Sarcomere is the smallest contractile unit in a muscle fibre which is limited by the area between
2 Z bands
According to sliding filament theory of muscle contraction,the actin and myosin filament slide past each other with the help of cross bridge to reduce the length of the sarcomeres.The smallest unit of muscle contraction is a sarcomere(which is delineated by Z-lines). As a muscle contracts,the Z Lines come closer together (shortening sarcomere), the width of the I bands decreases,the width of the H zones decreases but there is no change in the width of A band. During relaxation, cross bridges disappear and actin filaments slide back from A bands,the width of the I bands and H zones increases, but there is still no change in the width of the A band.