How can a muscle shorten without shortening its filaments?

An arm can lift a book as its muscles shorten, but the tiny force-producing filaments inside those muscles do not themselves shrink. Inside a muscle fiber, repeated units called sarcomeres contain thin actin filaments anchored to boundaries called Z-discs and thick myosin filaments nearer the center. Shortening brings the Z-discs closer because the thin filaments slide farther between the thick ones. Many sarcomeres arranged in series can combine their small changes into a larger shortening.

Myosin has projecting heads that temporarily bind to actin. During a power stroke, a bound head changes its orientation and pulls the thin filament toward the sarcomere's center. It then detaches and resets for another cycle. Repeated cycles by many heads increase overlap between the filaments; neither actin nor myosin has to shrink like a contracting rubber thread.

The interaction is controlled by other proteins. Calcium binds to troponin, changing the position of tropomyosin so that myosin-binding sites on actin become available. This connects a cellular calcium signal to access at the actual force-producing surface. When calcium is removed, the regulatory proteins can restrict that interaction again.

ATP has distinct jobs in the cycle. ATP binding allows a myosin head to detach from actin, and ATP hydrolysis helps reset the head for another cycle. ATP also powers pumps that return calcium to storage during relaxation. Energy supply is therefore involved in both repeated force production and recovery of the resting conditions, rather than being used only at the moment a filament moves.

In the invented geometry, each thin filament stays 110 drawing units long and the thick filament stays 160 while Z-disc separation changes. At separation 260, the thin-filament tip gap is 40 and overlap with the thick filament is 60 on each side. These lengths illustrate sliding, not physiological dimensions or a force calculation. A muscle can also develop tension without shortening when the load prevents movement: molecular pulling and visible motion are related but not identical. Holding that same book still also needs muscular force: the hidden molecular action need not produce visible movement.