MyosinThick filament

Myosin forms the thick filaments of the sarcomere. Each molecule has a head that binds actin, swivels to pull the thin filament inward, then detaches and resets — the cross-bridge cycle, repeated many times per contraction.

It matters because ATP powers that cycle at two points: it is needed to detach the head from actin and to re-cock it for the next stroke. Without ATP the heads stay bound, which is the mechanism of rigor mortis and part of why severe energy depletion in muscle feels like seizing rather than simple weakness.

In practice, the specific myosin variant a fibre expresses is what defines its type — the heavy-chain isoform is how researchers classify fibres as type I, IIa or IIx.