How does a muscle actually contract?

By filaments inside each cell sliding past one another, pulled by molecular motors — the muscle does not shorten by compressing, it shortens by overlapping.

The structure. A muscle is bundles of fibres; each fibre contains myofibrils made of repeating units called sarcomeres. Each sarcomere contains thick filaments of myosin and thin filaments of actin, interleaved.

The sliding filament mechanism:

A nerve signal arrives, triggering release of calcium inside the fibre.

Calcium unblocks the binding sites on actin, which are otherwise covered by regulatory proteins. This is the switch.

Myosin heads attach to actin, forming cross-bridges.

The power stroke. Each head pivots, dragging the thin filament past the thick one, then detaches using ATP and resets to repeat.

Millions of these cycles, asynchronously, produce smooth force. The sarcomere shortens because the filaments overlap further — neither filament changes length.

What follows from this:

ATP is needed to release, not only to contract. When ATP is unavailable, cross-bridges cannot detach — which is the mechanism of rigor mortis.

Force depends on overlap. There is an optimal length where the most cross-bridges can form; too stretched or too shortened and a muscle produces less force. This is why joint angle changes how strong you feel.

Eccentric contractions, where the muscle lengthens under load, produce more force than shortening ones and cause more muscle damage — which is why lowering a weight slowly is both effective and the main source of next-day soreness.

Force is graded two ways: recruiting more motor units, and firing them faster. Small precise muscles have few fibres per motor unit; large power muscles have many.

Why training works. Early strength gains are largely neural — better recruitment and coordination — which is why beginners get stronger before they get bigger. Later gains come from adding contractile protein, increasing the number of sarcomeres in parallel.