Concentric (Positive) Muscle Action

A concentric action (also called positive action) is a dynamic muscle action in which the whole muscle shortens and joint movement occurs — one in which the ends of the muscle are drawn closer together. In a repetition it is the concentric phase: the muscle shortens against a load.

It is the "positive" half of a dynamic exercise and decreases muscle length. The source recommends executing it without momentum:

• Refrain from using momentum (which reduces muscular loading).
• Use a deliberate, controlled manner.
• Ballistic-type movements compromise safety and effectiveness.

Concentric action produces less force than eccentric action: the muscles produce more force in lowering (eccentric) action than in lifting (concentric) action — the eccentric phase is roughly 40% stronger than the concentric phase. Typical prescribed tempos in the source's programs run about 1 to 2 seconds for the concentric (positive) phase.

• Concentric = positive action; the muscle shortens, ends draw closer, joint moves.
• The lifting phase of a repetition; decreases muscle length.
• Produces less force than eccentric action (concentric ≈ 40% weaker).
• Perform deliberately, controlled, without momentum or ballistic movement.

• eccentric-action — the stronger lowering counterpart.
• isometric-action — the static, no-movement action.
• muscle-action-terminology — why "action" rather than "contraction."
• positive-negative-resistance — positive/negative resistance framing.
• tempo — repetition timing of the concentric phase.