 Dear students, in this topic we shall discuss the sliding filament theory of muscle contraction. This theory was presented by H. E. Huxley and A. F. Huxley in 1954. It states that during muscle contraction, the thin and thick filaments in a sarcomere slide past each other and undergo shifting. When a muscle contracts, the thin filaments between the thick, flight between the thick myosin filaments and move closer to the center of sarcomere. As a result, the sarcomere becomes shorter. That is, thin filaments slide past the thick filaments. As a result, the length of sarcomere is shortened. When a muscle relaxes or it is stretched, the overlap between the thin and thick filaments is reduced. As a result, the sarcomere elongates. These changes in sarcomere length during stretch and contraction correspond to the changes in overall muscle length. Dear students, now we shall explain the sliding filament theory. Actually, in the relaxed muscle fiber, the thin and thick filaments overlap only at a small region of a bent. When sliding begins, there is formation of cross bridges between the thick and thin filaments due to the myosin heads binding with the actin protein at specific sites. During contraction, the A-bend maintains a constant length. A-bend maintains its length, it does not move anywhere and its overall length is constant. However, during contraction, the A-bend and H-zone become shorter as they slide past and move in that area, which is the zone of A-bend. Therefore, the A-bend becomes smaller and the H-zone becomes smaller as there are only myosin fibers, but even in that region, the thin actin fibers become shorter. When the muscle is stretched, then the A-bend again maintains its size, but the I-bend and H-zone become longer as the overlap region becomes shorter and the I-bend increases and only the thin fibers remain, whereas the A-zone, which was short because the part of the myosin where there were no thin fibers, where there were only myosin fibers, then this region expands. Dear students, in all this phenomena or all this process, neither the myosin thick fibers nor the actin thin fibers actually reduce or become shorter in their length. Actually it is the overlap that between the actin and myosin filaments, which is why the shortening of sarcomere is present. As we can see, the size of the fingers, if the overall area that they are taking, if they overlap on each other, then no finger is getting smaller, but actually the region that they covered is getting smaller, exactly this happens in the sarcomere. Dear students, the theory that Huxley and Huxley gave of the sliding filament model, its evidence, the practical evidence we get is by drawing the length tension curve. The length tension curve that is drawn explains the length tension relationship in which we measure the length of the sarcomere during contraction and compare it with the force generated because of this contraction and its result is drawn in the form of a length tension curve. This length tension curve actually explains the major assumptions of sliding filament theory. Dear students, we can see here a length tension curve. We shall explain the major points of this curve. You know that the tension produced by the muscles is maximum when largest number of cross bridges are formed between actin and myosin. These cross bridges form due to the overlap of thick and thin filaments. So this work thick and thin filaments make maximum cross bridges, that work the tension or contraction is maximum. Tension is reduced with increased length of sarcomere. The larger the sarcomere, the lower the tension is because the overlap of thick and thin filaments is reduced and the lower cross bridges are formed. This is the same way that the muscle tension or the tension of sarcomere reduces with decreased length. When the length of sarcomere is reduced, the tension is reduced because in this way the filaments of the actin start to collide with each other, so the cross bridges are not formed much. Actually this is the number of cross bridges which produce tension in sarcomere. Dear students, this curve also predicts a consequence of overstretching of sarcomere. So far that there remains no overlap between the actin and myosin filaments. As such there will be no possibility to form any cross bridges, so no active tension will dwell. Dear students, this curve shows that the tension produced by the contraction of sarcomere is proportional to its shortening which according to the sliding filament theory is due to sliding of thick and thin filaments and formation of cross bridges in the sarcomere during contraction.