 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, slide 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 because there were only myosin fibers, but even in that region, thin actin fibers enter. When the muscle is stretched, then the A-bend again maintains its size, but the I-bend and H-zone become longer because the region of overlap increases as the I-bend increases and only thin fibers remain in it, whereas the A-bend was short because the part of the myosin where there were not 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 lengths. Actually it is the overlap that between the actin and myosin filaments, which is why shortening of sarcomere happens. As we can see, the size of the fingers, if the overall area they are taking, if it overlaps on each other, then no finger is getting smaller, but actually the region they covered is getting smaller, exactly this happens in the sarcomere. Dear students, the theory that Huxley and Huxley gave about the sliding filament model, its evidence, the practical evidence we get is the length tension curve draw. This length tension curve is explained by the length tension relationship in which we measure the length of 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 the 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 the time when thick and thin filaments are formed, the maximum cross bridges are formed and the time when the tension or contraction is maximum. Tension is reduced with the increased length of sarcomere. The higher the tension, the lower the tension because the overlap of thick and thin filaments is reduced and the cross bridges are formed. Similarly, the muscle tension or the tension of sarcomere reduces with a decreased length when the length of sarcomere is reduced, the tension is reduced because in such a way, actin filaments start colliding with each other. So the cross bridges do not form much. Actually, this is the number of cross bridges that produce tension in sarcomere. Dear students, this curve also predicts a consequence of overstretching of sarcomere. So far, there remains no overlap between the actin and myosin filaments. There is also such a stage that if the muscle stretches too much, then there will be no overlap in the fibers or filaments of actin and myosin. In this way, there will be no possibility to form any cross bridges so no active tension will develop. 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.