How do muscle fibres shorten when they contract?

A muscle gets shorter because the tiny protein threads inside its fibres slide past each other. This is called the sliding filament theory. Each muscle fibre is packed with two kinds of filament: thin ones made mostly of a protein called actin, and thick ones made mostly of a protein called myosin. During contraction the myosin slides past the actin, while the filaments themselves stay about the same length.

How does the sliding happen? Myosin has little heads that grab onto the actin and pull it inwards, like a rope being hauled hand over hand, and the pull shortens each repeating unit of the fibre, called a sarcomere. Then the heads let go, reach forward and grab again. Letting go takes fuel: when ATP, the cell's energy molecule, binds to a myosin head, the head detaches from actin and can attach to another spot.

The command comes from a nerve. A motor neuron signals the muscle fibre, calcium is released inside it, and the calcium binds to a protein called troponin. That makes a blocking protein, tropomyosin, slide aside and uncover the grip points on the actin. The grabbing and pulling continue for as long as there is enough ATP and calcium. When calcium is pumped away, the grip points are covered again and the muscle relaxes.

This explains an eerie fact of life and death. After death the body runs out of ATP, so the myosin heads can no longer let go. They stay locked onto the actin in what is called rigor mortis.