MyosinThick filament

Myosin is the motor protein of muscle, and the thick filament of the sarcomere is made of it. Each myosin molecule has a long tail and a pair of heads, and those heads are what physically produce movement.

Contraction is a cycle. A head binds to the neighbouring actin filament, pivots to drag it along, releases when a fresh ATP molecule arrives, resets, and binds again further down. Billions of these cycles, slightly out of step with each other so the pull is smooth, shorten the muscle.

ATP has two jobs in that cycle, which is a detail worth knowing because it explains something unexpected: ATP is needed to release the head from actin, not only to power the stroke. That is why rigor mortis happens, since without ATP the heads stay attached.

Different myosin isoforms cycle at different speeds, and that is what fibre type physically means. Fast twitch fibres express faster myosin isoforms; slow twitch fibres express slower, more economical ones. Training shifts the proportions within a type more than it converts one to another.

The head is also where force is regulated from moment to moment. Calcium controls whether the binding sites on actin are exposed, so the nervous system controls contraction indirectly by controlling calcium release.

Myosin is one of the proteins damaged and rebuilt in the ordinary turnover that training accelerates, which is the tissue level reason protein intake and mechanical loading act together rather than separately.

In practice this is background biology, and the actionable version lives on our hypertrophy and mechanical tension entries: force production is the signal, and the machinery producing it is what gets rebuilt bigger.