Sliding-filament theory (A.F. Huxley, 1957): shortening of a muscle fibre occurs by thin filaments sliding over thick filaments toward the centre of the sarcomere.
Sarcomere Changes During Contraction
During contraction, A band length remains constant; H zone and I band decrease in width, and the Z lines move closer together.
Clinical
Constant A band length with narrowing H zone and I band is the structural evidence that shortening occurs by increasing filament overlap, not by shortening of the thick or thin filaments themselves.
High Yield
Sliding-filament theory (A.F. Huxley, 1957): muscle shortening occurs by thin filaments sliding over thick filaments toward the sarcomere centre.
A band length is constant during contraction.
H zone and I band decrease in width, and Z lines move closer together, during contraction.