A sarcomere shortens because thin filaments slide past thick filaments. The filaments themselves remain approximately the same length during an ordinary contraction.

This is the sliding-filament mechanism.

Myosin heads on the thick filament repeatedly interact with actin and pull the thin filaments toward the M line. Because thin filaments are anchored at the Z discs, inward sliding brings the Z discs closer together.

relaxed:     Z |----->   <-----| Z
                 =====

contracted: Z |-----> <-----| Z
                 =====

The arrows represent thin filaments moving farther into the region occupied by thick filaments.

What changes and what does not

Because thick-filament length is nearly constant, the A band stays approximately the same width during shortening.

As overlap increases:

• the distance between Z discs decreases;
• the I band narrows;
• the H zone narrows and can disappear;
• the A band remains approximately constant.

This geometric prediction was an important test of the sliding-filament model.

Many sarcomeres add their shortening

Sarcomeres are arranged in series along a myofibril. If each of 10,000 sarcomeres shortens by only $0.2\ \mu\mathrm{m}$, their total shortening is

$$10{,}000\times0.2\ \mu\mathrm{m}=2000\ \mu\mathrm{m}=2\ \mathrm{mm}.$$

Small molecular displacements can therefore sum to substantial shortening of a muscle fiber.

Shortening is not required for force

Actin–myosin interactions can generate tension even when the overall muscle length is held fixed. Sliding occurs only when the generated force is sufficient to move the imposed load.

Thus force generation and shortening are related but distinct: the sliding-filament model explains how relative filament motion changes sarcomere length, while the molecular cross-bridge cycle explains how myosin generates the force that can produce that motion.