Synergists

Muscles that assist the prime movers are called synergists, synergists stabilize joints from moving when the prime mover crosses two or more joints.
Example: making a fist the synergist stabilizes the wrist joint so the prime mover can act on the finger joints.

Antagonist Muscles

When a muscle causes movement that muscle is called the prime mover. The muscles that oppose or reverse movement are called antagonist. When the prime mover is contracting the antagonist is stretched and relaxed.

Fixators

Muscles that hold a bone still, or stabilize the area the prime mover is attached to are called fixators.
Example: muscles that stabilize the vertebral column.

3 Classes of Levers

For movement to occur muscles apply a force, bones act as levers, and joints act as the fulcrum. The body uses 3 different classes of levers to accomplish movement.