The extensor muscle is very strong. One reason for its strength is simply its size. In the photograph below, the cuticle on one side of the leg has been removed to show the muscles underneath. The extensor muscle (red outline) occupies most of the volume of the femur, while the flexor muscle (blue outline) is just a thin sheet of muscle lying along the bottom of the femur.
Another reason is its anatomy. The extensor muscle has a pennate structure. This can be seen in the "herrring bone" arrangement of the muscle blocks when the femur is viewed from above.
The strength of any muscle is proportional to its cross-sectional area. The pennate arrangement means that a greater cross-sectional area of muscle can be packed into the femur than would otherwise be possible.
If you stimulate the extensor muscle electrically at a frequency which will make it contract with the greatest force (a tetanic contraction), then you can measure the rate at which that force develops. The picture below shows the results of such an experiment in which one leg was stimulated while it was partially extended, while measuring the force developed by the muscle.
Amazing fact. The extensor muscle from the back leg of an adult female locust (Schistocerca gregaria) can develop a force of up to 1.4 kg. This means that the muscle from one back leg of a grasshopper (admitedly quite a large one) can lift almost a bag-and-a-half of supermarket sugar!
HOWEVER, force is only half the story. The thrust needs speed as well. And therein lies the problem...