Skeletal-muscle cross-bridge cycling is regulated by the Ca$^{2+}$ concentration in the fluid surrounding the myofibrils.

At low cytosolic Ca$^{2+}$, tropomyosin lies over important myosin-binding regions of actin. This reduces productive actin–myosin interaction even though ATP and both filament systems are present.

The troponin complex links Ca$^{2+}$ concentration to tropomyosin position.

Calcium exposes actin for cross-bridge cycling

One troponin subunit, troponin C, binds Ca$^{2+}$. Increased Ca$^{2+}$ occupancy changes the troponin complex and shifts tropomyosin away from its inhibitory position on actin.

cytosolic Ca2+ rises
        ↓
Ca2+ binds troponin C
        ↓
tropomyosin position changes
        ↓
more actin sites accessible to myosin
        ↓
cross-bridge cycling and force increase

Ca$^{2+}$ does not itself pull the filaments. It regulates access; myosin's ATP-driven cycle generates the force.

Removal of calcium turns the thin filament back toward the off state

When cytosolic Ca$^{2+}$ falls, Ca$^{2+}$ dissociates from troponin C. Tropomyosin again occupies a position that strongly inhibits productive myosin binding, and active tension falls as existing cross-bridges complete their cycles.

Force can be graded through calcium activation

Thin-filament regulation is not necessarily an all-or-none molecular switch. Intermediate Ca$^{2+}$ concentrations can activate only part of the available regulatory system, allowing the number of cycling cross-bridges and therefore muscle force to vary.

The essential separation is

$$\boxed{\text{Ca}^{2+}\text{ regulates access; ATP-driven myosin generates force}.}$$

This distinction is what allows an electrical signal to control contraction by first controlling intracellular Ca$^{2+}$.