An example of a first class lever in the human body is the head and neck during neck extension. The fulcrum (atlanto-occipital joint) is in between the load (front of the skull) and the effort (neck extensor muscles). The muscles are attached to the posterior part of the skull to allow for the greatest effort arm.
It is important to evaluate an exercise program before you participate because you don't want to severely injure yourself or push yourself beyond your exercise ability.