Isometric (Static) Muscle Action

An isometric action is one in which the ends of the muscle are prevented from drawing closer together, with no change in length. It is the static classification of muscle action: force is developed against a fixed or immovable resistance but the muscle neither shortens nor lengthens, and no joint movement occurs.

In the source's exercise classification, isometric is the muscle action belonging to "static" exercise, where muscle length shows no change (contrast with the dynamic concentric and eccentric actions). It also appears in practical contexts such as holding the spine in a semi-isometric contraction to keep it in an "S" shape during exercise, and in the mid-range pause at full muscle contraction during a repetition.

• Isometric = static action; ends prevented from drawing closer, no change in length.
• No joint movement; force developed against fixed/immovable resistance.
• The "static" row in the exercise/muscle-action classification (muscle length: no change).

• concentric-action — dynamic shortening action.
• eccentric-action — dynamic lengthening action.
• muscle-action-terminology — "action" vs. "contraction."
• exercise-muscle-action-classification — dynamic vs. static table.
• isometric-training — training method built on isometric actions.