Muscle Fiber
The sliding filament theory explains how muscles produce force. Myosin motor proteins bind to actin filaments and execute a power stroke, pulling the filament inward. ATP hydrolysis resets each cross-bridge — thousands of cycles per second shorten the sarcomere.
How it works
A sarcomere is the basic contractile unit of striated muscle. Thick filaments (myosin) interdigitate with thin filaments (actin). Each myosin head can bind actin, pivot through ~70°, release, and reset — the cross-bridge cycle.
The cycle requires ATP: (1) ATP binds myosin → head detaches; (2) ATP hydrolysis → head cocks; (3) Ca²⁺ exposes actin binding sites (via troponin-tropomyosin); (4) weak binding, then power stroke releases ADP+Pi; (5) head remains attached until new ATP arrives.
The Hill force-velocity curve governs the trade-off: maximum force is produced at zero velocity (isometric); maximum velocity at zero load. Tetanic stimulation summates individual twitches to produce sustained maximum force.