During concentric muscle action in the hamstring, which zone decreases while the Z discs are pulled towards each other?

During concentric muscle action, the muscle fibers contract, leading to a shortening of the overall muscle length. This action involves the interaction between the thick (myosin) and thin (actin) filaments within the sarcomere, the fundamental unit of muscle contraction.

In this process, the Z discs, which define the boundaries of each sarcomere, are pulled closer together as the myosin filaments grab onto the actin filaments and pull them inward. As this occurs, specific areas within the sarcomere change in size. The H zone refers to the area within the A band where there are only thick filaments (myosin) and no thin filaments (actin). This zone decreases during contraction, as the actin filaments slide over the myosin filaments, effectively reducing the space where only myosin is present.

The A band, which represents the length of the thick filaments and includes areas of overlap with thin filaments, remains consistent in length during contraction because the filament lengths do not change. The I band, which consists solely of thin filaments, decreases as the actin filaments overlap more with the myosin filaments. The zone of overlap increases as the actin filaments slide past