Back to the sarcomere. Small projections come off the thick bar in the middle. These are the heads.
A head is part of a myosin molecule. Bundle the tails together to make the thick filament and only the heads are left sticking out.
What they stick out towards is the thin filament. The next figure enlarges a single head to watch it grip and pull. Here, the heads that have gripped are drawn darker than the ones that have not.
One head, enlarged. The thick bar below is the thick filament and the wavy strand above is the thin filament.
The head goes round five states. Standing upright it binds the thin filament (attach), it swings down and drags its partner (power stroke), it stays locked on while down (rigor), it lets go (detach), and it cocks back up (recover).
It is the swing that moves the thin filament, about 10 nm to the left. That is where the force is. The other three states exist to set up the next one.
When the head lets go and stands up again, the thin filament does not return. Where it was dragged to becomes the starting point of the next turn.
Held at the third state, rigor. The head is still down and still locked onto actin.
In this state the head carries nothing. ADP and phosphate have both left, and it cannot release from actin on its own.
The only thing that releases it is ATP binding. Once ATP is bound, the affinity for actin drops and the head lets go. ATP is therefore needed not for pulling but for letting go.
After death, when no more ATP is made, every head in the body stops in this state. That is rigor mortis. The muscle does not stiffen by contracting; it stiffens because it can no longer release.
Running again, this time faster.
Ten nanometres a turn. On its own that moves nothing, but a single sarcomere holds several hundred pairs of heads, and all of them are pulling the same thin filament.
They do not turn together. While one head is detached another is holding on, so the filament is never let go and the pull never stops.
The faint heads drawn to the left and right of the main one are those neighbours. It is precisely because they are out of step that a muscle shortens smoothly.