What occurs within muscle fibers during contraction?

During muscle contraction, the fundamental action that takes place within muscle fibers involves the sliding of myosin and actin filaments. This phenomenon is described by the sliding filament theory, which explains how muscle fibers shorten and generate force when stimulated.

Myosin filaments have small projections called cross-bridges that attach to the actin filaments. When a muscle is stimulated to contract, calcium ions are released, which triggers the binding of myosin to actin. This interaction causes the myosin heads to pivot, pulling the actin filaments toward the center of the sarcomere, the basic contractile unit of muscle tissue. This sliding action effectively shortens the muscle fiber, leading to contraction.

While it is true that ATP is involved in this process, as it provides the necessary energy for the contraction through breakdown during the cycling of cross-bridges, the focus of the question is specifically on what occurs during the contraction process itself.