ABSTRACT
When muscle contracts, it can shorten by as much as a third of its original length. Information obtained from X-ray crystallographic (Section B2), and light-and electron-microscopic studies (Section A4) led to the proposal of the sliding filament model to explain muscle contraction. The thick and thin filaments were seen not to change in length during muscle contraction, but the length of the sarcomere was observed to decrease as the thick and thin filaments slide past each other (Figure 1). Thus, as muscle contracts, the sizes of the H zone and the I band are seen to decrease. The force of the contraction is generated by a process that actively moves one type of filament past neighboring filaments of the other type.
