What does a concentric contraction involve?

A concentric contraction involves shortening a muscle while it generates force, typically against a resistance. During this type of contraction, the muscle fibers actively shorten, which helps to lift or move a weight, overcoming the resistance presented by external forces. This action is commonly observed in exercises that require the muscle to lift a load, such as during a bicep curl when the elbow bends and the bicep muscle shortens to raise the forearm.

In contrast, lengthening against resistance describes an eccentric contraction, where the muscle extends while still producing force, typically when lowering a weight. Holding a position refers to an isometric contraction, where there is no change in muscle length, and relaxing without resistance indicates a passive state where the muscle is neither contracting nor exerting force. These distinctions clarify why the option focused on shortening against resistance is the correct identification of a concentric contraction.