The phrase "moment arm" refers to the distance between a force and the point where it is applied on an object. It helps determine how effective or strong the force can be in causing rotation or movement of the object. Full definition
The the superior and inferior fibers display the largest peak moment arm lengths, while the middle fibers display the smallest. (strengthandconditioningresearch.com)
This purpose of this section is to provide a summary of the muscle moment arms of the calf muscles in each of the main planes of movement. (strengthandconditioningresearch.com)