Muscles produce movement through contraction. The muscle contraction associated with movement is called an isotonic contraction. When a muscle contracts with no resulting movement, it is called an isometric contraction.
Isotonic contractions involve the shortening of muscles to produce movement. The word "isotonic" implies that the muscle has the same tone throughout its movement and never gets saggy while shortening.
Inside of the human body, muscles are arranged in groupings of agonist, antagonist and synergists that produce movement. The agonist muscles are the main muscles responsible for isotonic contractions because they produce motion by shortening the muscles, while agonist muscle is paired to the antagonist muscle, which is responsible for returning a limb to the previous position it was in before moving.
Finally, synergist muscles move in tandem with agonist muscles to produce movement around the joint. The biceps and triceps muscle groups are examples of agonist and antagonist muscle pairs; the triceps act as the agonist while the biceps act as the antagonist. Synergist muscles are sometimes referred to as neutralizers because they often act to reduce excessive force generated by the agonist muscles. They are useful because they fix certain joints to allow a range of muscle contractions.