When a skeletal muscle fiber contracts, (1) the H bands and I bands get smaller, (2) the zones of overlap get larger, (3) the Z lines move closer together, and (4) the width of the A bands remains constant. The contraction ends once the fiber has shortened by about 30 percent, which coincides with the elimination of the I bands. These observations make sense only if the thin filaments are sliding toward the center of the sarcomere, alongside the thick filaments (Figure 10-8). This explanation for the physical changes that occur during contraction is the sliding filament theory.

The sliding filament theory explains what happens to the sarcomere during a contraction, but it does not explain the mechanism involved. To do so, we must examine the contraction sequence in greater detail.