neurobiology, then go to the next page as normal...

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