This search only deals with the prime-mover muscles, this means that we only take into account the muscles that action the movement.
For example, when swinging a kettlebell, the knees, and hips move. The muscles that action those movements are the ones used for this search result. However, a swing uses many more muscles than those that action the movement. For example, the muscles that keep the spine erect or the ones that pull the scapula together and down. These muscles are accounted for in another search.