Muscle contraction

Muscle contraction is force generation by muscle fibers, usually through myosin interacting with actin. It can shorten a muscle, hold its length, or resist being stretched.

Sliding filament theory: The model in which actin and myosin filaments slide past one another as a muscle contracts. It explains how filament overlap changes while the filaments themselves retain their lengths.
Sarcomere: The repeating contractile unit of striated muscle, bounded by Z discs and containing organized actin and myosin filaments. Sarcomeres are the aligned units whose shortening produces striated-muscle force.
Isometric contraction: Muscle force production without a change in overall muscle length. It shows that contraction can generate force without shortening.
Length–tension relationship: The relationship between a muscle's length and the force it can produce. Filament overlap and elastic structures make force depend on starting length.
Ciliary beating: The coordinated movement of cilia, driven by dynein motors acting on microtubules. It uses motor proteins and ATP but relies on microtubules rather than actin filaments.
Cross-bridge cycle: The repeating sequence in which myosin binds actin, generates a power stroke, detaches, and resets. This molecular cycle produces force and filament sliding.
Actin: A protein that polymerizes into filaments and provides binding sites for myosin in muscle. Thin filaments supply the track that myosin pulls during contraction.
Concentric contraction: A muscle contraction in which the muscle shortens while producing force. It describes the shortening phase of many lifting movements.
Force–velocity relationship: The relationship between a muscle's shortening or lengthening speed and the force it produces. Cross-bridge behavior limits force as contraction speed changes.
Kinesin: A motor protein that usually transports cargo along microtubules using ATP. Like myosin, it converts ATP energy into movement, but works on a different filament system.

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