What describes the structure of muscle fibers?

The structure of muscle fibers is characterized by distinct functional units known as sarcomeres. Sarcomeres are the repeating units that make up myofibrils, which are the contractile elements of muscle fibers. Each sarcomere contains organized arrangements of actin and myosin filaments, which interact to facilitate muscle contraction. This organization allows for the efficient generation of force and movement when the muscle fibers contract. The arrangement of sarcomeres is crucial for the striated appearance of skeletal and cardiac muscles, contributing to their function in voluntary and involuntary movements respectively.

The other options do not accurately describe muscle fibers. Collagen is a component of connective tissues but not the primary structural component of muscle fibers. Elastic tissue is present in some types of connective tissue and allows for flexibility, but it is not the main composition of muscle fibers. Lastly, muscle fibers are not composed of a single layer of cells; they are multi-nucleated structures formed from the fusion of myoblasts during development, allowing for greater strength and coordination during contraction.