Skeletal muscle fibers are long, cylindrical cells that make up skeletal muscles, which are responsible for voluntary movements in the body. These fibers are multinucleated, meaning they contain multiple nuclei, and are striated, giving them a striped appearance under a microscope. They are designed for rapid contraction and can be classified into different types based on their contraction speed and endurance. Each skeletal muscle fiber is surrounded by a membrane called the sarcolemma and contains specialized structures like myofibrils, which are made up of repeating units called sarcomeres. These sarcomeres are the functional units of muscle contraction, containing proteins such as actin and myosin that interact to produce movement.