Muscles are constantly active. As a health and fitness professional, it is important to understand the actions muscles perform. When muscles contract, they vary from resting length (i.e., they shorten or lengthen) or they stay the same. The three muscle actions are: concentric, eccentric, and isometric. Their names indicate their length.

Concentric action

The muscle actively shortens when under tension. For example, the quadriceps shorten in the upward phase of a leg extension.

Eccentric action

The muscle actively lengthens under tension or returns to its resting position from a shortened position. For example, the biceps brachii lengthen in the lowering phase of a biceps curl as it returns the arm to the starting position.

Isometric Action

There is no visible change in length as the muscle contracts. A plank is an example of many of the major muscle groups performing isometric contractions.

The roles and responsibilities of muscles vary in movement. When in motion, muscles take on the role of agonist, antagonist, synergist, or co-contractor.

Agonist

• The main muscle or muscle groups responsible for a particular movement or action
• Also called a prime mover
• For example, the triceps brachii is the main muscle responsible for movement during a triceps extension.

Antagonist

• Moves in opposition to or opposes the agonist
• During a biceps curl, the opposing muscle group—the antagonist—is the triceps.

Synergist

• Assists the agonist in performing its action
• Stabilizes and neutralizes joint rotation (prevents joint from rotating as movement is performed)
• Becomes active as external force increases or when the agonist fatigues
• During a biceps curl, the synergists are the biceps brachii and brachioradialis, as the brachialis acts as the agonist. The first two assist the latter in stabilizing the elbow joint during the biceps curl exercise.

Co-contraction

• Agonist and antagonist contract at the same time to stabilize a joint
• Co-contracting muscles are “stabilizers” that protect the joint and help maintain alignment.
• For example, in abdominal bracing, muscles of the core—transverse abdominis and mutifidi—and the muscles of the pelvic floor contract together to help maintain postural alignment during exercise.