CVS Practice Test

The correct choice identifies that during contraction, I bands, which are the regions in a sarcomere of the muscle fiber that contain only thin filaments (actin), become thinner. This occurs because, during contraction, the actin filaments slide over the myosin filaments, causing the overlap between them to increase and the distance between the Z lines to decrease. As a result, the I bands shorten, giving the appearance of becoming thinner.

In contrast, during relaxation, I bands would not become thinner but return to their original state as the muscle fibers lengthen and the overlap between actin and myosin decreases. Stretching a muscle can also lead to an increase in the length of the I bands as the muscle fibers extend, while electrical stimulation typically induces contraction, thus leading to a decrease in the I bands. Understanding these concepts is essential for comprehending muscle physiology and the mechanics of muscle contraction.