Pull-ups are difficult because of the physics involved in the equation. Basically the work your arms have to do is described as the energy needed to move some mass over some distance. In teh case of doing a single pull-up, the work that your arms and back need to do is a function of your mass and how far you need to move it upwards to get your chin over the bar.
This lets you understand just what an achievement doing pull-ups is and why there is so much value in the bragging rights. This challenge takes you several levels up.