How is a relaxed muscle characterized at the microscopic level?

A relaxed muscle is characterized at the microscopic level by having actin and myosin myofilaments lying side by side, with maximum H zones and I bands. In this state, the sarcomeres—the basic contractile units of muscle tissue—exhibit the distinct regions that are visible under a microscope: the I band, which consists solely of actin filaments, and the H zone, which is the area where myosin is not overlapped by actin in a fully relaxed state. This arrangement maximizes the distance between the actin and myosin filaments, allowing for more room for contraction to occur when the muscle is stimulated. The presence of well-defined I bands and a pronounced H zone indicates that the muscle fibers are not currently contracting, which is essential for understanding muscle physiology and its functional states.

In contrast, fully extended or lengthened muscle fibers typically imply tension and an interaction of filaments that does not reflect a relaxed state. Tightly bound myofilaments and complete overlap of actin filaments by myosin are indicative of muscle contraction, not relaxation. Thus, the correct description of a relaxed muscle focuses on the arrangement of actin and myosin in a way that allows for potential contraction without them being actively engaged.