Answer
The lever arm is the shortest distance between the applied force (i.e., the line along which the force acts) and the rotational axis.
This is discussed on page 139, and examples are shown in Figures 8.19-8.20.
In the first case, the lever arm is the length of the pedal sprocket arm.
In the second case, the vertical position, the lever arm is zero. The line of action of the pushing force passes directly through the rotational axis.
