When muscles are first learned, it is classically the concentric (shortening) contractions that are first learned. Unfortunately, this is often as far as many students progress when learning about the musculoskeletal system. If a snapshot were taken at any one instant in time during a motion pattern, less than half of the contractions that are occurring in the body would actually be concentric contractions of movers (agonists). There are also the eccentric contractions of antagonists and the isometric contractions of fixators (stabilizers).
A nice exercise to do to examine and learn about the roles of muscles is to choose a specific joint action (and describe the position of the body at that time to determine the role of gravity), and then determine a possible mover, antagonist, fixator, neutralizer, and support muscle. Be aware though that some actions are simple and some are more complicated.