The photograph below shows the three legs from one side of a grasshopper.
The first thing to notice is how much bigger the back leg is than the middle or front leg (all these legs came from the same animal- a locust of the species Schistocerca gregaria). When fully extended the back leg is about 45 mm in length, which is twice as long as the others. Long legs are an advantage for jumping, because they increase the distance over which the jumper can push on the ground. This reduces the acceleration needed to achieve take-off velocity, which in turn reduces the amount of force needed (see Science stuff, which is the same as on the previous page).
The thick part at the top of the leg (femur) contains the muscles which make the thinner lower part (tibia) move. It's the tibia that has to extend as fast as possible to get a good jump, so this is kept as lightweight as possible. The foot at the end of the leg has sharp claws, which give the grasshopper a good grip so that it doesn't skid when it pushes on the ground.
Because grasshoppers are insects, and insects have their skeletons on the outside of the bodies (an exoskeleton), all the muscles are inside the hard shell. The two main muscles are the extensor tibiae muscle which causes the leg to extend, and the flexor tibiae muscle which causes the leg to flex. These muscles pull on tendons which are attached to the tibia on either side of the joint pivot. These tendons can be seen by removing the cuticle from one side of the leg in the joint region
Note that the flexor tendon does not have a direct line of pull, but rather loops over a structure in the femur called the "lump". We will come back to the importance of this later.
When one of the muscle contracts, it pulls on its tendon and moves the tibia one way, when the other muscle contracts, it moves the tibia the other way.
Both the extensor and flexor muscles are working through a long lever - the tibia. This means that during a jump the force produced by the extensor muscle has to be much greater than the force with which the foot at the end of the tibia actually pushes on the ground.
Science stuff: levers and the extensor muscle
The lever ratio between the extensor muscle and the end of the tibia is about 1 to 35. Thus to produce 15 g of thrust at the end of the tibia (which is what each leg produces when it is about half extended in the jump), each muscle must be contracting with a force of at least 500 g. However, because the tibia starts off completely flexed, the muscle force at the beginning of the jump has to be considerably higher - and may be as much as 1.5 kg.
So now you know how a contraction of the extensor muscle is converted into a push on the ground. But remember, this push has to be both strong and fast. Let's see how the grasshopper manages with that ...