The thin filament seen from above. A long strand called tropomyosin lies in the groove of the actin, with troponin attached to it in triplets.
The small rectangles are the binding sites for the myosin heads. Right now the tropomyosin strand lies over them, and no head can bind.
The filaments are there, the heads are there, the ATP is there, and still the muscle does not contract. What decides is this cover.
What lifts the cover is calcium. Here is the route that gets the calcium released.
The band across the top is the fibre membrane, and the tube dropping from it into the cell is a T tubule. The inside of the tubule is still the outside of the cell: the membrane has simply been folded inwards.
The sacs drawn on either side are the sarcoplasmic reticulum, the store where calcium is kept. Where they meet the T tubule they widen, and that is where the release channels sit.
An action potential in the motor neuron reaches the neuromuscular junction, acetylcholine is released, and the fibre membrane depolarises. That signal runs in from both sides, turns down the T tubule and reaches the interior. Voltage sensors there open the release channels, and calcium pours into the cytosol.
Back to the thin filament. Send the signal, and the calcium binds troponin.
Troponin changes shape, and the tropomyosin attached to it shifts deeper into the groove of the actin. In the figure the strand moves down.
With the strand out of the way, the binding sites underneath are exposed. Only now can a head bind, and the five-state cycle from the previous article begins to turn.
Calcium in the cytosol rises from about 0.05 µM at rest to 10 µM, some two hundred times. That change in concentration is the switch that decides whether the muscle contracts.
Stop the signal. The calcium returns to the reticulum, the strand slides back, and the binding sites are covered again.
That return does not happen by itself. A calcium pump in the membrane of the sarcoplasmic reticulum spends ATP to draw the calcium back in.
So ATP is spent on releasing the head and on pumping the calcium back. Neither job is contracting. Both are stopping.
Shortening and relaxing both cost ATP. That is why a body that has run out of it stiffens rather than going slack.