Fitness Certifications ▶
Continuing Education ▶
Fitness Pro Resources ▶
My ACE Account ▶
About ACE ▶
ACE Store ▶
Need Help? Call Us ▶ (888) 825‑3636
Share this page
Pin It

October 25, 2013 | Exam Preparation Blog

Muscles in Motion

Muscle contractionMuscles are always in motion. As a trainer, it is important to understand the actions muscles perform. When muscles move or contract, they change length—they shorten, lengthen or stay the same.

The three muscle actions or contractions are: concentric, eccentric and isometric. Their names indicate their length.

Concentric Action: The muscle actively shortens in length when under tension. For example,  the quadriceps shortens 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 triceps lengthens in the upward phase of a biceps curl, while the biceps lengthens in the lowering phase as it returns to rest.

Isometric Action: There is no visible change in length as the muscle contracts. A plank is an example of an isometric contraction.

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


-The main muscle or muscle groups responsible for a particular movement or action
-Also called the prime mover
-For example, the biceps brachii is the main muscle responsible for movement during a biceps curl.


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


-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 brachialis and the brachioradialis. They both assist in stabilizing the elbow joint during the biceps curl.


-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—the transverse abdominis and mutifidi—and the muscles of the pelvic floor must contract together to help maintain postural alignment during exercise.

By Makeba Edwards

Makeba is a Study Assistance Representative for ACE, and holds a B.S. in Exercise Science and Sports Studies. She is an ACE Certified Personal Trainer Group Fitness Instructor. She also a Les Mills Certified Group Fitness Instructor and teaches several modalities.