Force in a sarcomere is generated by repeated cycles in which a myosin head binds actin, changes conformation, releases and resets. The interacting actin–myosin connection is called a cross-bridge.

The cycle is coupled to ATP chemistry.

One cross-bridge cycle

A useful sequence begins immediately after ATP binds myosin.

1. ATP binding detaches myosin from actin. ATP binding lowers the affinity of the myosin head for actin.
2. ATP hydrolysis primes the head. Myosin hydrolyzes ATP to ADP and inorganic phosphate ($P_i$). The products remain bound while the motor adopts a higher-free-energy conformation.
3. Myosin binds actin. If the actin site is accessible, the primed head forms a cross-bridge.
4. Phosphate release strengthens binding and promotes the force-generating conformational change. The myosin head rotates relative to its actin attachment, pulling the thin filament. This is commonly called the power stroke.
5. ADP is released. Myosin remains strongly attached until another ATP molecule binds and begins the next cycle.

ATP binds
   ↓ detach
ATP hydrolysis
   ↓ prime
actin binding
   ↓ Pi release
force-generating stroke
   ↓ ADP release
strongly bound state
   ↓ new ATP
repeat

ATP has more than one mechanical role

It is misleading to say simply that 'ATP supplies the energy for the power stroke.' ATP binding is also required for detachment, while hydrolysis prepares the head for another force-generating cycle.

If ATP is absent, myosin can remain strongly bound to actin rather than cycling normally.

Many asynchronous heads produce sustained force

A thick filament contains many myosin heads. They do not all bind and detach simultaneously. At any instant, some heads can be producing force while others are detached and resetting.

This overlapping activity permits relatively smooth tension despite each individual molecular stroke being small and discrete.

The cross-bridge cycle therefore converts repeated ATP turnover into directed actin–myosin force and, when the load permits, filament sliding.