Which statement accurately describes the resting state of a muscle fiber before contraction?

The correct statement regarding the resting state of a muscle fiber before contraction is that myosin heads are relaxed and unattached. In the resting state, the muscle fiber is not actively contracting, which means that the myosin heads, which are part of the thick filaments, are not engaged with the actin filaments. This state is characterized by the fact that the myosin heads are in a low-energy configuration and have not yet attached to the actin, which is essential for muscle contraction to occur.

During this resting phase, the myosin-binding sites on actin are typically not exposed because the protein tropomyosin covers these sites, preventing interaction between actin and myosin. Additionally, in this state, calcium ions are not bound to troponin; when calcium binds to troponin, it causes a conformational change that shifts tropomyosin away from the myosin-binding sites, allowing for contraction. Therefore, since no cross bridges (the connections between actin and myosin) are formed in a relaxed muscle fiber, this further supports that the myosin heads are unattached and prepared for potential contraction.