Which action occurs when a muscle shortens during movement?

The action that occurs when a muscle shortens during movement is called concentric action. This type of contraction happens when the muscle fibers generate tension and pull on the bones, resulting in movement. A classic example of concentric action is lifting a weight, such as during a bicep curl, where the biceps contract and shorten to lift the forearm. This shortening creates force and allows the joints to move through their range of motion.

In contrast, isometric action occurs when the muscle generates tension without changing in length, such as holding a weight steady without moving. Dynamic action encompasses both concentric and eccentric movements, but does not specifically refer just to shortening. Eccentric action, on the other hand, involves the muscle lengthening while still under tension, like when lowering a weight back down. Thus, concentric action is specifically identified by the muscle shortening, making it the correct choice.