During muscle contraction, which structure shortens?

During muscle contraction, the sarcomere is the structural unit that actually shortens, leading to the overall contraction of the muscle. The sarcomere is the smallest functional unit of muscle fibers, composed of thick and thin filaments.

When a muscle contracts, the thick filaments (myosin) pull on the thin filaments (actin) through the process of cross-bridge cycling. This action leads to the shortening of the sarcomere as the thick and thin filaments slide past each other. As the sarcomeres within a muscle fiber shorten, the overall length of the entire muscle decreases, resulting in contraction.

Though myofibrils are made up of many sarcomeres arranged in series, it is the sarcomere itself that is responsible for the shortening. The sarcolemma, which is the membrane surrounding the muscle fiber, does not shorten but instead transmits the electrical signals needed for contraction. Thick filaments are part of the sarcomere's structure and do not shorten independently; they participate in the sliding mechanism that facilitates the sarcomere's shortening.