Pectoral AND glute flex

You need to have a model in a pose that shows both frontal upper body and backside. Side views (profile shots), or backside visible with torso twisted towards camera work best. Photos with front or backside only will apply only the movement for the respective body part.

Seated characters (kneeling, deep squat, split) will work best; characters standing up might start squatting (especially if the behind is less pronounced)