The phrase "main movers" refers to the key people or things that are responsible for causing big changes or making significant progress in a situation or project. They are the main driving force behind the success or advancement. Full definition
Primary authorship is highly valued because it usually indicates who had the idea, who was the «main mover» in the work, or both, Kosslyn says. (sciencemag.org)
When you aim very high, the hip flexors become the main movers and the abs take a secondary movement role, which is totally counter-productive if you plan on doing this exercise to isolate the lower abs. (fitnessandpower.com)