Kinesiology

Lesson 2 of 16

Muscle roles and contractions

Muscles cooperate to create, control, or steady motion. A muscle's role is specific to the task: the same muscle may be an agonist in one action and an antagonist in another. Contraction describes active tension, which can occur with or without visible shortening. Understanding these distinctions improves movement observation and safe positioning.

Key points

• The agonist is the prime contributor to a particular action.
• An antagonist opposes or brakes the action; it need not be inactive.
• Synergists assist an action or reduce unwanted movement.
• Fixators stabilize a base so another segment can move efficiently.
• Concentric contraction produces tension as a muscle shortens; eccentric tension accompanies lengthening.
• Isometric contraction creates tension without appreciable joint movement.

Key terms

Agonist

The muscle contributing most directly to a named action.

Antagonist

A muscle that opposes or controls the action of an agonist.

Synergist

A muscle assisting the desired action or limiting extra motion.

Fixator

A stabilizing muscle that steadies a segment during movement elsewhere.

Eccentric contraction

Active tension while a muscle lengthens under load.