Which of the following represents a class I lever system?

A) Crossing your legs
B) Hyperextension of the head
C) Standing on your tiptoes
D) Flexion of the elbow to elevate the hand
E) Lifting weight with your arm

In a class I lever system, the fulcrum is located between the effort and the load. Hyperextension of the head on the neck is an example of this, where the fulcrum is the atlanto-occipital joint, the effort comes from the posterior neck muscles, and the load is the front part of the head.