Question

Which statement is correct for muscle contraction?

• A. Length of H-zone increases.
• B. Length of A-band remains constant.
• C. Length of I-band increases.
• D. Length of two Z-line increases.

Answer 1

The correct option is B Length of A-band remains constant.

During muscle contraction, the laterally projecting heads (cross bridges) of the thick myosin myofilaments come in contact with the thin actin myofilaments and rotate on them. This pulls the thin myofilaments toward the middle of the sarcomere, past the thick myofilaments. The Z lines come closer together and the sarcomere becomes shorter. Length of the A band remains constant. Myofilaments (both actin and myosin) stay the same length. Free ends of actin myofilaments move closer to the centre of the sarcomere, bringing Z lines closer. I bands shorten and H zone narrows. A similar action in all the sarcomeres results in shortening of the entire myofibril and thereby of the whole fibre and the whole muscle which results in contraction.