Mechanism of Muscle Contraction
Contraction is the function of a muscle. It is a physical activity generating force. It causes shortening of the muscle. The widths of the I-bands and H-zone both decrease but the width of the A-band remains constant. Apparently the lengths of the myosin and actin filaments also do not change. This shows that muscle shortens in contraction without either of the kinds of filaments changing length.
Figure: The sliding-filament hypothesis
Instead contraction of the muscle fibre is brought about by sliding of the filaments past each other in their region of overlap. These observations were first made in 1954 by two independent teams, H.E. Huxley and Hanson and A.F.Huxley and Niedergerke, and they proposed it as "sliding filament model" which has been experimentally confirmed. In Figure you can note the relation of the myofilaments during shortening of two sarcomers.