A skeletal-muscle fibre contains parallel myofibrils assembled from repeating sarcomeres between Z discs. Thin actin filaments extend from the Z discs; bipolar thick myosin II filaments occupy the central region. Overlap between them allows many myosin heads to interact with actin in parallel.
A rise in cytosolic calcium exposes actin binding sites through the troponin–tropomyosin regulatory system. ATP binding releases a strongly attached myosin head; hydrolysis primes its lever arm, attachment and product release permit a force-generating stroke, and another ATP allows detachment. The cycle converts chemical free energy into mechanical work rather than consuming actin or shortening the filaments themselves.
Relative filament sliding draws the Z discs together. The I band and H zone shorten as overlap increases, whereas the thick-filament A-band length is approximately unchanged. Calcium removal restores inhibition and permits relaxation. The many-head ensemble sustains force even though individual cross-bridges repeatedly attach and detach. This sliding-filament description concerns myosin II in muscle; the processive two-head walking of Myosin V in a is a related but distinct motor arrangement.
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