Myofibrils: The Building Blocks of Skeletal Muscle Contraction

The answer is myofibrils.

Myofibrils are the basic contractile units of skeletal muscle. They are made up of thick and thin filaments, which are arranged in a repeating pattern called a sarcomere. The thick filaments are composed of the protein myosin, while the thin filaments are composed of the protein actin. When a muscle contracts, the thick filaments slide past the thin filaments, causing the muscle to shorten.