Muscles don't shorten their filaments. They slide them past each other. Myosin heads grab actin and pull it inward, step by step. This pulls the Z-lines together. The filaments stay the same length, but the sarcomere gets shorter. Watch it.
The sarcomere
Sliding filamentrelaxed
💡 During contraction the A-band stays the same width, because it is the length of myosin. But the I-band and H-zone shrink as actin slides in. That's the clue to look for on a test.
📷 Reference image
A real labeled figure to compare with the interactive above.
Sliding Filament Contraction — OpenStax, CC BY 4.0 (via Wikimedia Commons)
