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In the sliding filament model of muscle contraction, the (thin) actin myofilaments [red] (attached to the Z-line) slide (actually, are pulled) inward along the (thick) myosin myofilaments [blue], and the sarcomere (measured from one Z line to the next) is shortened. Note on the measuring scale that as contraction is occuring, both the sarcomere length and the I band (area where thin filaments do not overlap with thick filaments) length decrease, but the A band (area containing thick filaments) length remains constant.
animation ©1998 Mark Meyer / 3 Dot Studio