= Solution
A skeletal-muscle fibre contains parallel <Myofibril>[myofibrils] assembled from repeating <sarcomeres> between Z discs. Thin <actin filaments> extend from the Z discs; bipolar thick <myosin II> <Myofilament>[filaments] occupy the central region. Overlap between them allows many <Myosin>[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 <Myosin lever arm>[lever arm], attachment and product release permit a force-generating stroke, and another <ATP> allows detachment. The cycle converts <Gibbs free energy>[chemical free energy] into <Work>[mechanical work] rather than consuming <actin> or shortening the <Myofilament>[filaments] themselves.
Relative filament <Facilitated target search>[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 <6/a> is a related but distinct motor arrangement.
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