Muscle fibers refer to the individual units that make up a muscle. These fibers are responsible for generating movement and force in our bodies, as they contract and relax in response to nerve impulses. There are three main types of muscle fibers found in humans - type I (slow-twitch), type IIa (fast-twitch oxidative), and type IIx (fast-twitch glycolytic) - each with different characteristics, such as their speed of contraction, endurance, and ability to generate force.